UW NKLr
METEOROLOGICAL GLOSSARY
FFICE.
(Fourth Issue)
In continuation of The Weather Map, (M.O. 225 i).
fcg tf)c authority of tfje iJHetforologiral Committrr.
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; i . LIMITED, J.
AND .'JIOM
| M OEOLOGHCAL
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M.O. 225 ii.
METEOROLOGICAL OFFICE.
METEOROLOGICAL GLOSSARY
(Fourth Issue)
In continuation of The Weather Map, (M.O. 225!,)
tg ifje &tttf)orttg of tije jj&eteoro logical Committee.
LONDON :
PBINTED UNDER THE AUTHORITY OF HIS MAJESTY'S STATIONERY
OFFICE
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AND TO BE PURCHASED FROM
TH-E METEOROLOGICAL OFFICE, EXHIBITION ROAD, LONDON, S.WJ,
Price Is. net.
For units or systems of units.*
F.P.S. is equivalent to foot-pound-second system of units.
C.G.S ,, centimetre-gramme-second system of units.
B.T.U. ., British Thermal units.
.The following are also used :
For the
expression of
Abbrevia-
tion.
Meaning.
Angle
O 1 II
Degrees, minutes, seconds of arc.
Density
g/m 3
g/cc
Ib/cu. ft.
Grammes per cubic metre.
Grammes per cubic centimetre.
Pounds per cubic foot.
Length
mm
cm
m
k
Millimetre.
Centimetre.
Metre.
Kilometre.
Mass ...
&
Gramme.
Kilogramme.
Pressure
mb
cb
Millibar.
Centibar.
Temperature
a
C
F
Absolute scale of temperature.*
Centigrade scale of temperature.
Fahrenheit scale of temperature.
Velocity
. rn/s
mi/hr
Metres per second.
Miles per hour.
Volume
cc
m 3
Cubic centimetre.
Cubic metre.
* The abbreviation "a" is al^o us'-vi '-o represent a unit or degree of
temperature en the absolute scale. Fov aa iacerval of temperature la is the
same as 1 0.
METEOROLOGICAL GLOSSARY.
TABLE OF CONTENTS.
The entries marked (I) are also referred to in The
Weather Map. (M.U. 225 i.)
Page. Page.
Absolute Extremes ...
12
Aneroid barometer ..
30
Absolute Humidity ...
290
Aneroidograph
30
Absolute Temperature
12
Anthelion
30
Accumulated Temper-
293
Anticyclone (I)
30
ature.
Aqueous-vapour
31
Actinometer ...
14
Atmosphere (I)
33
Adiabatic
15
Atmospheric Electri-
294
Aerology ...
16
city.
Aeroplane Weather ...
16
Audibility
33
Air(i)
19
Aureole
36
Air-Meter
20
Aurora...
298
Airship- Weather
20
Autumn
36
Altimeter
27
Average
37
Altitude
28
Azimuth
37
Alto-cumulus ...
28
Alto-stratus
28
Backing
37
Anabatic
29
Ballon sonde ...
39
Anemobiagraph
29
Balloon Kite ...
42
Anemogram .,.
29
Bar
43
Anemograph ...
29
Barogram
43
Anemometer ...
29
Barograph
43
Anemoscope ...
29
Barometer
43
(13204r 12.) Wt. 26779464. 7000. 3/18. D & S. (S.) Gr.
3.
423582
Glossary.
I
'age. i
Page.
Barometric Tendency
44 Condensation ...
... 70
Beaufort Notation (J )
44 Conduction
... 71
Beaufort Scale (1)
45
Contingency ...
... 302
Bishop's Ring ...
46
Convection
... 71
Blizzard
46
Corona ... ...
... 71
Blue of the Sky
46
Correction
, ... 12
Boiling Points
300
Correlation
... 74
Bora
47
Correlation Ratio
... 302
Breeze ...
48
Cosecant
... 76
Brontometer ...
49
Cosine ...
... 76
Buoyancy
49
Cotangent
... 76
Buys Ballot's Law (I)
57
Counter Sun ...
... 76
Cumulo-stratus
... 77
O.G.S
57
Cumulus
... 77
Calm
57
Cyclone (I)
... 77
Calorie...
301
Cyclostrophic ...
... 77
Celsius ...
57
Centibar
58
Damp Air
... 77
Centigrade
58
Day Breeze
... 77
Centimetre
58
Debacle -
... 77
Cirro-cumulus
58
Dekad
... 78
Cirro-stratus ...
58
Density...
... 78
Cirrus ..-
58
Depression
... 82
Climate
58
Dew
... 82
Climatic Chart
59
Dew-point
... 82
Climatology
60
Diathermancy
... 82
Clouds
60
Diffraction
... 83
Cloud Burst
67
Diffusion
... 84
Clouds, Weight of ..
67
Diurnal
... *5
Col
68
Doldrums
... 88
Compass
68
Drought
... 88
Component
69 Dry Air
... 88
Table of Contents.
Page, i
Page.
Dry Bulb
... 88
Forecast
... 117
Duration of Rainfall
... 303
Freezing
... 118
Dust
... 304
Frequency
... 118
Dust-counter ...
... 305
Friction
... 122
Dynamics
... 89
Frost
... 123
Dynamic* Cooling
... 89
Gale ...
... 124
Earth Thermometer
... 89
Gale Warning...
... 128
Eddy
... 89
Gas
... 129
Electrification of Water- 337
Geostrophic
... 129
drops.
Glacier ...
... 307
Electrometer ...
... 91
Glazed Frost ...
... 129
Energy
... 9*
Glory ...
... 130
Entropy
... 94
Gradient
... 130
Equation of Time
... 100
Gradient Wind
... 134
Equator
... 100
Gramme
... 138
Equatorial
... 101
Grass Temperature
... 139
Equilibrium ...
... 102
Gravity
... 308
Equinox
... 103
Great Circle ...
... 139
Error ...
... 104
Gulf Stream ...
... 139
Evaporation ...
... 106
Gust
... 140
Expansion
... 109
Gustiness
... 142
Exposure
... 109
Extremes
... 110
Hail
... 142
Halo
... 143
Fahrenheit
... 110
Harmattaii
... 145
Fall
... Ill
Harmonic Analysis
... 311
False Cirrus ...
... 306
Haze
... 145
Fluid
... Ill
Heat
... 145
Fog
... 112
High (I)
... 152
Fog Bow
... 116
Hoar Frost
... 152
Fohn
... 115 Horizontal
... 152
(jrlossary.
Page. | Page.
Horse Latitudes
... 153 I Katabatic
182
Humidity (I)
... 154 i Khamsin
182
Hurricane
... 155
Kilometre
183
Hydrometer . . .
... 158
Lake
183
Hydrosphere ...
... 158
Land Breeze
1*3
Hyetograph
... 158
Lapse ...
183
Hygrograph ...
... 158
Lenticular
185
Hygrometer ...
... 159
Level ...
185
Hygroscope
... 159
Lightning
186
Hypsometer ...
... 160
Protection against
324
Line Squall
188
Ice ...
. 321
Liquid
189
Iceberg
... 161
Low (I)
189
Incandescence
... 161
Lunar ...
169
Index ...
... 162
Index Error ...
... 162
Mackerel Sky ...
190
Insolation
... 162
Magnetic Needle
190
Inversion
... 163
Magnetism
327
Ion
... 164 Mammato-Cumulus ...
190
lonisation
... 322
Mures 1 Tails ...
191
Iridescence
... 166
Maximum
191
Irisation
... 166
Mean ...
191
Isabnormals ...
... J66
Meniscus
191
Isanomalies
... 166
Mercury
192
Isentropic
... 167
Meteor ...
192
Iso
... 168
Meteorograph ...
193
Isobars ...
... 168
Meteorology ...
193
Isohels 1
Metre ...
193
Isohyets 1 g
Tcsn
Microbarograph
193
Isopleths [ k
JLbU.
Millibar
194
Isotherm J
Millimetre
195
Isothermal
. 181
Minimum
195
Table of Contents.
Page.
Page.
Mirage ...
... 195
Pocky cloud 212
Mist
... 196
Polar 212
Mistral ...
.., 197
Pole 212
Mock San
...'197
Potential 213
Mock Sun Ring
... 197
Potential Temperature 213
Monsoon
... 197
Precipitation 329
Moon
... 198
Pressure ... ... 213
Prevailing winds ... 213
Nadir
. 198
Probability 214
Nephoscope
... 198
Prognostics ... ... 215
Nimbus
... 198
Psychrometer 216
Normal
... 198
Pumpimr 216
Purple Light ... ... 217
Observer
... 203
Pyrheliometer ... 217
Ombrometer ...
... 203
Orientation
... 203
Radiation 330
Orographic Rain
... 203
Rain ... 217
Ozone
... 204
Hainband . ... 218
Rainbow .. ' ... 218
Rainday 219
Pampero . .
. 204
Raindrops, size of, &c. 334
Paranthelion . .
... 204
Rainfall ., ... 219
Paraselenae . .
... 204
Rainfall, duration of . . . 303
Parhelia , .
... 204
Raingauge 219
Pentad
... 205
Rain-spell ... ... 219
Periodical
... 205
Reaumur 219
Persistence . .
... 206
Reduction 220
Persistent Rain
... 206
Reduction to Sea Level 220
Phases of the Moon
... 208
Refraction 222
Phenology
... 210
Registering balloon ... 223
Pilot-balloon ...
... 210
Regression-equation ... 339
Pluviograph ...
... 211
Relative humidity ... 223
Pluviometer ...
. 212
Reversal 224
Glossary.
Page.
Page.
Ridge
225
Solar Radiation Ther- 238
Rime ...
225
mometer.
River
225
Solstice
... 238
Roaring Forties
226
Sounding
... 238
Spells of Weather
... 238
Spring ...
... 239
St. Elmo's Fire
226
Squall
.,. 239
Saturation
226
Stability
... 239
Scotch Mist
340
Standard Time
... 239
Screen
226
State of the Sky
... 239
Scud
226
Statics
.. 240
Sea-breeze
227
Station
... 240
Sea-level
227
Statoscope
... 240
Seasons
227
Storm ...
... 241
Secant ...
231
Storm Cone
... 241
Secondary
231
Strato-cumulus
... 241
Seismograph ...
231
Stratosphere ...
... 241
Serein ...
231
Stratus
... 241
Shamal
231
Summer
... 241
Shepherd of Banbury
232
Sun
... 241
Silver Thaw
235
Sun-dial
... 343
Simoon
235
Sun-dogs
... 242
Sine
235
Sun Pillar
... 242
Sine curve
236
Sunset Colours
... 242
Sirocco
237
Sunshine
... 242
Sleet
237
Sunshine-recorder
... 243
Snow ...
237
Sun spot Numbers
... 243
Snow crystals
237
Surge
... 244
Soft hail
343
Synoptic
... 244
Solar Constant
237
Solar Day
238
Tangent
... 244
Solarisation
238
Temperature (I)
... 244
Table of Contents.
Page.
Page.
Temperature Gradient 246
Vapour Tension
... 266
Tension of Vapour .... 247
Vector ...
... 266
Terrestrial 247
Veering
... 267
Terrestrial Magnetism 327
Velocity
... 267
Thaw 247
Vernier
... 268
Thermodynamics ... 247
Vertical
... 268
Therm ogram ... ... 247
Viscosity
.,, 269
Thermograph 247
Visibility
... 269
Thermometer 248
Vortex
... 347
Thunder 248
Thunderstorm ... 249
Time ... 252
Water
... 271
Tornado 252
Water-Atmosphere
... 273
Torricelli 253
Waterspout
... 274
Trade Winds 253
Water-Vapour
... 274
Trajectory ... ... 262
Waves ...
... 274
Tramontana 263
Waves of Explosion
... 276
Transparency ... ... 263
Weather (I) ...
... 276
Tropic ... ... ... 263 Weather maxim
... 276
Tropical 263 Wedge
... 279
Tropopause ... ... 263
Weight
... 279
Troposphere 263
Wet Bulb
... 279
Trough 263
Whirlwind
... 280
Twilight 344
Wind
... 280
Twilight Arch ... 264
Wind Rose
... 351
Type 264
Winter ...
... 288
Typhoon 265
Wireless Telegraphy
... 288
Upbank Thaw 265
Zenith ...
...288
V-Shaped depression... 266
Zodiac ...
... 289
Vapour Pressure ... 266 j Zodiacal Light
... 289
10
TABLE OF CONTENTS OF THE WEATHER MAP.
AN INTRODUCTION TO THE GLOSSARY.
(Now issued as a separate volume.)
PAGE.
Meteorology and Military Operations 3
Weather Records nd Climate 5
The Necf ssity for Forecasts of Weather ... 7
Modern Meteorology the Work of an Organisation, not of an
Individual ... 8
The Meteorologist at Headquarters ... ... ... ... 8
A Map of the Weather ... 9
The Beaufort Notation ... .., 10
A Map of the Winds 12
The Beaufort Scale 13-14
The Atmosphere . , 15
Water Vapour : Evaporation and Condensation .,. ... 17
Temperature and Humidity , 19
A Map of Temperature
Pressure and Its Measurement 21
The Barometer 23
A Map of the Distribution of Pressure
Isobars ... ... ... ... ... ... ... ... 25
A Map of all the Elements together 26
Lessons from Weather Maps 26
Buys-Ballot's Law and the General Relation of Pressure
to Wind .., 26
Weather and Temperature 30
The Sequence of Weather 31
The Travel of the Centres of Cyclonic Depressions ... 35
Barometric Tendency ... ... ... ... ... ... 41
Veering and Backing of Wind 42
Types of Pressure Distribution . 43
The Upper Air. The Dynamics and Physics of the
Atmosphere ... ... ... ... ... ... ... 44
Distribution of Rain and Cloud in Cyclonic Depressions... 50
Climatic Supplement, Charts and Diagrams ... 55
11
METEOROLOGICAL GLOSSARY.
CONTAINING INFORMATION IN EXPLANATION OF
TECHNICAL METEOROLOGICAL TERMS.
Details as to the use of meteorological instruments are
given in' the Observers Handbook, and as to the numerical
computations in the Computer's Handbook, to both of
which reference is made in the Glossary when required.
In accordance with the practice of the Oxford Diction-
ary the initial word of each article is in black type, and
words in the body of the text are printed in small capitals
when they are the subjects of articles in another part of
the glossary.
For the articles in this glossary I am principally
indebted to the Staff of the Observatory at Benson,
W. H. Dines, F.R.S., and E. V. Newnham, B.Sc. ; and of the
Branch Office at South Farnborough, Captain C. J. P.
Cave, R.E., and R. A. Watson Watt, B.Sc., with Major Taylor,
Professor of Meteorology, R.F.C. ; and to the staff of the
Forecast Division, especially F. J. Brodie, E. L. Hawke,
B.A., and Second Lieutenant T. Harris, R.E., who have
passed the MS. through the press, and W. Hayes who
prepared many of the illustrations.
The revision of the work for the present fourth issue
has been carried out by Dr. C. Chree, F.R.S., Superintendent
of the Observatory, Richmond. Some new articles have
been added at the end of the volume, pp. 290-354.
NAPIER SHAW.
Meteorological Office,
26 October, 1917.
12
METEOROLOGICAL GLOSSARY.
Absolute Extremes. The word extreme is often
used with reference to temperature to denote the highest
and lowest temperatures recorded at an observing station
in the course of time. As the observations are generally
summarised for a year, extreme temperatures come to
mean the highest and lowest temperatures of a year.
When the survey is taken over a longer period, 10 years,
20 years, 35 years or 200 years, according to the duration
of the observations, the highest and lowest temperatures
observed during the whole period are called the absolute
extremes. The journalistic expression would be u the
record," high or low as the case may be.
The absolute extremes for the British Isles are, highest
310'8a., (100 F.) ; lowest 245'8a., (-17 F.). Those
for Belgium, highest 311'2a., (]QO-8 F.) ; lowest
243-2a., (-21-6 F.). The Surface of the Globe, highest
329-7a., (134 F.) ; lowest 203'2a., (-93-6 F.). In the
Upper Air, lowest 182'Ja., (- 131'6 F.) at a height of
16^ km. over Java.
Absolute Temperature. The temperature of the
centigrade thermometer, increased by 273, more properly
called the temperature on the absolute, or thermodynamic
scale. The absolute scale is formulated by reasoning
about the production of mechanical work at the expense
of heat (which is the special province of the science of
thermodynamics, see ENTROPY), but for practical purposes
the scale may be taken as identical with that based on
the change of volume and pressure of one of the per-
manent gases with l^eat. For thermometric purposes
aiming at the highest degree of refinement the hydrogen
scale is used, but for the purposes of meteorological
Absolute Temperature. Ui
reckoning the differences of behaviour of the permanent
gases, Hydrogen, Oxygen, Nitrogen are unimportant. In
physical* calculations for meteorological purposes the
absolute is ihe natural scale ; the densities of air at any
two temperatures on the absolute scale are inversely
proportional to the temperatures. .Thus the common
formula for a gas,
P _ ' Po ^
P (273 + " PO C^ 73 + *o)
where p is the pressure, p the density and t the tempera-
ture Centigrade of the gas at one time, p Q , p , to the corre-
sponding values at another, becomes
JL ffo
where T and Jo are the temperatures on the absolute centi-
grade scale. Its most important feature for practical meteor-
ology is that from its definition there can be no negative
temperatures. The zero of the absolute scale is the tempera-
ture afc which all that we call heat would have been spent.
In the centigrade scale all temperatures below the freezing
point of water have to be prefixed by the negative sign .
This is very inconvenient, especially for recording obser-
vations in the upper air, which never gives tsmperatures
above the freezing point in our latitudes at much above
4 kilometres (13,000 feet), and often gives temperatures
below the freezing point nearer the surface.
The absolute temperature comes into meteorology in
other wavs ; for example, the rate at which heat goes out
into space from the earth depends, according to Stefan's
Law, upon the fourth power of the absolute temperature
of the radiant substance. See RADIATION.
14
Glossary,
Temperatures can also be m expressed in an absolute
scale of Fahrenheit degrees of 'which the zero is approxi-
mately 45',' below the Fahrenheit zero.
Some common temperatures on the absolute scales and
their equivalents in Centigrade and Fahrenheit are :
Centigrade.
Fahrenheit.
af.
The boiling point of
4
269
-452-2
7-2
helium.
The boiling point of
77
-196
-320-8
138-6
nitrogen.
The freezing point of
234-2
-38-8
-37-8
421-6
mercury.
The freezing point of
273
32
491-4
water.
The mean temperature
282-7
9'7
49'5
508-9
of London.
" Temperate " as shown
285-8
12-8
55
5H'4
on an ordinary ther-
mometer.
The best temperature
290
17
62-6
522-0
for a living 1 room.
A hot summer day
300
27
80-6
540-0
The temperature of the
3io
37
98-6
558'o
human body.
The temperature of the
6,000
10.000
Sun.
Actinometer. An instrument for measuring the
intensity of RADIATION received from the sun. In
Michelson's Actinometer, for example, the essential
element consists of two strips of different metals fixed
together. These are heated by the solar radiation which
they absorb, and the amount of bending which results
Actinometer. 15
from their unequal expansion is a measure of the rate at
which they are receiving radiant energy.
AdiabatiC The word which is applied in the science
of thermodynamics to the corresponding changes which
may take place in the pressure and density of a substance
when no heat can be communicated to it or withdrawn
from it.
In ordinary life we are accustomed to consider that
when the temperature of a body rises it is because it takes
in heat from a fire, from the sun or from some other
source, but in the science of thermodynamics it is found
to be best to consider the changes which occur when a sub-
stance is compressed or expanded without any possibility
of heat getting to it or away from it. In the atmosphere
such a state of things is practically realised in the interior of
a mass of air which is rising to a position of lower pres-
sure, or sinking to one of higher pressure. There is, in
consequence, a change of temperature which is called
mechanical or dynamical, and which must be regarded as
one of the most vital of meteorological phenomena because
it accounts largely for the formation and disappearance of
cloud, and probably for the whole of our rainfall.
Tyndall' illustrated the change of temperature due to
sudden compression by pushing in the piston of a closed
glass syringe and thus igniting a piece of tinder in the
syringe. The heafcing of a bicycle pump is a common
experience due to the same cause.* On the other hand
the refrigeration of air is, often obtained simply by ex-
pansion, particularly in the free atmosphere.
* Dangerous heating- may result on firing a gun from the sudden
compression of gas within the bursting charge of the shell if there
are cavities in the explosive therein.
16
Glossary.
To plan out the changes of temperature of a substance
under compression and rarefaction alone, we have to
suppose the substance enclosed in a case impermeable to
heat the word adiabatic has been coined to denote
impermeable to heat in that sense. The changes of
temperature thereby produced are very great, for example :
For adiabatic change of pressure
decreasing 1 from 1000 nib. by
The fall of temperature from
29oa, 62-6 F.. is
nib.
in.
<J.
F.
10 c
r 0*30
0-9 c
r 1*6
100 ,
2'95
8-7
I5'7
200
S'9 1
18-2
32-8
300
8-86
28-4
51*1
400
11-81
39*9
71-8
500
14-77
52-8
95*o
600
17-72
67-6
121*7
700
20*67
85-5
153-9
800
23-62
108-1
, 194-6
900
26-58
I4 1 "3
> 254-3
Aerology. The study of the free air; a word that has
come into use recently to indicate that part of meteorology
which is concerned with the study of the upper air.
Some of the results are given under BALLON-SO.NDE and
PILOT-BALLOON.
Aeroplane weather. The weather most suitable
for aeroplanes is calm clear weather with little or no
wind. The only conditions which make it impossible
for a good pilot to fly a modern aeroplane are a strong
To face p. 16.
Diagram showing the pressure in the upper air corresponding with the
standard pressure (1013-2 mb.) at the surface and adiabatic lines for
saturated air referred to height and temperature. (From Neuhoff
Smithsonian Miscellaneous Collections, Vol. 51, No. 4, 1910.)
The pressure is shown by full lines crossing the diagram, and the adia-
batic lines for saturated air by dotted lines. Temperatures are given in the
absolute scale.
The short full lines between the ground and the level of 1,000 metres
show the direction of the adiabatic lines for dry air.
Aeroplane Weather. 17
GALE or a FOG. On the other hand many weather con-
ditions may prevent useful work from being done by an
aeroplane when it is in the air. The weather affects
civilian and military flying in quite different ways.
When testing aeroplanes, with a view to finding their
rate of climb, top speed when flying level, landing speed,
or other aerodynamical quantities it is usually necessary
to choose a calm day, when eddies, or large ascending or
descending currents, or other conditions prejudicial to
accurate testing, are unHkely to occur.
In flying across country the chief danger is that the
engine will stop when the aeroplane is over ground on
which it is impossible to land. When the engine has
stopped the aeroplane must come down somewhere in-
side a circle whose radius is about equal to five times the
height of the aeroplane above the ground. An aeroplane
flying at a height of one mile will have an area of about
75 square miles in which it may choose its landing ground,
while at a height of 2,000 feet on a calm day the machine
has less than 12 square miles to choose from.
In England it is almost always possible to pick out a
possible landing ground in a circle containing 75 square
miles ; but it is frequently impossible to do so in an area of
12 square miles. For this reason clouds below 6,000 feet
are one of the chief dangers of cross-country flying, and
the lower they are the more dangerous they become.
In flying under war conditions eddies and vertical
currents are almost immaterial provided they are not so
violent as to impede observations. On the other hand
low clouds make observations over an enemy's lines almost
impossible, owing to the accuracy of modern anti-aircraft
gnus. Detached clouds iiupede, but do not put a stop to,
reconnaissance. On days when the clouds are too low to
20 Glossary : Lithoplate IX.
the upper regions, collections of water globules which are
called clouds ; in meteorology the dust is regarded as an
impurity, and the clouds as an addition to the air, not
part of it. The dust, though an impurity, is important, as
it makes the formation of cloud and rain possible when-
ever the temperature of a mass of air gets below the
DEW-POINT.
Air-meter. The name given to an apparatus designed
to measure the flow of air. It consists of a light wheel
with inclined vanes carried by the spokes, and a set -of
counting dials to show the number of revolutions of the
wheel. Its accuracy can be tested on a whirling table.
As generally sold it is the most portable form of ANEMO-
METER, its box not being more than four inches each way.
But it cannot be used with success by a careless observer.
Airship-weather. Favourable weather.- The most
favourable conditions for airships are calms or light airs,
with good seeing from above, extending over the whole
area to be traversed, and persistent for the whole period, say
24 hours. Detached low clouds may be an advantage, but
precipitation, whether in the form of rain, snow or hail,
would spoil the occasion. The favourable conditions
thus defined are characteristic of the central part of an area
of high barometric pressure which, in technical language,
is called an ANTICYCLONE. Thus, for operating across
the North Sea, the primary meteorological conditions will
be favourable when the barometer readings at Helder,
Yarmouth and Grisnez are higher than those at surround-
ing places, because the three places named will then be
in the central region of an anticyclone, cf. Fig. 3, p. 25.
/ During an anticyclone the pressure is, as a rule, above
the normal for the place ; thus, pressures above 1,020
AIRSHIP WEATHER. FIG. 1.
DISTRIBUTION OF TEMPERATURE, WEATHER, WIND, AND
PRESSURE, 6 P.M. 6th SEPTEMBER, 1915.
Normal Ttroperature ^^^*^~* ~~
of the Se*tS*pt.*il
ISOBARS are drawn for interrals of fire milH- WEATHER. Shown by lh following symbols -
bar*. f\ olear sky. f) iky i clouded.
bl " rowa
the scale 0-12, is iodi-
o%Ud by the number of father,
l
overcatt sky. A rain falling
^ow. A bail* S f OR.
Tthwodtr. "K thunderstorm.
TEMPERATUR.-OiTen in dfrta Fahrenheit
Airship - Weather. 21
millibars (30*1 inches) are generally to be found in anti-
cyclones, and this has given rise to the statement that a
high barometer in itself indicates favourable conditions
for airships and vice versa. This is often, but not always,
the case. If one plots the pressure on a map the favour-
able area extends outward from the central region where
the highest pressures are found until the pressure begins
to fall away rapidly, arid then one finds strong winds and,
possibly, also rain or snow.
An anticyclone is indicated on a map by drawing lines
of equal pressure, ISOBARS, which naturally enclose the
area of highest pressure. The lines of an anticyclone,
generally speaking, run in roughly parallel curves and are
easily recognisable on a map, such as the one reproduced
here, Fig. 1. The shape is sometimes that of a regular
curve, more or less like a circle or an oval, as in this
instance, but often it is quite irregular and straggles over
a large region. Anywhere near what may be called the
top of it, ^.e., the region of highest pressure^ there are
calms or light airs. Further away the winds begin to
range themselves in circulation round the central region
easterly winds on the uouth side, westerly on the north.
Further out, as one gets towards the regions of low pres-
sure, the winds become brisker, and on the margins of an
anticyclone they may be very strong, but on the eastern
side they are generally steady, not changeable or squally.
It is characteristic of an anticyclone that when it is
once set up and well marked it generally lasts two or
three days ; sometimes it is persistent for a week or
10 days, occasionally even more. An anticyclone is, in
fact, typical of settled weather, and consequently the
setting in of a large anticyclone over the area of
22 Glossary : Lithoplate X.
operations may be regarded as providing an ample period
of favourable weather.
An anticyclone in our neighbourhood generally drifts
eastward or north-eastward. It has northerly winds on its
eastern or front side, so that the setting in of a northerly
wind, veering to N.E., generally means that an anticyclone
is coming over, and as it will take two or three days at
least to pass, the navigator is practically sure of a few days
of fair conditions, and while the central region is going
over and the wind is slacking down from north-east to
calm, and then changing to south or south-west, there is
practically certain to be a perfect day, possibly two or
three, for operating an airship.
All that an airship navigator has to do, therefore, to hit
upon a favourable time fo/ a raid is to choose the occasion
when there is an anticyclone with its central area over
southern England or the Channel, advancing slowly east-
ward, as most of them do. He may then reckon on two or
three days' favourable weather, and if he watches the map
may extend his forecast day by day as he notes the
behaviour of the anticyclone. An anticyclone is such a
well-recognised creature in meteorological maps that
observations from one half of it are sufficient to go upon
in forming a judgment as to its existence. Its end comes
with the southerly wind that marks its western side, so
that a southerly, wind, even a light one, is a warning
which no hostile navigator is likely to disregard.
In winter it is often foggy in anticyclonic weather,
particularly when the anticyclone is going away, and
sometimes it is cloudy and gloomy, but there is never
heavy rain in the central region, and seldom any rain at all.
Unfavourable weather. The most unfavourable
weather for hostile operations with airships is cyclonic
Plate X.
AIRSHIP WEATHER. FlG. 2.
DISTRIBUTION OF TEMPERATURE, WEATHER, WIND, AND
PRESSURE, 7 A.M. 1st NOVEMBER, 1915.
ISOBARS are drawn for intervals of five milli- WEATHER. Shown by the following gym bols :
^. bftrs - C\ clear ky. O k T i clouded.
WIND.- Direction Is shown by arrcrws fljing X ;s: .
with the wind. dD 8R y ^ clouded. (Jj) sky j clouded.
Force, on the scale 0-12, is indi- /Tix overcast sky. A rain falling
Umber f I8atheri - Y ow. A hail s fog
=uiist, T thunder. 15 thunderstorm.
TEMPERATURE. t/iven in degrees Fahrenheit.
x
f J
A irsh ip- Wea ther. 23
weather represented on the map by a region of relatively
low barometer round which strong winds circulate. (See
figure 2.) It is the opposite of anticyclonic weather.
The cyclonic depression passes rapidly across the map
and the weather goes through a well-known cycle of
phases in the course of twelve to twenty-four hours. A
well-developed cyclonic depression makes successful air
raiding impossible, partly because of the strength of the
wind, which may reach 50, 60 or 70 miles an hour in the
upper regions, and still more because of its variability
(the wind is gusty and squally and is also liable to regular
changes), which may give the ship as much lee-way as
traverse. This, in darkness, means losing the course and
probably losing the bearings. Besides, there is often
heavy rain or snow with cyclonic weather.
In the South of England cyclonic weather generally
begins with a southerly or south-westerly wind, and as
there is frequently a succession of depressions passing
along the same track there are successive fallings and
risings of the barometer and successive phases of southerly
wind, veering to N.W. with the rising barometer, and
backing again to S.W. with a falling barometer.
Between two successive depressions there is often a day
of perfect weather, light, transparent airs and clear skies.
An airship commander who started at the right time
might use this brilliant interval to make a raid, but with-
out extremely expert forecasting, which would require
ample telegraphic information, it is too dangerous. He is
more likely to wait until the setting in of a northerly or
north-easterly wind marks the beginning of an anticyclone.
Risky weather. Between these two extremes of easily
identifiable weather, favourable or unfavourable, there
are a number of conditions which may be called risky,
24 Glossary.
periods of slow transition between anticyclonic and cy-
clonic, or periods of vague type without marked features.
, These require an expert knowledge of meteorology if
they are to be dealt with successfully. -So far as we know,
hostile aircraft have used special meteorological observa-
tions (with pilot balloons) to identify a case in which a
strong north-easterly wind, too strong for easy navigation,
fell off and became much lighter in the upper air. That
is characteristic of easterly and north-easterly winds, but
there are exceptions. To make use of the proper occasion
in this particular is certainly clever, but it is risky ;
because we can only take advantage of such cases when
we happen to find them, and we do not know any law
of their distribution. In this connexion it may be re-
marked that airships are not likely to take the air at
night in a strong wind. Not knowing precisely the dis-
tribution of air currents, it is impossible to lay a course
for an objective across wind, so the objective must be
approached ultimately up wind. That means the slowest
speed at the most critical point.
The most risky weather for an airship is when a cyclonic
depression with southerly wind in its front advances
rapidly eastward and replaces the light airs in front of it.
Light airs, it has been remarked, are characteristic of the
central region of an anticyclone, but they are also
characteristic of the region in front of an advancing
depression. Depressions sometimes advance at a rapid
rate, say 25 miles an hour 600 miles a day always in
that case from the W 7 est or south-west. In winter the risk
which an airship runs depends a good deal upon the
position of the centre of the depression. On the northern
side of it, or in its rear, there is often snow, in the latter
case with strong northerly winds.
Airship- Weather.
FIGURE 3.
25
Chart of Barometer Reading's
Variations of the Barometric pressure at five stations during the
passage of an anticyclone in September, 1915.
26
Glossary.
FIGURE 4.
Chart of Barometer Readings
Oct.- Nov., 1915
1020mb
Helde-r
Yarmouth
IQIQmk
Holqhead
C.GrisNer
woo*
Skudesnaes
Variations of Barometric pressure during the passage of two consecutive
cyclonic depressions in October-November, 1915.
Airship- Weather. 27
This description is quite typical, and is exactly applic-
able to the case of February 17th, 1915, when two airships
were lost.
Prognostication. It will be understood from what has
been written above that forecasting favourable weather
for air raids for a few days ahead is an easy matter when
an anticyclone establishes itself on the map. It requires
only the most elementary knowledge of weather-study.
Similarly it is quite easy to recognise a day or two in
advance the periods of unfavourable weather. The best way
of doing this is to have a daily map upon which the
positions of the anticyclone or cyclonic depressions are
easily recognised. But if a chart of consecutive barometer
readings is preferred, the charting of the readings at
Helder, Yarmouth, Holyhead, Grisnez, Skudesnaes may be
recommended. Figure 3 shows the chart for the
passage of the anticyclone of Figure 1 , and Figure 4 the
chart for the passage of the cyclone of Figure 2.
Forecasting for the more risky weather is a matter for
experts, and entails a careful study of weather maps.
What was curious about the late summer and autumn of
1915 was the frequency of anticyclonic periods, and their
coincidence with times of new moon.
A succession of depressions is generally characteristic
of the weather of North-Western Europe, and, consequently,
a competent meteorological establishment would naturally
lay itself out to catch the occasional opportunity. But
this was not at all necessary in the season of 1915. No
particular skill in forecasting was required.
Altimeter. An aneroid barometer graduated to show
height instead of pressure. The most that an aneroid
barometer can do is to give a satisfactory measure of the
28 Glossary.
pressure of the air. The pressure is very largely affected
by the height, and, therefore, whatever indicates the
pressure gives a rough indication also of the height.
The accurate determination of the height of a position
requires a knowledge of the temperature of the air at
successive steps, so that a mean temperature may be
obtained for the column between the position and the
earth. There are various short ways of making estimates
of the temperature of the column, but in any case the
temperature at the top and bottom should be noted.*
Altitude. The angle in a vertical plane subtended at
the eye of the observer by the line drawn from the top of
an object to the horizon. The word is also used commonly
as synonymous with height.
AltO-CUmulus. A form of cloud of middle height
(10,000 feet to ^5,000 feet). It consists of fleecy groups
of cloudlets called by the French " gros-inoutons."
The separate cloudlets are thick enough to show a
darkening of the white ; the similar groups of smaller and
higher clouds, cirro-cumulus, show no shadow. See
CLOUDS.
AltO-StratUS. A sheet of continuous cloud of middle
height, of considerable size and moderate thickness, some-
times covering the whole sky. It must be distinguished
from cirro-stratus which is higher and thinner, and stratus
without any prefix, which is the lowest form of cloud-
sheet.
* The pressure at the foot of the column must also be known, and
the lack of this knowledge is a source of error with a machine
travelling over great distances. A special note on the subject prepared
for the Handbook of Meteorology can be obtained from the Meteorological
Office.
Anabatic. 29
Anabatic. Referring to the upward motion of air
due to convection. A local wind is called anabatic if it is
caused by the convection of heated air ; as, for example,
the breeze that blows up valleys when the sun warms the
ground. See BREEZE.
Anemobiagraph. See ANEMOGRAPH. .
Anemogram. The record of an anemograph.
Anemograph.. An instrument for recording the
velocity or force, and sometimes also the direction of the
wind. The best known forms of anemograph are
the Robinson Cup anemograph similar to that designed
by Beckley for Kew Observatory, the Tube anemograph
with direction recorder similar to that designed by
Dines for Benson Observatory (which might be called
the Harpagraph or gust-recorder), the anemobiagraph
designed by Halliweli for Negretti and Zambra, the Dines
Tube recorder, with direction recorder designed by
Rooker for R. W. Munro.
The Royal Observatory at Greenwich and the Observa-
tory of the Mersey Docks and Harbour Board, near
Liverpool, have pressure-plate anemographs by Osier.
Anemometer. An instrument for measuring the
velocity or force of the wind. Anemometers register in
various ways ; by counting the number of revolutions of
cups in a measured time, by the difference of water level
in a tube, and in other ways. Information as to the
construction and use of various anemometers is given in
the Observer's Handbook,
Anemoscope. An instrument for indicating the
existence of wind and snowing its direction. The one
30 Glossary.
best known to the Meteorological Office is that designed
by Mr. J. Baxendell which is provided with recording
mechanism. It is an observatory -instrument, not a
portable one.
Aneroid Barometer. An instrument for deter-
mining the pressure of the atmosphere. It consists of a
shallow air-tight metal box, usually nearly exhausted of
air. The distance between opposite faces of the box
alters with change in the surrounding atmospheric
pressure, the alteration being shown on a dial by a hand
actuated by a suitable train of levers. An aneroid is light,
portable and convenient, but should be compared occasion-
ally with a mercury barometer, as an appreciable change
of zero sometimes occurs. It is also subject to "creep,"
e.g., after a recent large fall of pressure such as may
occur when it is used as an ALTIMETER it will, though
under a really constant pressure, show a small spurious
further fall, which in the course of an hour may amount
to 1 or 2 per cent, of the previous fall. u Creep " in the
same direction may be perceptible for several hours, but
its rate continually diminishes.
Aneroidograph.. A self-recording aneroid. An
aneroid-barometer provided with mechanism for record-
ing the variations of pressure of the atmosphere. See
BAROGRAPH.
Antlielion. A colourless MOCK SUN (see HALO)
appearing at the point of the sky opposite to and at the
same ALTITUDE as the sun.
Anticyclone. An anticyclone is a region in which
the barometric pressure is high, relatively to its surround-
ings, and is generally shown on the weather charts by a
series of closed isobars, the region of highest pressure
Anticyclone. 31
being the central region of the anticyclone. In a well-
marked anticyclone the isobars are roughly circular or
oval curves, the wind blows spirally outwards in accord-
ance with Buys Ballot's Law, and the pressure in the
central parts is very seldom under 1,015 millibars or
30-00 inches. See Plate XIII.
Certain parts of the earth, notably large parts of the
latitude belts of about 30 N. and 30 S., also continental
areas in the winter in temperate latitudes, are anti-
cyclonic regions. In the Azores-anticyclone in summer
the pressure is usually about 1,025 millibars, or 30*25
inches, and in winter about 1,020 millibars, or 30*10
inches, and in the Siberian anticyclone of winter the
pressure is often as high as 1,050 millibars, or 31 '00 inches.
Anticyclones are characterised by calms and light
winds and an absence of rain; the desert regions of the
earth are anticyclonic regions. But in the temperate
zones, short of gales and strong winds, almost any
weather may occasionally occur in an anticyclone. In
England they are generally accompanied in winter by
dull, cheerless weather and fogs, and in summer by
bright, hot weather.
The causes of anticyclones are still unknown. We
have learnt in recent years that the temperature of the
air in them between the heights of 2 and 10 kilometres
(1-6 miles) is higher, but at still greater heights lower
than its environment.
For the anticyclone in relation to weather refer to the
Weather Map, and see also AIRSHIP-WEATHER and
ISOBARS.
Aqueous Vapour.* Aqueous vapour isalways present
in the atmosphere, and, although it never represents more
. * See also HUMIDITY, p. 154, and ABSOLUTE HUMIDITY, p. 290.
32
Glossary.
than a small fraction of the whole, it has physical properties
that give it great importance in meteorology. In a closed
space wherever there is a free surface of ice or water,
evaporation takes place until the water- vapour exerts a
definite pressure of saturation, depending only upon the
temperature, and not upon the pressure of the surrounding
air. This pressure of saturation is very much greater at
high than at low temperatures as is shovm in the following
table :
Temperature.
Pressure
of Saturation
in millibars.
Temperature.
Pressure
of Saturation
in millibars.
F a.
10 260-8
20 266'3
30 271-9
40 277-5
50 283-0
2-4
3*7
5'8
8-5
I2'2
l? a.
60 288-6
70 294-1
80 299-7
90 305-2
100 310-8
17-6
24-7
34-6
47*8
65-0
A cubic metre of dry air at 1,000 mb. and 289a. weighs
l,206g.
The mass of water contained in saturated air at different
temperatures is given in the following table :
Mass in grammes
Mass in grammes
Temperature.
of water vapour
per cubic metre
Temperature.
of water vapour
per cubic metre
of saturated air.
of saturated air.
F. a.
F. a.
32 273-0
5
70 294-1
18
40 277-5
7
80 2997
25
50 283-0
9
90 305-2
34
60 288-6
13
100 310-8
45
Vapour. 33
It is easily seen from these figures that saturated air
must at once yield rain or snow if cooled, and even air
that does not contain all the aqueous-vapour possible will
ultimately deposit moisture if sufficiently cooled.
In the passage from the liquid to the gaseous state great
quantities of heat are absorbed, 536 calories for every
gramme ^evaporated at the boiling point, and even more
if the water is initially cold. Conversely much heat is
yielded up when condensation occurs. See p. 70.
Tyndall has shown that the heat radiated from a black
body at the boiling point of water is readily absorbed by
aqueous vapour, which must, therefore, have a correspond-
ing power of radiation. Spectrum analysis shows also that
some of the visible radiation of the sun is strongly absorbed
by the earth's atmosphere.
Atmosphere. See Weather Map. M.O. 225 i., p. 15.
Atmospheric Electricity. See p. 294.
Audibility. The audibility of a sound in the atmos-
phere is measured by the distance from its source at
which it becomes inaudible. On a perfectly clear, calm
day the teound of a man's voice may be heard for several
miles, provided there are no obstructions between the
source of sound and the listener ; but- quite a small
amount of wind will cut down the range of audibility
enormously.
The sound is not cut down equally in all directions;
to leeward, for instance, a sound can usually be heard
at a greater distance than it can to windward of the
source. This is accounted for by the bending which the
sound-rays undergo, owing to the increase in wind-velocity
with height above the ground, the rays to leeward of the
source being bent downwards while those to windward
are bent upwards so that they pass over the head of an
34 Glossary.
observer stationed on the ground. The decrease in all
directions in the range of audibility of a sound when
there is a wind appears to be due chiefly to the dissipation
in the energy of sound as it passes through eddying air.
A plane wave-front becomes bent in an irregular manner
when it passes through air in irregular or eddying
movement. It, therefore, ceases to travel uniformly for-
ward. Part of its energy is carried forward, while the
rest is dissipated laterally.
If there were no dissipation of energy in a sound-wave
the intensity of the sound would decrease inversely as
the square of the distance from the source. Experiments
show that, under normal conditions when there is a light
wind blowing, the rate of decrease in intensity of sound
at a distance of half a mile or more is considerably greater
owing to the dissipation of energy than would be expected
from the inverse square law. If, for instance, a whistle
can be heard at a distance of half a mile, four whistles
blown simultaneously should be audible at a distance of
a mile ; but the range is actually only increased to about
| of a mile. 4
Sounds are usually heard at greater distances during
the night than during the day. On calm nights the range
of audibility of a sound may be as much as 10 or 20 times
as great as it is during the day. This effect is clue partly
to the increased sensitiveness of the ear at night owing to
the decrease in the amount of accidental disturbing
waves, partly to the inversion of temperature which com-
monly occurs on calm, clear nights, and has the effect of
bending the sound- waves downwards, but chiefly to the
diminution of the amount of disturbance in the atmos-
phere at night.
Between the source of sound and the extreme range of
Audibility. 35
audibility areas of silence sometimes appear, in which
the sound cannot be heard. This effect has in some cases
been attributed to a reversal in the direction of the wind
-in the upper layers of the atmosphere. The lower wind
would bend the sound rays upwards to windward of the
source. On entering the reversed upper wind current
these, rays would be bent down to the earth again, and
would reach it at a point separated from the source of
sound by an area of silence. This explanation is quite
adequate in many cases in which the places, where the
sound is heard again, are to the windward of the source.
There are, however, many cases in which areas of silence
appear to leeward of the source, and many others in
which an area of silence occurs in the form of a ring
enclosing the source and surrounded by an area of distinct
audibility. In most of the cases where a ring-shaped
area of silence has been observed the outer region of distinct
audibility begins at a distance of about 100 miles from
the source, and may extend to 150 miles or more. The
well-known Silvertown explosion is a good example of a
case in which a detached area of audibility was separated
from the source of sound by an area of silence. In the
accompanying map, which is reproduced by permission
from the Quarterly Review, the two areas of audibility
are shown. It will be seen that the outer area, which
includes Lincoln, Nottingham and Norwich, lies about 100
miles from the source of sound. The inner area sur-
rounding the source is not symmetrical, being spread out
towards the north-west and south-east. Definite evidence
was obtained that no sound was heard at various towns
within the area of silence.
No very satisfactory explanation of these cases has so
far been offered. The wind-distribution necessary to
13204 B 2
36 Glossary.
explain them on the wind-refraction theory would be
very complicated and would, moreover, in some cases, be
of a type which no meteorologist has yet observed.
The effect of FOG on the audibility of sound has been
the subject of a considerable amount of discussion. The
idea that sound is muffled by a fog seems to be commonly
accepted ; but on the other hand the experiments of
Henry and Tyndall have failed to give any indication of
such an effect. They seem rather to show an increase in
audibility in a fog. The effect of the waterdrops them-
selves has been shown to be too mall to affect the
propagation of sound waves to an appreciable extent,
while the weather conditions usually associated with the
production of fog, the homogeneous state of the atmos-
phere and the INVERSION of temperature, are such as to
give rise to increased audibility.
In calm weather the direction of a hidden source of
sound may be estimated to a few degrees by turning the
head till the sound appears to come from the point
towards which the observer is facing. The .observer,
however, is seldom confident that he has attained such
accuracy. In windy weather it is more difficult to esti-
mate the direction of sound.
Aureole. The luminous area surrounding a light
seen through a misty atmosphere.
Aurora. See p. 298.
Autumn. Autumn, in meteorology, comprises the
three months of September, October and November, the
first three months of the farmer's year. In astronomical
text-books it is defined as the period commencing with
the autumnal equinox and ending with the winter
solstice, i.e., from September 23rd to December 22nd, but
Audibility
To face p. 36.
^Stfrtv
Canter &pry c
(Reproduced "by permission from the Quarterly Review. July, 1917.")
38
Glossary.
CURVES SHOWING CHANGE OF TEMPERATURE WITH HEIGHT ABOVE SEA- LEVEL
OBTAINED FROM BALLON- SONDE ASCENTS 1907-8.
V PYRTON HILL
4 PYRTON HILL
iPITCHAMWftK
pwnw HILL
pTDMMMK
[MANCHESTER
r; toe. MANCHESTER
8 DEC 3, (MANCHESTER'
'
SELLACK
MANCHESTER
PYRTON HiUL
WTCHAMfMK
MANCHESTER
CRiNAN'HARfiS
MANCHESTER
CRINAN HARB<
(&CNNANHUW
OITCHAM F*RX
JPYRTON HILL
pITCHAM PAW
(MANCHESTER
Jwa. PYRTON HILL
. PYRTQN HILL
flfae. MANCHESTER
IWRTON HILL
[MANCHESTER
PYRTON HILL
MANCHESTER
icpTifSELLACK
(OITCHAM f
5. OITCHAM R^RH
ISELLACK
icpre. 01
SOTit PYRTON HIU
Z. PYRTON HILL
5. [Oil
ISELLACK
OITCHAM WRK Z7Ae. PYRTON HILL
-40 -EO 2.Q
TEMPERATURE IN DEGREES FAHRENHEIT ,
3^0 40 250 2^0 -270
fEMPCRATURE ABSOLUTE
300
r The separate curves represent the relation between temperature and height
in miles or kilometres in the atmosphere. The numbers marking the separate
curves indicate the date of ascent at the various stations as shown in the tabular
columns. The difference of height at which the isothermal layer is reached, and
the difference of its temperature for different days or for different localities, is also
shown on the diagram by the courses of the lines.
Brdlon sonde. 39
Ballon SOnde. A small balloon usually made of india-
rubber, inflated with hydrogen, and used for carrying
self -registering instruments into the free atmosphere and
thus obtaining records of the pressure, temperature and
humidity aloft. The balloons used in England usually
have a diameter of about one metre at starting (40 inches
nearly), those on the continent nearly two metres. The
balldons generally rise until they burst, on account of the
diminished external pressure of the air, which may not
happen until they have reached a height of 20 kilometres
(12 miles) or more.
After the balloon has burst, the material acts as a kind
of parachute and breaks the fall of the instruments, so that
they reach the ground without injury. A label is attached,
offering a reward to the finder for the return of the
instrument, and in that way valuable records are secured.
Sometimes the balloon fails to burst, but develops pin-
holes through which the gas leaks. A long TRAJECTORY
is the result. One of our records was returned from a
Bavarian forest.
In other countries, where much heavier recording
instruments are used, two balloons in tandem are em-
ployed, one of which bursts, and the other regulates the
fall. This mode of arranging the apparatus, with a
simple modification, is available for use at sea and many
soundings of the air over the sea have been obtained.
We have not taken part in that side of the inquiry.
Very remarkable facts about the temperature of the
free air have been disclosed by soundings with ballons-
sondes. Their general characteristics are shown in the
diagram on page H8 which exhibits graphs of tempera-
ture and height for 45 soundings obtained for the
Meteorological Office in 1907-8. The reader should notice
that, with one or two exceptions, the balloons reached a
40
Glossary.
O
O
C^ O co t-H CO O co i-i O C
M coo <^ co VQ O ^ O v
-d- vo CTiOO H-I CT> co n O t- **"> O "3-OO co t^
CO -rf-O O\ coo MOi-it^^ coOO>O-'
v-O VOOOOCOO COCOO OO O I *j"> t^ O ir >
O i^)O ^ O "st" t^ ) ~ | O
O i ' t^- -j- N O O^ O i
PL,
i_i (_| n i_ i co co cocO'^-'^- vo O t> l~>- O^ O
CO co -rj- ^ *~t~> O O !> OO O
M 00 co C
PH O
II
s
C7\ rh O O co
M CO N OO^C^iOO
-QOco i-i^O^O^" '
) rj- rj- vD VO O t> OO O
JBallon sonde.
H
SI
H
02
W
P
to
feq
O
O
fe
o
PQ
EH
3 ri
^ g
o
02
^ 2
OQ 3
O -g
w S k
a g ^?
> M >
* M "S S
P ^ 03
I I I-
.CD 3
S
r^ -
_ f ' ^
to
43'
I
Qs LO M
I>*GO M LOMGO
M M 01 ^
GC O COOO VOM
* M M
OO rt-MGO lOMOO
^ n ^ ro co rj- 10 uox
M MSQ COM
n M w co rf
-Mt^ COCO M
\O
oo
M w \o co w oo voc^co -Nha^coaN
C< 01 W CO -t- rt- LO^O vO !>. r^OC GC
^M i>,ir)i-i i>. coco co O
co TJ- TJ- LO\O so r^ r^oo GO
M iriMGO LOCI OMOMVO C^OO
01 01 CO CO rf IT) LO^O l>s ^00 GO
O Tt- OvO H OvO 01 GO CO ON LO
^1 01 01 CO rf rf- LOvO ^O t^ X^GC
O 01 \O M OvO M CNJ^ O ^O 01
01 M 01 01 co co *-j- ir) LO\O t^ r^oo
OMT^O --t-o r^^
M M 01 01 co co *t-
01
x r^ o
M M 01 01 Ol
-f-o r^coo rx ro ix H \o
M M rj 01 ro co -f- i^i i
n M o
42 Glossary*
position (somewhere near ten kilometres), after which the
temperature ceased to fall, yet the range of temperature
at that level for the whole series is larger than the range
at the surface.
Additional soundings have enabled us to put forward
a table of the average pressure and temperature of the
free air at different levels in the several months of the
year, which is given on pp. 40 and 41.
Particulars as to the variation of the meteorological
elements, temperature, wind and humidity derived from
observations with ballons-sondes, kites and pilot balloons
are given in Geophysical Memoirs, No. 5, by Major E. Gold,
D.S.O. (published by the Meteorological Office).
Balloon Kite. For dealing with the general features
of the relation of temperature to pressure or height in the
upper air of all parts of the globe the ballon-sonde is
most effective. In this general inquiry the details due to
the smaller irregularities of diurnal or seasonal variation
may be disregarded. Such irregularities are specially
noteworthy in the lowest kilometre of the atmosphere
and are of importance in aviation and gunnery, because
changes in the distribution of temperature are necessarily
accompanied by changes in the distribution of pressure,
and consequently of wind. The first kilometre or 3,000
feet, therefore, requires special attention. Observations
of temperature, humidity and wind can be got by means
of kites, when there is wind enough ; by captive balloons,
when there is little or no wind ; and by the observation
balloon or balloon kite in all weathers, except a gale.
Special instruments are required for these observations.
A. special form of meteorograph has been designed by
Mr. W. H. Dines for use with kites, but suitable provision
has still to be made for kite-balloons and captive balloons.
Bar. 43
Bar. The unit of atmospheric pressure, being equal
to the pressure of one million dynes (one megadyne) per
square centimetre. The BAR is equal to the pressure of
29-5306 inches, or 750-076 mm. of mercury at 273a (32F)
and in latitude 45. The name was introduced into prac-
tical meteorology by V. Bjerknes, and objection has been
raised by McAdie of Harvard College on the ground that the
name had been previously appropriated by chemists to
the C. G. S. unit of pressure, the dyne per square
centimetre. The meteorological bar is thus one million
chemical bars, and what chemists call a bar we should call
a microbar. One bar is 100 CENTIBARS or 1,000
MILLIBARS. See p. 194.
Barogram. The continuous record of atmospheric
pressure yielded by a self-recording barometer. See p. 156.
Barograph. A self-recording barometer, an instru-
ment which records automatically the changes of.
atmospheric pressure. In one form of mercury barograph
the movements of the mercury in a barometer are com-
municated by a float to a pen in contact with a moving
sheet of paper carried by a revolving drum which is
driven by clockwork.
The portable barographs which are in common use are
arranged to record the variations of pressure shown by an
aneroid barometer, and on that account they are some-
times referred to as ANEROIDOGRAPHS. Particulars as to
the method of using these instruments are given in the
Observer's Handbook. (M.O. Publication 191.)
Barometer. An instrument for measuring the pres-
sure of the atmosphere. The mercury barometer has
been found to be the most satisfactory form for general
use, The principle underlying this type of instrument
44 Glossary.
is quite simple. If a glass tube 3 feet long, closed at one
end, is filled with mercury, and the open end is tempor-
arily stopped up and immersed in an open vessel of the
same liquid, then if the tube is held in a yertical position
and the immersed end is re-opened, the mercury will fall
until the level inside the tube stands at a height of about
30 inches above the mercury in the trough. The pressure
of the atmosphere on the lower mercury-surface balances
the tendency of the enclosed column to fall, and the
height supported in this way represents the atmospheric
pressure at the time. In order to cojnpute the pressure,
the length or height of the column of mercury has to be
measured. Different mercury barometers vary as regards
the method of reading this height, and in all the tem-
perature of the mercury and the latitude of the place
mast be taken into account.
In the aneroid barometer the pressure of the atmosphere
causes deformations in a spring inside a closed metallic box,
which has been exhausted of air, and these are communi-
cated to a pointer moving over a suitably engraved scale.
For the purposes of meteorology the pressure of the
atmosphere has to be determined to the ten-thousandth
part, which is a much higher degree of accuracy than is
required in other meteorological measurements. Special
contrivances and precautions are therefore required, whicfr
are duly set out in the Observer's Handbook.
Barometric tendency. The change in the baro-
metric pressure within the three hours preceding an
observation. See Weather-Map, p. 35.
Beaufort notation. A table of letters for weather.
See Weather-Map, p. 10.
Beaufort Scale. 45
Beaufort Scale. The scale of wind force devised by Admiral
Beaufort in 1805. An explanation has been given in the
Weather-Ma/), p. 13.
Table of Equivalents in Force and Velocity.
Pressure of Wind
Equiva-
|
Limits of Velocities.
on a Plate
lent
a
velocity
&
^
in Ibs.
in Milli-
in
miles
f-\
EH
Q
Statute
Miles
Nautical ^ ,
Miles i Metres
Feet
per
square ft.
bars
( i o 3 dynes
per
hour.
s
1
per
Hour.
per
H P , S d '
per
Second.
per cm. 2 ).
CQ
O
Less
Less
Less
Less
1 than i
than i
than 0*3
than 2
01 'or
2
1
i-3 i-3
o'3-i'S
2-5
08 '04
5
2
4-7 4-6
i -6-3-3
6-1 1
28
13
10
3
-12 7-10
S^-SH
12-18
67
32
15
4
13-18 11-16
5'5-S-o
19-27
I-JI
62
21
* 5
19-24
17-21
8-1-107
28-36
2'3
i-i
27
6
25-31 22-27
10-8-13-8
37-46
3-6
17
35
7
3^-38
28-33
I 3'9- T 7' 1
47-56
5'4 2 ' 6
42
8
39-46 '
34-4
17-2-207
57-68
77 37
So
9 47-54
41-47
20*8-24-4
69-80
10*5 5-0
59
10
55-63
48-55
24-5-28-4
8i-93
14/0 67
68
11
64-75
56-65
28-5-33-5
94-110
Above : Above
Above
12
Above
Above
3 3 -6 or
Above
17-0 8-1
75
75
65
above.
no
46 Glossary.
Bishop's ring, so named after its first observer, is a
dull reddish-brown ring of about 20 outer radius seen
round the sun in a clear sky even at mid-day. That it
is a CORONA, and not a HALO, is proved by the fact that
at times it has been seen to have a red outer margin (see
CORONA). It appeared after the great eruption of
Krakatoa in 1883, and remained visible till the spring of
1886, and was no doubt due to. minute particles shot out
by the eruption ; these remained suspended in the atmos-
phere for a considerable time. The great radius of this
corona is explained by the smallness of the particles, and
its intensity by their great number. The non-appearance
of the other colours of the corona is explained by the
presence of particles of many different sizes. Bishop's
Ring was seen again after the eruptions of the Souffriere
in St. Vincent and Mont Pelee in Martinique in 1902.
Blizzard. A gale of wind with the temperature
below freezing, the air being filled with fine dry snow.
The snow may not be actually descending from the
clouds but be merely raised from the snow-covered
ground. The fine powdery snow peculiar to these
storms is formed only at very low temperatures, so that
the phenomenon is practically confined to the polar
regions and the large land areas of the temperate zone.
During a blizzard the temperature often rises, possibly
because the violent wind causes a mixing of the lowest
layers of the atmosphere and brings down air that is
often warmer than the excessively cold air lyinof near
the ground.
Blue Of the Sky. Light rays striking particles which
are smaller than the wave length of the light are scattered,
that is turned aside in all directions. But the short wave's
Blue of the Sky. 4?
composing the blue and violet end of the spectrum are
more completely scattered than the long red and yellow
waves. Hence light passing through a medium contain-
ing a great number of such particles is left with an excess
of red, while light emerging laterally has an excess of
blue. It is for this reason that soapy water looks yellow-
ish when one looks through it at a source of white light,
and bluish when one looks across the direction of
illumination. The greater part of the sky appears blue
because the light from it consists mainly of light scattered
laterally from minute particles in the atmosphere. The
smaller the particles the less intense is the light but the
greater the proportion of it that is blue. When the
particles are larger the proportion of blue is less, as in
the whiter sky of a haze. Near the horizon the sky is
whiter than at the zenith because the rays of light from
that region have passed through a greater thickness of
the lower air where large particles are relatively more
numerous, Sunset colours are reddish because the rays
reaching ue directly have lost much of their blue light
by lateral scattering. The sky as seen from high moun-
tains and from aeroplanes at a great height is of a deeper
but purer blue because there are fewer large particles
than at lower altitudes.
Boiling* Points. See p. 300.
Bora. A cold wind occurring in the northern
Adriatic, very violent, which blows from the high
plateaus which lie to the northward. These plateaus
may become extremely cold in clear winter weather, and
passing cyclonic systems allow the air to flow down to
lower levels. The actual violence of the wind in a bora
is largely due to the weight of the cold air of the plateau
causing it to run down the slope like a torrent or cataract
48 Glossary.
of water. The wind experienced may therefore be in-
dependent of Buys Ballot's Law. The adiabatic warming
due to the increased pressure below is riot sufficient to
prevent the resulting wind from being cold. In the
Meteorological Office it is proposed to call local winds of
this character " katabatic " in order to distinguish them
from the winds which show the normal relation to the
distribution of atmospheric pressure and are called
" geostrophic " winds.
Breeze. A wind of moderate strength.
Glacial-breeze. A cold breeze blowing down the course
of a GLACIER, and owing its origin to the cooling of the air
in contact with the ice. The movement of the air is due
to the gravitation of the air made denser by the cold
surfaces on a slope, and a glacial breeze may be classed
as a typical example of a katabatic wind.
Lake-breeze. A breeze blowing on to the coast of a lake
in sunny weather during the middle of the day, part
of the convectional circulation induced by the greater
heating of the land than of the water.
Land-breeze. An off-shore wind occurring at the
margin of a sea or lake during a clear night, due to the
more rapid cooling of the air over the land than over
the water. During the day the conditions are reversed
and the wind blows from the sea to the land, constituting
a SEA-BREEZE. These phenomena are most marked in the
tropics, where the wind arising from other causes is
usually not strong enough to mask the convectional
effect. See also p. 183.
Mountain-breeze. A night breeze blowing down the
llrew. 49
valleys, due to the flowing downward of the air chilled
by the cold ground.
These also, as being due to convection in which the
colder air takes the leading part, would be classed as
katabatic winds, whereas the one next following in which
warmed air plays the leading part would be classed as
an u anabatic " wind.
Valley -breeze. A day-breeze that blows up valleys when
the sun warms the ground.
Brontometer, from bronte, a thunderstorm, a com-
bination of apparatus for following and noting all the
details of the phenomena of weather during a thunder-
storm.
Buoyancy. Used generally with regard to ships or
balloons to indicate the load which a ship could carry
without being completely submerged, or the weight which
a balloon or airship can carry without sinking.
In the case of the balloon or airship the buoyancy is
due to the displacement of air by hydrogen which is
lighter than air. A cubic metre of perfectly dry air at
1000 mb. and 273 a. weighs 1*28 kilogramme, whereas a
cubic metre of hydrogen under the same conditions
weighs only *09 kg. It follows that a cubic metre of
hydrogen in air at 1000 mb. and 273a. will have a
buoyancy represented by the difference, i.e., 1*19 kg.
Part of the buoyancy will have to be devoted to sup-
porting the envelope which contains the hydrogen and
which adds so little to the volume of air displaced by
the hydrogen that, for purposes of calculation, we may
regard the displacement of the air by hydrogen as the
source of the buoyancy, and count the envelope and oth<T
accessories of the same kind as " dead weight."
50 Glossary.
The gas that is used under the name of hydrogen for
filling balloons always contains some impurity that re-
duces its buoyancy. Water-vapour in the hydrogen and
the surrounding air will reduce the buoyancy by an
amount varying from 0*2 per cent, to 2 per cent, in the
common range of circumstances and the impurities in-
cidental to the manufacture of the gas, or to leakage.
may easily reduce the buoyancy by 5 or 10 per cent.
Instead, therefore, of taking the buoyancy of a cubic
metre of working hydrogen at 1*19 kg. the theoretical
figure for pure dry hydrogen in dry air, we may take it
at 1*10 kg. per cubic metre at 1000 mb. and 273 a.
The volume of a large airship may be 25,000 m 3 ,
the dimensions being 140 m. in length and 15 in. in
diameter. The gross buoyancy at 1000 mb. and 273 a.
is, in that case, 25,000x1-10 kg., or 27,500 kg. In those
conditions the relation of pressure to temperature is
3*66:1, see p. 53. In other conditions of pressure and tem-
perature the relation, and therefore the buoyancy, will be
different.
The relation between the displacement and the weight
supported is given by the equation
where Q is the volume of air displaced, p the density of
the air, a the specific gravity of the "hydrogen " referred
to air at the same temperature and pressure, W the dead
weight, L the portable load, and B the ballast.
BUOYANCY IN DIFFERENT ATMOSPHERIC CONDITIONS
AND THE LIMIT OF HEIGHT THAT AN AIRSHIP
CAN REACH.
The density of air p becomes less and less as one
ascends in the atmosphere because the pressure di-
minishes. The temperature diminishes also, and, on that
Buoyancy. 51
account, there is some compensation for the fall of pres-
sure, but not enough co preserve the buoyancy.
We may suppose that <r, the specific gravity of the
hydrogen, remains the same throughout, because, in an
airship, the pressure of the hydrogen changes with
that of the air in which it floats. Its temperature changes
likewise, and, if we leave out of account the heat received
or lost in the form of radiation, the fall of temperature of
the hydrogen of an ascending balloon is generally more
rapid than that of the surrounding air. It would take
time for the temperature to become equalised, whereas
the adjustment of pressure is practically immediate.
Therefore, the hydrogen in a rising airship is rather
denser than the result of calculation would give. Thus,
to suppose <r to remain constant, is a little more favour-
able to the navigator than actuality, unless he takes ad-
vantage of sunshine.
The buoyancy at any level is determined by the
density p.
With the assumption that <r remains constant the
buoyancy can be computed from the density of the air at
the level of standard pressure and temperature by the
ordinary gas-equation
Where p, p, T are the pressure, density and absolute
temperature of the air at the selected level p n9 p , T are the
standard pressure, density and temperature.
The equation of buoyancy becomes
0xx 'poO-T)^ W+ + B.
52 Glossary.
For the figures which we have quoted,
jp u = 1000 mb, T,, = 273a, Po (1 - *) = 1-1 kg/m 3
and the equation becomes
273 x 1-1 p _ TF+ +
1000 T Q
W and L, the dead weight and the portable load, cannot
be altered during a voyage without sacrificing something,
but the ballast B is carried for the purpose of adjusting
the level. The maximum height will be reached when
the ballast is exhausted, that is when B is zero.
In this equation the value of the ratio p\T which
determines the density depends upon the pressure and
temperature of the air at the time of the flight, but for
aeronauts the most important cause of the variation in
these elements, and therefore in their ratio, is the change
of pressure and temperature of the atmosphere with
height. These are so considerable that they overshadow
altogether the changes at any chosen level from day to
day or from month to month. We can therefore use a
table of average monthly values with advantage. The
following tables give the average values ofpjTfor different
heights computed from observations of ballons-sondes.
From the values pjT the density in grammes per cubic
metre can be found by multiplication by 348. (The first
'table is computed from the pressures in the accompanying
table and the temperatures in Table IV. 2, Computer's
Handbook, Part II., 2, p. 56, : the second has been pre-
pared by Mr. W. H. Dines, F.R.S., for the Handbook of
Meteorology).
Buoyancy.
53
. ir> -f- ro Ol I-H O
ONOO r-0 n
^04 hH^
03 01
1
ir< -}- moo O4 OO
vno t-^oo O *-
rj-04 O O C4
\n 04 M -sf- O
coo ON O4 O
a l i
CD
Tf- CO "^-O l-l O
vno r^oo O -H
c^vnRo,^
O co rj- t~>. vn
cOO ON O4 O
- g
ii i
1 1 hH
M hH I-H l-l 04
O4 04 Ol CO co
~ST in <&
vno r^oo O "i
coxnr^oo HH
cOO ON 04 O
^
hH I-H
,_ _ W tH 04
O4 O4 O4 CO CO
2 ^pts
1
O vno ON cooo
-* KH O 00 O
ri- O 00 O *>.
cOO OO CN vn
'I jg 1
'1
j vno !> ON O i-i
rh 04 O 00 O
co vn t^oo I-H
co ,ONOO O vn
co vnoo 04 vn
43
rt H
BU
I-H I-H I-H M O4
O4 04 O4 CO CO
c3 "S bo
bi)
p
kj
co vno oo O
04 OO t^ t^ 04
co vnoo M vn
<1
fhH hH
_ HH hH I-H 04
O4 O4 O4 CO CO
S, |-S
*
t-*.O r- O ^h O
co vno oo O
O4 OO t^.OO CO
co vnoo HH vn
4-3 rH vr>
5 - M
1-4 M I-H I-H O4
O4 O4 04 CO CO
43 p
|
3 vr*vn
p vno t^oo O >-<
Svn^as
co O t> ONO
coo oo hH vn
-2&
-a
M l-l
|_| |_| |_4 hH 04
C4 N O4 CO co
~^ vn vn vnoo co ON
vn 04 ON i-i M
co vno ON i-i
rh 04 hH COOO
coO ON 04 vn
| IS
fl
HH M
M hH M I-H 04
04 O4 O4 CO CO
O!
d,
vno J>-OO O -i
a as as
O l-l O4 vn 04
coO ON cX O
S
S3ZSS5
rj- I-H CT> HH co
coo ON c>4 O
02 bo
S
HH ^H
|-| hH I-H I-H 04
04 04 O4 CO CO
"- so ^ '-i
s
-1- co coO O t~>.
vno t^oo O '-i
co vn j> ON M
00 ^f- vo O 00
cOO ON coO
b
l-l I-H
|_, HH M hH 04
O4 O4 O4 CO CO
fl Id *-i *
p
SKg8S
8>S8
co O ON co O
^11
M I-H
M M hH M 04
O4 O4 O4 CO CO
<j3 ^ K^g
^ m ^f- co 01 i-i o
0.00 t-o vn
^-COOlhH^
Q^ ^
Glossary*
> O 1-^00
O^ O c-i
O woo W to eo co <
co fO vnoo co -H
oo co -H ON
C^O t-i M
co t-i OO to W O\ vr>
-
O 1/- > O
O O^> O
O
^ *$ co TJ- j> cxi o c^ ^ vn oo vo O^ oo co vn co oo co oo co t^
Q d M co *$ -rf- vnvo 1^.00 O *- co
xnTj-vnoo w OO -^-Mintxivncooo Ooo -rh^rl-^'^-
^r^vnvO f>.co O M -rh<o O>W>O O^^Ot-^fOi-i criCTiO>-<
t- vnC\rf-OvO xn T}- vnoo c^oocoO O>C^>-<vni-" w -^O
^ vn rj- vnoo WOO'^hCNir^-i-icow t> ONOO vn O*> vn O 'rh
)_, _< I I h- 1 - C-l Cl CO CO -rf -rf- 1/~>VQ VO t^ s ! O
^1 jjh-ii-H>-it-(t-H(-ii II-HI iciMc<ifO"^- f- vnvD vO t^ J>.oo
be
I
Buoyancy. 55
With these tables the limiting height of flotation can be
approximately determined when the dimensions and par-
ticulars of the dead weight, portable load and ballast are
known.
For example, in the case of an airship which displaces
25,000m 3 of air with a dead weight 11,500kg. 9,000kg.
ballast, including 2,000kg. of fuel which can be spared,
7,000kg. portable load, including crew, clothing and food,
fuel for the return journey> oil armament and working
tackle, the right hand side of the equation is reduced to
its minimum when the ballast is disposed of, that is when
B = o, in that case p\T works out to be 2*46. This will be
found in the table between 3k and 4k, rather nearer 3k in
summer than in winter.
THE EFFECT OF WEATHER UPON BUOYANCY.
The changes of weather affect the buoyancy because
they alter the value of p\T at the starting point and at
every stage of the course. The value at the starting point
determines the amount of ballast that can be carried to
begin with, and the value at other stages of the journey
determines the height to which the airship can rise with
all its ballast expended.
Assuming that the limit of ascent is determined by the
value 2'4 for the ratio pj T 7 , we find that the variation in
the course of the year 1913 was from 3'5k to 4'lk.
The effect of low pressure or high temperature can
easily be seen but it is not very pronounced. The differ-
ence between the worst occasion and the best from January
to May 1913 was about 1,600 ft. It is now recognised
that above one kilometre, when pressure is low, tempera-
ture is also low, and, consequently, at any level the value
54 Glossary.
of p\ J^is steadier than the variations at the surface would
lead one to expect. In these calculations no allowance
has been made for leakage, nor, on the other hand, for
compressed hydrogen carried to replace losses.
DEPRESSION PRODUCED BY THE WEIGHT OF A
DEPOSIT OP RAIN OR SNOW.
The catchment area may be about 1000 m 2 , and the
figures quoted from Josselin by Modebeck for the weight
on that area are
Maximum
depression.
k.
Dew (light)
Dew (heavy)
Rain ...
Heavy rain
Storm rain
Weight.
Snow
kg.
15
80
200
250
4OO
800
to
to
to
to
to
to
kg.
50
240
290
360
480
1000
03
15
18
21
3
60
According to this table the worst effect, even of snow,
would be a depression of 2,000 feet.
DEPRESSION PRODUCED BY DESCENDING WIND.
The calculation is very hazardous ; supposing the
vertical component of the wind in ordinary circum-
stances to be from 0'5 to 1*5 m/s, and assuming the
ordinary law of resistance, which is true for small areas,
the downward force would bo '01 nib. = 10 dynes per
cm 2 , say 500 x 111 1 x 10 dynes on the envelope (iakmt- ihe
area to be f)00 m 2 ), or 5 x 10 7 dynes. 'This is equivalent
to a weight of about 50 kg. and is, therefore, unim-
portant, but it would become important if the downward
Buoyancy. 57
velocity reached 5 m/s, which it probably may do in a
disturbed atmosphere, and in the downrush of a line
squall this value might be largely exceeded.
Buys Ballot's Law (see The Weather Map). The law
is that if you stand with your back to the wind the
atmospheric pressure in the northern hemisphere de-
creases towards your left and increases towards your
right. In the southern hemisphere the reverse is true.
The law is a necessary consequence of the earth's rotation.
C.G.S. Abbreviation for Centimetre Gramme Second,
used to denote the organised system of units for the
measurement of physical quantities by units which are
based upon the centimetre, the gramme, and the second
as fundamental units.
Calm. Absence of appreciable wind ; on the Beau-
fort Scale 0, i.e., less than 1 mile an hour, or three-tenths
of a metre per second.
Calorie. See p. 301.
Celsius, Anders : an astronomer and physicist, the
inventor of the Centigrade thermometer, born at Upsala
in 1701. The name Centigrade arises from the division
of the interval between the freezing point and boiling
point of water into one hundred parts. In continental
countries the scale is generally named after the inventor
Celsius. It is also not infrequently referred to as the
centesimal scale.
Note. Celsius divided the thermometric interval between freezing
and boiling points of water into 100 parts, but he made the former
100 and the latter 0.
The Centigrade scale now in use was introduced by Christin of
Lyons in 1743 (possibly earlier). It has also been claimed for Liiine,
but the priority is extremely doubtful.
58 Glossary.
Centibar. A hundredth part of a "bar" or C.G.S.
" atmosphere." See BAR, also see Table of equivalents at
the end of this volume.
Centigrade. A thermometric scale introduced by
ANDERS CELSIUS (q.v .), which has zero at the melting
point of ice, while 100 represents the boiling point of
water at a pressure of 760 mm. of mercury. A centigrade
degree is 9/5 Fahrenheit degrees. On the centigrade
" absolute " scale the freezing point is at 273a, the unit
being the same as a centigrade degree.
Centimetre the hundredth part of a metre. The
unit of length on the Centimetre-Gramme-Second system
which is universally employed for electrical and magnetic
measurements. A metre was originally defined as one
ten-millionth part of the earth's quadrant, that is, the
distance from the equator to the pole. 1 centimetre is
equal to *394 inch, and 2*54 centimetres to 1 inch, to a
high degree of accuracy.
Cirro-cumulus. A group of fleecy cloudlets or small
" flocks " of cloud formed at great heights and showing
no shadows. They are called " moutons " in French. See
CLOUD.
CirrO-StratUS. A layer of thin, transparent cloud at
a high level. See CLOUD.
Cirrus. Clouds at great height, generally formed into
" wisps " of thread-like structure. See CLOUD.
Climate. A general summary of the weather for any
particular locality. When the weather has been observed
for a sufficiently long time in any locality we are able to
make a useful statement as to the weather which may be
To face p. 58.
Cirrus clouds, Thread or Feather clouds at a height of
from five to six miles, and generally of a white colour. They
are composed of ice-crystals.
The picture gives an idea of rather more massive struc-
ture than is usual with cirrus clouds, but the sweeps and
^vvisps are very characteristic.
Climate. 59
experienced at any particular time of the year in that
locality. Technically, the climate of a place is represented
by the average values of the different meteorological
elements, which should include means for each month, as
well as means for the whole year. Average extreme values
for different periods and ABSOLUTE EXTREMES are of in-
terest, and also the number of rain-days, days of snowfall,
frost, hail, thunderstorm, gale, &c., and the frequency of
occurrence of winds from different directions. Included
also under this heading would be the earliest and latest
dates of frost and snow, the average depth of snow lying
at different times, and particulars of the temperature of
the soil at various depths. In places where the type of
weather at a given time of year varies greatly, a long
series of observations is needed in order to obtain a fair
idea of the different climatic elements. Various kinds of
climate are characterised, chiefly with regard to moisture
and temperature, as continental which is dry, with great
extremes of temperature ; insular or oceanic which is
moist and very equable in temperature ; tropical in which
the seasons depend chiefly upon the time of occurrence of
rainfall ; temperate in which the seasons are chiefly
dependent upon the variation of the daily course of the
sun in the sky ; arctic in which the year is mainly
two long periods of sun and no sun. Local climates, such
as those of the Mediterranean in general and of the
Riviera in particular, are dependent upon the geographical
conditions of land and water. Every climate is to some
extent affected by the geographical nature of the sur-
roundings of the locality.
Climatic Chart (see Weather Map, p. 88). A map
showing the geographical distribution of some element of
60 Glossary.
climate ; temperature, rainfall and sunshine are the most
frequently charted. Charts of normal values of these
elements for the British Isles for long periods of years
ended 1910 are issued by the Meteorological Office. The
word also covers any diagram representing the periodic
or secular variation of a climatic factor.
Climatology. The study of CLIMATE (q.v.).
Clouds. Clouds have been classified into certain
typical forms, but there are so many intermediate types
that it is sometimes difficult to decide to what class any
cloud belongs.
Certain types of cloud and the direction from which
they are moving indicate certain states of weather ; cer-
tain types of cloud being usually within certain heights,
a knowledge of cloud forms enables the observer to make
a rough estimate of the height of clouds.
For practical purposes clouds may be divided into
cloud sheets and cloud heaps.
CLOUD SHEETS.
Many forms of cloud are obviously in more or less
extended sheets, sometimes covering the whole sky,
sometimes only covering a small patch. They vary
much in thickness, sometimes no blue sky is visible
through the sheet, sometimes small patches of blue are
visible, sometimes half the sky is blue, at other times the
sheet of cloud is represented by a few detached clouds
in the blue.
The formation of a sheet of clouds is not well under-
stood. But the following may be on some occasions the
method of formation ; it is known that the atmosphere is
to a certain extent stratified, and that there may be damp
Cloud*. 61
and dry layers ; if the pressure over any area is diminished
the air in the region will be cooled by expansion ; if
sufficiently cooled the dew point will be reached, and any
damp layer will become a cloudy layer.
Cloud sheets are often seen to be broken up into waves ;
the waves may not be formed in the cloud itself ; if
waves are set up in the atmosphere they will be propa-
gated upwards and downwards ; in a thin cloud layer the
air that rises on the wave crest is cooled by expansion,
more condensation occurs and the cloud is thicker ; the
air that descends in the hollow is warmed by compression
and some of the cloud is evaporated, leaving a clear or
nearly clear space.
Cloud sheets may sometimes be seen forming at several
levels at the same time ; over a cyclonic depression there
are probably sheets of cloud at several levels.
Cloud sheets may be divided up into, three classes
which differ in appearance and height. The upper layer,
the cirrus clouds, are at heights of from 25,000 to 30,000
feet ; the middle layer, the alto clouds, are from 10,000 to
25,000 feet ; the lower layer clouds are below 10,000 feet.
THE UPPER LAYER.
The clouds of the upper layer, the CIRRUS CLOUDS,
are composed of ice particles, not of water drops as
all other clouds. They are of a pure white colour
with no shadows except when seen when the sun is
low down. They are usually streaky and have a
brushed out appearance (mares' tails); they are frequently
spoken of afe windy looking clouds, but in spite of this
appearance they do not necessarily indicate wind.
They are often seen in long streaks stretching across
the sky, sometimes in parallel bands, serming by
62 Glossary.
the effect of perspective to converge, on the horizon
to a V-point. These clouds often move away from the
centres of depressions ; when coming from North- West,
West or South-West they indicate a depression in a
westerly direction, which will probably spread over the
observer ; when seen coming from the North they indi-
cate a depression to the Northward which will probably
move away to the North-East and so not influence the
weather. To observe the motion of clouds, especially of
the high clouds, get a patch of cloud " on " with the
point of a branch or some point of a building; if one
moves so as to keep the cloud on the point, one's direction
of motion will be towards the direction from which the
cloud is coming. It is often difficult to determine the
motion of the high clouds without a careful observation
of this kind.
Bands of cloud do not necessarily move in the direction
of their length, nor do parallel bands of clouds necessarily
move from their V-point.
HALOS and MOCK SUNS (sun dogs) are seen at times
in cirrus clouds but in no other forms ; a halo is a ring,
sometimes coloured, seen at some distance (22 or more
rarely 46) from the sun or moon.
Besides the wisps and streaks of cloud of common
cirrus there are other forms :
CIRRO-CUMULUS : lines or groups of cloud, some-
times detached globules, sometimes waves ; mackerel
sky (French : moutons) ; these clouds are pure white, with
110 shadows.
ClRRO-STRATUS : a bank of tangled web sometimes
overspreading the whole sky ; no shadows ; sun pale and
" watery."
Cloud*. G;>
ClRRO-NEBULA : similar to last but no visible structure ;
a veil of pale white cloud. Precedes depressions.
THE MIDDLE LAYER, ALTO CLOUDS.
The alto clouds are composed of water droplets, not ice
particles, therefore halos are never seen ; CORON^E may be
seen ; these are coloured bands quite close round the sun
or moon. The alto clouds are not such a pure white as the
cirrus clouds, and shadows are visible on them. They may
move away from centres of low pressure, like the cirrus
clouds. They are, on the average, only half the height of
the cirrus clouds, being from 10,000 to 25,000 feet high.
The following are the most important varieties :
ALTO-CUMULUS. Very like cirro-cumulus, but the
cloud masses are larger, and shadows are visible ; some-
times arranged in globular masses (French : Gros
moutons), sometimes in waves.
ALTO-CUMULUS CASTELLATUS, TURRET CLOUD. The
globular masses of Alto-cumulus are developed upwards
into hard-edged clouds like miniature cumulus ; some-
times a large number of clouds almost of exactly the
'same shape are seen. When coming from a southerly or
westerly point, after fine weather, Turret cloud is a sign
of approaching thundery conditions.
ALTO-STRATUS. Very like cirro-stratus, but a thicker
and greyer cloud ; halos never seen ; precedes depres-
sions, and does not usually extend so far from the centre
as cirro-stratus.
, THE LOWER LAYER.
The clouds of the lower layer are below 10,000 feet ;
thicker clouds with not such a fine structure as the
higher clouds.
(54 Glossary.
STRATO-CUMULUS. Masses of cloud with some vertical
structure appearing in rolls or waves, sometimes covering
the whole sky ; sometimes the whole sky is covered with
cloud, but the hollows of the waves are lighter and
obviously thinner ; blue sky is occasionally seen more or
less plainly through the thinner parts. This cloud is
common in quiet weather in winter, and sometimes per-
sists for many days together. In summer it is more
broken up, and tends to turn into cumulus clouds.
STRATUS. A uniform layer of cloud which resembles
a fog, but does not rest on the ground. Small masses of
stratus, more or less in the shape of a lens (lenticular
cloud), are often seen near thunder clouds ; they appear
dark when seen against the white sides of the thunder
clouds.
NIMBUS. A dark shapeless cloud without structure,
from which continuous rain or snow falls ; through open-
ings in this cloud, should such occur, layers of cirro-stratus
or alto-stratus may almost always be seen.
SCUD. Small shapeless clouds with ragged edges ;
sometimes seen without other cloud, but usually asso-
ciated with nimbus and cumulus.
. HEAP CLOUDS.
Clouds with considerable vertical structure, not forming
horizontal sheets of cloud. Their formation is due to
rising currents of air ; as the air rises it is cooled by
expansion till it reaches the dew point when cloud begins
to form ; as soon as condensation takes place heat is
liberated, and though the air cools as it rises still further,
it does not cool so rapidly as it would were no heat
liberated by the condensing water vapour. The rising
current is due to air heated above its surroundings, and
the condensation enables the current to rise considerably
Between pp. 64 and 65.
CLOUDS SEEN FROM BELOW : FIGURE 1.
:Strato-Cumulus from below.
Cumulo-Nimbus (Thunder-cloud) with "Anvil" extension
of. false cirrns. 30th October. 1915.
CLOUDS SEEN FROM ABOVE : FIGURE 2.
Strato-cuinulus from an aeroplane (4,000 feet).
filled with fog, 300 feet deep, in early morning after
still night.
Clouds. 65
higher than it would have done had no extra heat been
available.
SIMPLE CUMULUS. This may consist of small clouds
with flat base and rounded top or large cauliflower shaped
clouds (Woolpack cloud). These clouds commonly form
on a summer day, beginning as small clouds, and growing
larger by the early afternoon, when the rising currents
due to the sun's heat are at a maximum ; they usually
disappear before sunset.
CUMULO-NIMBUS ; SHOWER CLOUD ; THUNDER
CLOUD. Sometimes when cumulus grows to large propor-
tions the upper edge is seen to become soft and brushed
out into forms somewhat like cirrus (false cirrus) ; at the
same time from the under edge of the cloud, which has
been growing very dark, rain begins to fall. Sometimes
only a slight shower may result ; but if the cloud is large
there may be heavy rains and violent thunderstorms.
Sometimes the false cirrus is brushed out round the top
of the cloud giving it the shape of an anvil ; the anvil
cloud is usually associated with very violent thunder-
storms and falls of hail. Thunder clouds due to the rising
currents of a hot day usually disappear about the time of
sunset, but often leave the false cirrus.
Cumulus often forms in long lines of cloud presenting
the appearance of a succession of clouds or of a wall of
cloud extending along the horizon. Any cause that
brings masses of air of different temperatures near together
brings about the formation of cumulus cloud.
It should be noted that rain, hail, and snow only fall
from nimbus or cumulo-nimbus clouds, except the
slightest and most transient showers, which may some-
times fall from alto-cumulus or strato-cumulus. Nimbus
causes persistent rain or snow, cumulo-nimbus more or
less severe showers of rain, hail or snow.
13204 C
66
Glossary.
TABULAR STATEMENT OF THE SEVERAL TYPES OF CLOUDS.
I. CLOUD SHEETS.
CIRRUS. Mares' tails ; wisps or lines of pure
white clouds with no shadows.
Upper cloud layer
about 30,000 feet.
Clouds composed of
ice crystals. With
these are sometimes
seen halos, or rings,
at some distance from
the sun and moon.
Middle cloud layer ;
10,000 feet to 25,000
feet. Clouds com-
posed of minute drops
of water. Coloured
rings sometimes seen
quite close to sun and
moon, but never halos.
Lower cloud layer.
Below 7,000 feet.*
CIRRO-CUMULUS. Small speckles and flocks
of white clouds ; fine ripple clouds : mackerel
sky.
CIRRO-STRATUS. A thin sheet of tangled web
structure, sometimes covering the whole
sky ; watery sun or moon.
CIRRO-NEBULA. Similar to last, but a veil of
cloud with no visible structure.
ALTO-CUMULUS. Somewhat similar to cirro-
cumulus, but the cloud masses are larger,
and show some shadow.
ALTO-CUMULUS CASTELLATUS. Turret cloud;
alto-cumulus with upper margins of the
cloud masses developed upwards into
miniature cumulus, with hard upper edges.
(Sign of thunder.)
ALTO-STRATUS. Very like cirro-stratus and
cirro-nebula, but a thicker and darker cloud.
STRATO-CUMULUS. Cloud masses with some
vertical structure ; rolls or waves some-
times covering the whole sky.
STRATUS. A uniform layer of cloud
resembling a fog but not resting on the
ground.
NIMBUS. Shapeless cloud without structure,
from which falls continuous rain or snow.
SCUD. Small shapeless clouds with ragged
edges ; sometimes seen without other
cloud, especially in hilly country ; but
more commonly seen below other clouds,
such as cumulus and nimbus.
*xThe heights given are only approximate. Thus lower clouds
of the strato-cumulus type may rise above 7,000 feet and attain at
times the height of the middle cloud layer.
Clouds. 67
II. HEAP CLOUDS.
CUMULUS. (Woolpack clouds) ; clouds with flat base and con-
siderable vertical height. Cauliflower-shaped top.
FKACTO-CUMULUS. Small cumulus with ragged tops.
CUMULO-NIMBUS. (Anvil-, thunder- or shower-cloud). Towering
cumulus with the top brushed out in soft wisps or larger masses
(false cirrus) and rain cloud at base.
The height of the heap clouds is very variable, Mean height of
base, about 4.500 feet ; the height of the top varies from about 6,000
to 25,000 feet.
Reproductions of photographs of Strato-cumulus seen from below
and from above, also of Cumulo-Nimbus and of a Valley filled with
fog are inserted between pages 64 and 65. A photograph representing
Cirrus faces page 58, and one representing Mammato-cunmlus, page 190.
Cloud-burst. A term commonly used for very heavy
thunder-rain. Extremely heavy downpours are sometimes
recorded, which in the course of a very short time tear up
the ground and fill up gulleys and watercourses ; this
happens in hilly and mountainous districts, and is
probably due to the sudden cessation of conyectional
movement, caused possibly by the supply of warm air
from the lower part of the atmosphere being cut off as
the storrn moves over a mountain range. With the
cessation of the upward current, the raindrops and hail-
stones which it had been supporting must fall in a much
shorter time than they would have done had the
ascensional movement continued.
Clouds, Weight of. Measurements on the Austrian
Alps of the quantity of water suspended in clouds have
given 0*35 g/m 3 to 4- 8 g/m 3 . The water suspended as
mist, fog, or cloud may be taken as ranging from 0*1 to
5 g/m 3 . (See Wegener's Thermodynamics of the Atmo-
sphere, p. 262.)
13204 C 2
68 Glossary.
Col. The neck of relatively low pressure separating
two anticyclones (see ISOBARS). One of the most
treacherous types of barometric distribution, as it some-
times marks a locality of brilliantly fine weather and
sometimes is broken up by thunderstorms. See Plate XIV.
Compass. A circumference, or dial, graduated into
thirty-two equal parts by the points N., N. by E., N.N.E.,
N.E. by E., N.E. and so on. The cardinal points of the
compass are North, South, East and West. The points of
the compass are often called ORIENTATION points. A
MAGNETIC COMPASS, or MARINER'S COMPASS, is a compass
or orientation-card having attached to it one or a series of
parallel magnets and supported so that it may turn freely
in a horizontal plane. Any magnet sets itself parallel to
what is termed the magnetic meridian, but it is only some
few countries that have a magnetic meridian the same as
the geographical meridian ; they include narrow strips
in Arabia, European Russia, Finland, and North Lapland.
Even at places within temperate latitudes the angle
between the two may amount to 50 or more, and in the
Arctic or Antarctic regions the direction of the compass
needle may be the opposite of the geographical north
and south line. In the neighbourhood of London the
needle points about 15 to the west of north, and the
amount is slowly decreasing.
In the west of Ireland the declination, or variation, as
it is called, is more than 20 W.
Public wind-vanes are often incorrectly set according
to magnetic north instead of true north ; caution must be
exercised accordingly.
All maps are set out according to true north ; often an
orientation mark is given to show the variation of the
compass or magnetic declination.
Compass. 69
A compass-needle is influenced by magnetic material in
its neighbourhood, and therefore in practice should not
be used near to articles made of iron or steel.
The variation of the compass and correction of its error?
are matters of primary importance for aircraft pilots and
are provided for by a special manual.
Component. A word used to indicate the steps, in
their various directions, which must be compounded or
combined geometrically in order to produce a given
displacement. For example a man going ten steps upstairs
arrives in the same position as if he took ten treads
forward on the level and then ten rises straight upwards,
or equally if he first went up ten rises and then took ten
treads forward. The actual distance travelled is the
combination of the horizontal distance and the vertical
rise. We call the horizontal distance and the vertical
rise the components arid the actual distance the geometrical
sum or the resultant of the components.
It is evident that it is not necessary that the components
should be at right angles to each other as the horizontal
and vertical are. Any displacement AC is the geometrical
sum of the two components AB and BC,, wherever B
may be.
This analysis of displacement into components, or
combination of component displacements to form a
resultant displacement, finds an effective illustration in
the effect of leeway on the performance of aircraft.
The aircraft must necessarily be carried along by the air
in which it travels. If we suppose AB to be the travel
of the aircraft through the air in an hour, and BC to
represent the travel of the wind in the hour, AC, the
resultant, will represent the travel of the aircraft with
reference to the fixed earth, or its performance. BC is
the leeway, AB is the headway made through the air.
70 Glossary.
Hence the performance is the geometrical sum of the
headway and the leeway. See VECTOR.
The law of composition of displacements which is thus
set out is equally applicable to the composition of
velocities, accelerations, and forces. The resultant is
always the geometrical sum of the components, and to
obtain the geometrical sura, set out a step or line
representing the first component, then another step
representing the second component : the resultant is the
step from the beginning of the first component to the end
of the second.
The simplest way of dealing with the magnitudes and
angles which come into questions involving geometrical
composition is to make a scale-drawing and measure them
with a rule and protractor. That is generally accurate
enough for most purposes, but the methods of the solution
of triangles can be employed if high numerical accuracy
is wanted.
Condensation. The process of formation of a liquid
from its vapour. See AQUEOUS VAPOUR.
Conduction. 71
Conduction. The process by which heat is trans-
ferred by and through matter, from places of high to
places of low temperature, without transfer of the matter
itself, the process being one of '* handing on " of the heat-
energy between adjacent portions of matter. It is the
process by which heat passes through solids ; in fluids,
although, it occurs, its effects are usually negligible in
comparison with those of CONVECTION. See also p. 146.
Convection. In convection heat is carried from
one place to another by the bodily transfer of the matter
containing it. In general, if a part of a fluid, whether
liquid or gaseous, is warmed, its volume is increased, and
the weight per unit of volume is less than before.
The warmed part therefore rises and its place is taken
by fresh fluid which is warmed in turn. Conversely,
if it is cooled it sinks. Consequently, if heat is
supplied to the lower part of a mass of fluid, the heat
is disseminated throughout the whole mass by convection,
or if the upper part is cooled the temperature of the
whole mass is lowered by a similar process.
There are two apparent and important exceptions in
meteorology. Fresh water, when below the temperature
of 39*1 F., 277a., expands instead of contracting on being
further cooled. Hence a pond or lake is cooled bodily
down to 39*1 but no further, as winter chills the surface
before it freezes. Secondly, heat applied to the bottom
of the atmosphere may stay there without being dis-
seminated upwards when the atmosphere is exceptionally
stable in the circumstances explained under ENTROPY.
Corona. A coloured ring, or a series of coloured rings,
usually of about 5 radius, surrounding the sun or moon.
The space immediately adjacent to the luminary is
72 Glossary.
bluish-white, while this region is bounded on the outside
by a brownish red ring, these two together forming the
" aureole." In some cases the aureole alone appears, but
a complete corona has a set of coloured rings surrounding
the aureole, violet inside followed by blue, green, yellow
to red on the outside. This series may be repeated several
times outwards. The corona is produced by diffraction,
that is by the bending of rays of light round tl^e edge of
small particles, in this case minute water drops, but
sometimes dust (see BISHOP'S RING). If the diffracting
particles are of uniform size the colours are pure ; a mix-
ture of many sizes may give the aureole only. The more
numerous the particles involved, the greater is the
intensity of the colours, while the radius of the corona is
inversely proportional to the size of the particles. Thus
a corona whose size is increasing indicates that the water
particles are diminishing in size, and vice versa. The
corona is distinguished from the halo, which is due to
REFRACTION, by the fact that the colour sequence is
opposite in the two, the red of the halo being inside, that
of the corona outside.
Correction. The alteration of the reading of an
instrument in order to allow for unavoidable errors in
measurement. The measurement of nearly all quantities
is an indirect process, and generally takes the form of
reading the position of a pointer or index on a scale.
When we wish to know the pressure of the atmosphere
we read an index on the scale of a barometer ; when we
wish to know the temperature of the air we read the
position of the end of a thread of mercury in a ther-
mometer ; to determine a height we use the pressure,
though the scale may be graduated in feet or metres.
Correction. 73
Almost all measurements are, in fact, ultimately re-
duced to reading a position or length on a graduated
scale and, generally speaking, the reading depends mainly,
it is true, on the quantity which the instrument is in-
tended to measure, but also partly upon other quantities.
Thus the readings of barometers are generally affected by
temperature as well as pressure, those of thermometers
by alterations in the glass containing the mercury or spirit.
It is the object of the designer and maker of instruments
to get rid of these disturbances of the reading as far as
possible, either by the selection of special materials or by
introducing some device whereby the disturbing effect is
automatically corrected. In that case the error, and often
the instrument, is said to be compensated.
But in most cases the amount of the error has to be
determined and allowed for by a suitable correction. An
ANEROID BAROMETER is often compensated for tempera-
ture, and for a mercury barometer the effect of temperature
is made out and tabulated and a correction introduced,
which is derived from a table, when the temperature of the
" attached thermometer " has been noted. In a similar
manner the correction of a barometer reading for the
variation of GRAVITY at different parts of the earth's sur-
face is worked by means of tables, the variation of
gravity with latitude having been previously reduced to a
formula, by means of observations from which the figui e
of the earth has been determined and the change of
gravity has been ascertained.
In some measurements, such as the determination of
height by the use of an aneroid barometer, corrections
are numerous and complicated : the uncorrected reading
may even be only a rough approximation not sufficiently
accurate for practical purposes.
74 Glossary.
Correlation. Two varying quantities are said to be
correlated when their variations from their respective
mean values are in some way or other mutually connected
with each other.
A kind of measure of this connection is given by the
" correlation coefficient." This is a decimal lying between
+ 1 and 1, which is easily calculated. Values of + 1
and of 1 show that the two quantities -are directly or
inversely proportional ; that is to say, for any departure
from the normal of the one quantity there is a corre-
sponding departure from the normal of the other, always
in the same sense or always in the opposite sense. On the
other hand values that are nearly nothing show that there
is little if any connexion.
It may be of interest to state that the coefficient of
correlation between the phases of the moon and the
barometric pressure at Greenwich is insignificant.
Modern statistical methods have been frequently applied
to meteorological problems. Mr. W. H. Dines, F.R.S.,
has used the method of correlation with great success in
his work on the upper air. His results are published by
the Meteorological Office in Geophysical Memoirs No. 2.
As examples of correlation coefficients we may give those
found by Mr. Dines between the pressure at a height of
9 kilometres and the mean temperature of the air column
from 1 to 9 kilometres. For different sets of observations
the values found were '88, '96, 1 90, -90, 94. The inference
to be drawn from such correlation coefficients is that the
variations of temperature of the air column from 1 to 9
kilometres are directly dependent upon the pressure at the
top of the air column.
Mr. Dines gives correlation coefficients from work on
the upper air in various other papers.
Correlation.
75
Sir" Gilbert Walker, of the Indian Meteorological
Service, has used the method of correlation to predict the
amount of the Indian Monsoon Rainfall. He has also
investigated the connection between sunspots and tem-
perature, sunspots and rainfall, sunspots and pressure by
statistical methods.
From, various papers the following examples of correla-
tion coefficients have been selected :
Correla-
tion
Coefficient.
Number of
Observations.
Variables Correlated.
Reference.
*49
42
(1867-1908).
Height of Nile flood
and South Ameri-
can mean pressure
in March, April,
May.
G. T. Walker, Cor-
relation in Seasonal
Variations of
Weather. (Memoirs
of the Indian Met.
Dept., Vol. XXI.,
Part II.)
-'47
30
(1880-1909).
Rainfall at Java,
October to March,
with rainfall at
Trinidad the fol-
lowing six months.
R. C. Mossman. South-
ern Hemisphere
Seasonal Correla-
tions. (Symons Met.
Mag., Vol. 48, 1913.)
-'43
34
(1877-1910).
Annual mean tem-
perature at Cairo
and annual mean
temperature in Eng-
land, S.W., and
South Wales.
J. I. Craig, a See-Saw
of Temperature.
(Quar. Journal, Roy.
Met. Soc., April
1915.)
-62
36
(1875-1910).
Mean pressure for
March at Azores
and in Iceland.
J. P. Van der Stok.
76
Glossary.
Correla-
tion
Coefficient.
78
-81
80
Number of
Observations.
20
(1891-1910).
47
(1869-1915).
20
(1896-1915).
Variables Correlated.
March barometric
gradient at Zikawei
(China') and July-
August air tem-
perature at Miyako
in N.E. Japan.
Mean pressure for
March at Kew and
rainfall total for
same month at
Kew.
Sunspot numbers
and level of Lake
Victoria.
Reference.
T. Okada (Monthly
Weather Review,
U.S.A., Jan,, 1916).
E. H. Chapman, The
Relation between
Atmospheric Pres-
sure and Rainfall.
(Quar. Journal, Roy.
Met. Soc., 1916.)
M.O. MS.
Cosecant. In a right angled triangle the ratio of the
hypotenuse to one side is the cosecant of the angle opposite
to that side ; the ratio is the reciprocal of the sine. See
SINE.
Cosine. In a right angled triangle the ratio of one
side to the hypotenuse is the cosine of the angle between
them. See SINE.
Cotangent. In a right angled triangle the ratio of
the two sides that form the right-angle is the cotangent
of the angle opposite the side taken as the divisor. See
SINE.
Counter sun. See ANTHELION.
Cumulo-stratus. 77
CumulO-StratUS. The name given to a certain com-
bination of cloud forms which is no longer used in the
international classification. See CLOUDS.
Cumulus. The technical name of the woolpack cloud.
See CLOUDS.
. A name given to a region of low barometric
pressure ; now usually spoken of as a DEPRESSION or a
LOW. See ISOBARS.
Cyclostrophic. See GRADIENT WIND.
Damp Air. As distinguished from dry air in me-
teorology, damp air implies a high degree of RELATIVE
HUMIDITY (q.v.). When its relative humidity equals or
exceeds 85 per cent, of saturation air may fairly be called
damp. It will deposit some of its moisture in dry
woollen fabrics, cordage or other fibrous material, though
its water will not condense upon an exposed surface
until 100 per cent, is reached. Even the driest air of the
atmosphere contains some water vapour, and its relative
dampness or dryness can be changed by altering the
temperature. Thus the same air may be very dry at
2 o'clock in the afternoon, and very damp, even cloudy,
at 8 o'clock in the evening, simply because its tempera-
ture has been lowered. At any time of the year, but
especially in summer, the dampness of the air is subject
to great changes.
Day Breeze. See SEA BREEZE or BREEZE.
Debacle. Breaking up of the ice in the spring in
rivers and seas ; it lasts from two to six weeks, and takes
place between the end of January and the beginning of
78 Glossary.
May, varying according to locality. The waters are
usually free from ice by the end of April-May. At
" debacle " the water in rivers rises to the extent of
inundating the country for miles around, sometimes
stopping all carriage traffic ferry-boats taking their place.
This condition may last for as long as three weeks.
In Russia there are some 110 stations at which obser-
vations of the "debacle" are taken. The event takes
place earliest in the Caspian, Black and Azov Seas : it
commences at the end of February, and the sea along the
coast is free of ice by the end of March the open sea
being clear by the end of February. In the Pacific the
phenomenon takes place in April, and the sea is clear by
May ; in the Baltic usually in March or the beginning of
April, the sea being cleared by the beginning of May, but
at Reval and Libau this may occur even by the end of
March. At Uleaborg the "debacle" is later, there being
ice in the sea till the end of May ; and in the Arctic Sea
still later, about the end of April, and the sea is not clear
till the beginning of June.
In Canada, in Ontario, the occurrence takes place in
March, freeing the waters by April, and the same is true
in the case of the Maritime provinces. In the St. Lawrence
it ia a little later, the river being free of ice in May.
Dekad, in Meteorology, a period of ten days, but
decade is often, used for ten years.
Density. The density of air is one of the things you
have to know when you want to calculate the lifting
power of a balloon of given size. As applied to air,
density is a difficult word to explain because the numeri-
cal value depends partly upon the composition of the air,
partly upon its pressure, and again partly upon its tern-
Density. 79
perature. Thus it is often said that moist air or damp
air is lighter than dry air, warm air is lighter than cold
air, and rarefied air is lighter than compressed air, and
all these statements are true provided that in each case
we remember to introduce the condition "other things
being equal."
By the density of a sample of air is to be understood
the weight, or better, the mass of a measured volume, a
cubic foot, or a cubic metre ; and moist air is lighter than
dry air in this sense that a cubic metre of perfectly dry
air weighs 1,206 grammes when its barometric pressure
is 1,000 millibars, 29*53 inches, and its temperature is
289a. (60*8 C F.), whereas if it were saturated air instead
of dry air the cubic metre would weigh 1,197 grammes.
But if the barometric pressure rose while the change from
dry air to saturation was being effected the density would
rise in like proportion ; and if the temperature changed
the density would change in reversed proportion to the
absolute temperature. The formula for the density of
air is, therefore, a complicated one
A-
where
A is the density to be computed,
A is the density of perfectly dry air at pressure^ and
temperature T .
If p = 1,000 mb., 29-53 in., and T = 290a. then
A = 1,201 g/m 3 .
p is the barometric pressure in mb. of the sample.
e is the pressure of aqueous vapour in the sample.
The following is a complete example of the deter-
mination of the density of a sample of air from a reading
of the barometer and wet and dry bulb thermometers.
80 Glossary.
Barometer corrected for temperature 1 ,010*1 mb. (29*83 in.).
Dry bulb 285*8a. (55*1 P.).
Wet bulb 281-3a. (47*0 P.).
Vapour pressure from humidity tables = 8*2 mb.
(0*239 in.).
A = 1,201 g/m3,
1,010-1 - 8*2 x 3/8 290
A = 1,201 x - i )() 00 x 285^8 g / m '
= 1,227 g/m,
or 0*0766 Ibs. per cubic foot.
Note, One grain per cubic foot is equivalent to 2-29 g/m 3 .
Since the reading of a barometer gives the pressure at
the level of the mercury in the barometer cistern it must
be understood that the density refers to the sample of air
at that level. For the value at any other level a correction
for level must be made.
The variation of the density of air with pressure and
temperature is of great importance in meteorology.
Pressure is of course higher in an anticyclone than in a
cyclonic depression, and it has recently been made cer-
tain that above one kilometre (3,281 feet) the air in the
high pressure is warmer than in the low pressure, so
the effect of difference of pressure may nearly counter-
balance the effect of temperature, and in consequence the
density of a column of air in a cyclone may be very little
different from that in an anticyclone.
The influence of moisture upon the density of air is
not regarded as having the importance in meteorology
which used to be attributed to it when it was held to explain
the difference between high and low pressure with the
accompanying weather.
Density.
81
The following are the average pressures, temperatures
and densities of air at different levels above high pressure
and low pressure respectively in the British Isles accord-
ing to the results obtained with registering balloons by
Mr. W. H. Dines, F.R.S.
TABLE o^ AVERAGE VALUES OF THE PRESSURE, TEM-
PERATURE AND DENSITY OF AIR IN A REGION OF
HIGH AND OF LOW PRESSURE.
High Pressure.
Low Pressure.
Height.
Pres-
sure.
Temp.
Density.
Pres-
sure.
Temp.
Density.
lOOO-ft. k.
mb.
a.
g/m 3
mb.
a.
g/m 3
32*809 IO
273
226
421
247
225
382
29-528. 9
317
2 33
474
288
226
444
26 * 247 8
366
240
53i
335
227
SH
22 * 966 7
422
247
595
388
232
583
19-685 6
483
254
662
449
240
652
16-406 5
552
261
736
5i6
248
724
13-124 4
628
267
818
59i
255
807
9^43 3
713
272
911
675
263
893
6-562 2
807
277
IOI2
767
269
992
3-28l I
913
279
H37
870
275
IIOO
o o
1031
282
I27O
984
279
1226
The densities quoted above are calculated on the
assumption that the relative humidity is 75 per cent.
82 Glossary.
The following are the densities of a few other substances.
'Hydrogen (dry) 88'74 g/m 3
*Carbonic acid gas (dry)... 1,953 g/m 3
Water 1-000 g/cc (at 277a.)
Sea water I'Ol to T05 g/cc.
Mercury 13'596 g/cc (at 273a.)
Petrol ... ... 0-68 to O72 g/cc.
See also under BUOYANCY.
Depression. A region of low barometric pressure
surrounded on all sides by higher pressures. See
ISOBARS, and Plate XL
Dew. The name . given to the deposit of drops of
water which forms upon grass, leaves, &c., when they
become cooled, by radiating heat to the sky on a clear
night, to such an extent that their temperature is below
the saturation or DEW POINT of the air which surrounds
them.
The last part of the process of the formation of dew is
in no way different from that which operates when a
glass of ice-cooled water covers itself with water drops
indoors, or when a deposit of moisture is formed by
breathing on a window-pane.
Dew-point. The temperature of saturation of air,
that is to say, the temperature which marks the limit to
which air can be cooled without causing condensation,
either in the form of cloud if the cooling is taking place
in the free air, or on the sides of the vessel if it is
enclosed. See AQUEOUS VAPOUR.
Diathermancy. Diathermanous. The power of
allowing heat in the form of radiation to pass in the
same way that light passes through glass. Rock salt is
* At a pressure of 1,000 mb. and temperature of 273a.
Diathermancy. 83
peculiarly diathermanous, water, on the contrary, and
glass are not.
All forms of energy which are " radiated " and in
connection with which the word " ray " is used, such as
rays of light, heat rays, X-rays, radio-telegraphic rays,
travel by wave motion and have some properties in
common. One of the most characteristic is that described
as transparency and opacity with regard to light. But
the same substances are not similarly transparent for all
kinds of rays. X-rays and electric rays make no difficulty
about going through walls which stop light ; and sound-
waves often find a way where light cannot follow. The
question of transparency and opacity for different kinds
of rays is one of bewildering complexity. The study of
DIATHERMANCY deals with that part of the subject which
is concerned with the transmission of heat in the form of
wave motion.
Diffraction. The process by which rays of different
colour are separated one from another when a beam
of light passes an obstacle of any shape. In reality the
shape of the obstacle must be carefully chosen in relation
to the shape of the beam to make the phenomenon easily
apparent. Perhaps the simplest experiment is to draw a
greasy finger across a plate of glass and to look through
the glass at the bright line of an incandescent electric
lamp, taking care that the plate is turned so that the lines
left by the finger on the glass are parallel to the bright
line. Those who are unfamiliar with the experiment
will be surprised at the brilliancy of the colours which
are produced by the simple process. In more scientific
form when the lines are ruled regularly on the glass by a
suitable dividing engine we get a diffraction-grating, one
of the most delicate of all optical instruments.
84 Glossary.
For scientific experiments in diffraction either a bright
line of light formed by a slit in front of a lamp with a
linear obstacle, or a bright point of light with a circular
obstacle may be used, and many remarkable results can
be shown with these simple means. For example, when
the distances are properly adjusted a bright spot will be
found at the central point of the shadow of a circular
disc, thrown by a bright point of light, and again on
looking past a needle at a line of light parallel to the
needle a great play of colours will be seen.
The phenomena of diffraction are explained by the
hypothesis that the actual transmission of light is not a
direct projection of the light along straight lines from
any luminous point, but the spreading out of waves with
a spherical wave-front central at the luminous point.
They are exhibited in the atmosphere principally by the
formation of CORONAE round the sun and moon, and
sometimes also by the IRIDESCENCE of CLOUDS.
Diffusion. The slow molecular process by which
supernatant fluids mix in spite of the differences in
their density. The molecular forces or motions which
come into play in diffusion are perhaps most effectively
illustrated by the tenacity with which the mixture main-
tains its composition when once the mixing has taken
place. For example, whisky is lighter than water and a
separate layer of the spirit can be floated on the top of
water by judicious manipulation. The spirit and water
will then slowly mix by diffusion, even if there be 110
stirring, due to thermal convection or mechanical opera-
tion. But when the spirit and water have become mixed
no amount of allowing to stand will cause the water to
settle to the bottom and leave the whisky in a separate
Diffusion. 85
layer at the top. Once mixed they are mixed for ever,
owing to the power of diffusion.
The process of diffusion follows certain definite rules,
which are similar in type to those for the diffusion of
heat by thermal conduction, and the diffusion of velocity
through a viscous mass, but it takes so long a time for any
appreciable effect to be produced, that in practice diffu-
sion only completes the process of mixing which has been
begun by stirring or convection. Major G. I. Taylor has
recently shown that in the atmosphere mixing by tur-
bulent motion (see EDDY) follows a similar law, but with
a different characteristic constant that brings diffusion
by turbulent motion among the operative forces in
Meteorology.
Diurnal. The word, which means " recurring day by
day," is used to indicate the changes in the meteorological
elements which take place within the twenty-four hours of
the day. Thus, by the diurnal change of pressure is meant
a slight rise of the barometric pressure between about
4 a.m. and 10 a.m., and between 16 h. (4 p.m.) and 22 h.
(10 p.m.) with corresponding falls between. In this
change it is the " semidiurnal " variation which is the
most striking because it occurs (with different intensity)
all round a whole meridian simultaneously, and sweeps
round the globe from meridian to meridian about three
and a half hours in front of the sun.
Other elements also show noteworthy diurnal variations.
We give here diagrams showing the diurnal variation of
pressure, temperature, humidity and wind velocity at
Kew for January and July as representing winter and
summer respectively.
86
DIURNAL VARIATION IN SUMMER.
HOURS
c so-G
CD
O
c 70-ti
60-:
3 6 9 12 15 18 21
HLMIDITY
i\
i-60
HOURS
so
.310145-1
-2996u
-/-
WIN
_ o
6
-
--4
<U
294
<28St
286-
TEMPERATURi:
:-55
From the Normals for July.
Averages of not less than twenty-five years of Hourly
Readings at Kew Observatory. For the Diurnal Variation
of wind-speed through the different months at the summit
of 4 the Eiffel Tower, Paris, see p. 287,
87
DIURNAL VARIATION IN WINTER.
HOURS
12 15 18 21 2t HOUR5
HUMIC
(D
"
90-E
80-:
70-:
JIOI65.E
fioi&al
PRESS
ITY
E-so
: -70
-30005
/VIN
O ^ j
cu O 4~"
C Q ~
1-3+
a;280^
^275
PERATURE
<274- :
40 ^
35
From the Normals for January.
Averages of not less than twenty-five years of Hourly
Readings at Kew Observatory.
88 Glossary.
Doldrums.- The equatorial belt of calms and light
variable airs, accompanied by heavy rains, thunderstorms
and squalls. The belt follows the sun in his movement
North and South, but its movement is not so great as that
of the sun, and lags behind it by one to two months.
Drought. Dryness due to lack of rain. According to
the classification of the British Rainfall Organization an
absolute drought is a period of more than 14 consecutive
days without one-hundredth of an inch of rain on any one
day, and a partial drought is a period of more than
28 consecutive days the mean rainfall of which does not
exceed * 01 inch per day, or the total fall for the 28 days
at most barely exceeds a quarter of an inch.
Dry Air. The words are used in two senses. In a
book on physics or chemistry dry air means air that
carries no water- vapour at all, but in ordinary practice it
is used for the atmosphere when it contains a smaller
proportion of water-vapour than usual. Water evaporates
from wet surfaces exposed to the air unless the atmos-
phere is completely saturated. If we call air containing
85 per cent, or more of the possible amount of water-
vapour damp, we may call air with less than 60 per cent,
dry, and understand thereby that EVAPORATION is rapid
and roads, grass, &c., dry quickly.
The letter " y " has recently been added to the Beaufort
Notation (see Weather Map, p. 10) to signify air with less
than 60 per cent, of the possible amount of water-vapour.
The following table shows for different dry-bulb tem-
peratures the smallest depression of the wet-bulb which
would justify the use of the letter "y" in reporting the
" present weather."
Dry Bulb. 89
Dry Bulb. Depression of Wet-bulb.
a a
271 or below 1
273 (F.P.) or la above or below 2
275281 3
282292 4
293305 5
above 305 5^
It is found from the hourly readings of the Meteorological
Office Observatories that out of a thousand hours Aberdeen
has 37 hours of " dry air " thus defined, Valencia 6,
Falmouth 19 and Kew 90. Had the limit been set at 50
per cent, instead of 60, the figures would have been only
4, 1, 1 and 19 respectively (see p. 224).
Dry bulb. A curious name given to an ordinary
thermometer used to determine the temperature of the
air, in order to distinguish it from the wet bulb. No
special precautions are taken to keep a " dry bulb " dry
beyond protecting it from falling rain in a screen. But
it is true enough that if the dry bulb gets a film of water
upon it by condensation it will not be in a proper con-
dition to record the temperature of the air until it has
got quite dry again.
Dynamics. The study of the motion of bodies in
relation to the forces which control the motion. The
fundamental principle of dynamics is that if a moving
body is let alone it will go on moving. It is a vulgar
error to suppose that it will stop.
Dynamic Cooling*. The fall of temperature which
occurs automatically throughout a mass of air when it
expands on the release of pressure. (See ADIABATIC.)
Examples of the expansion under reduced pressure and
consequent cooling are to be found in the flow of air up
a mountain slope with the formation of cloud at the top.
90 Glossary.
Earth - Thermometer. A mercury - thermometer
suspended in a tube sunk into the earth, or an electrical
resistance thermometer buried in a trench usually at
depths of one foot and four feet for measuring the
temperature of the ground. See Observer's Handbook.
Eddy. " The water that by some interruption in its
course runs contrary to the direction Of the tide or current
(Adm. Smyth) ; a circular motion in water, a small
whirlpool," according to the New English Dictionary.
Eddies are formed in water whenever the water flows
rapidly past an obstacle. Numbers of them can be seen
as little whirling dimples or depressions on the surface
close to the side of a ship which is moving through the
water. In the atmosphere similar eddies on a larger
scale are shown by the little whirls of dust and leaves some-
times formed at screet corners and other places which
present suitable obstacles. The peculiarity of these wind
eddies is that they seem to last for a little while with an
independent existence of their own. They sometimes
attain considerable dimensions and, in fact, they seem to
pass by insensible degrees from the corner eddy to the
whirlwind, the dust-storm, the waterspout, the tornado,
the hurricane, and finally the cyclonic depression. It
is not easy to draw the line and say where the
mechanical effect of an obstacle has been lost, and the
creation of a set of parallel circular isobars has begun, but
it serves no useful purpose to class as identical phenomena
the street corner eddy twenty feet high and six feet wide
and the cyclonic depression a thousand miles across and
three or four miles high.
The special characteristic of every eddy is that it must
have an axis to which the circular motion can be referred.
The axis need not be straight nor need it be fixed in shape
or position. The best example of an eddy is the vortex-
Eddy. 91
ring or smoke ring which can be produced by suddenly
projecting a puff of air, laden with smoke to make the
motion visible, through a circular opening. In that case
the axis of the eddy is ring shaped ; the circular motion
is through the ring in the direction in which it is travelling
and back again round the outside. The ring-eddy is very
durable, but the condition of its durability is that the axis
should form a ring. If the continuity of the ring is
broken by some obstacle the eddy rapidly disappears in
irregular motion.
It is on that account that the eddy motion of the
atmosphere is so difficult to deal with. When air flows
past an obstacle a succession of incomplete eddies are
periodically formed, detached, disintegrated and reformed.
There is a pulsating formation of ill-defined eddies. The
same kind of thing must occur when the wind blows on
the face of a cliff, forming a cliff -eddy with an axis,
roughly speaking, along the line of the cliff and the
circular motion in a vertical plane.
Whenever wind passes over the ground, even smooth
ground, the air near the ground is full of partially formed,
rapidly disintegrating eddies, and the motion is known as
turbulent, to distinguish it from what is known as stream-
line motion, in which there is no circular motion. The
existence of these eddies is doubtless shown on an
anemogram as gusts, but the axes of these eddies are so
irregular that they have hitherto evaded classification.
Irregular eddy motion is of great importance in meteoro-
logy, because it represents the process by which the slow
mixing of layers of air takes place, which is an essential
part in the production of thick layers of fog. Moreover,
all movements due to convection must give rise to current
and return current which at least simulates eddy motion.
Electrometer. An instrument for measuring elec-
92 Glossary.
tromotive force, or potential difference. An ordinary
battery has an electromotive force of at least a volt
and shows a corresponding potential-difference on an
electrometer when the poles of the battery are connected
to the electrodes (connecting clamps) of the electrometer.
A fully charged secondary battery shows a potential-
difference of about two volts. In the atmosphere near
the ground there is, on the average at most stations, a po-
tential-difference exceeding 100 volts for a difference of
level of one metre, due to the electrification of the air.
It can be measured by an electrometer using a burning
match or a water-dropper or a radio active substance as
" collector." The potential-difference in the atmosphere
measured in this way is very variable, especially during rain.
The potential-difference necessary to cause a spark
between two metal balls through one centimetre of air
at ordinary pressure is about 30,000 volts, which suggests
that the potential-differences necessary to produce a dis-
charge of lightning are enormous.
Energy. Used frequently in meteorology in the
general sense of vigour or activity. Thus, a cyclone is
said to develop greater energy when its character, as
exhibited by a low barometer, steep gradients and strong
winds, becomes more pronounced. But there is a technical
dynamical sense of the word, the use of which is some-
times required in meteorology, and which must become
more general when the physical explanation of the
phenomena of weather is studied, because all the pheno-
mena of weather are examples of the " transformations of
energy " in the physical sense.
The most important conception with regard to energy
is its division into two kinds, kinetic energy and potential
energy, which are mutually convertible. A clock-weight
gives a good idea of potential energy. When the clock
Energy. 93
is wound up the weight has potential energy in virtue of
its position ; it will utilise that energy in driving the
clock until it is " run down " and can go no further.
Potential energy must be restored to it by winding up
before it can do any more driving. The potential energy
in this case is measured by the amount of the weight and
the vertical distance through which it is wound up. In
dynamical measure the potential energy of the raised
weight is mgh, where m is the mass of the clock- weight,
h the vertical distance through which it is wound up,
g the acceleration of gravity. It is to the mysterious
action of gravity that the energy is due : hence the
necessity for taking gravity into account in measuring
the energy.
Using the simple product mgh as a measure of the
potential energy of gravitation, by a simple formula for
bodies falling freely under the action of gravity, we
have
mgh = fynv 2
where v is the velocity acquired by a body falling through
a height 7i, or, speaking in terms of energy, by losing the
potential energy of the height h. It thus obtains a
certain amount of motion which represents kinetic energy,
in exchange for its potential energy. The kinetic energy
is expressed by the apparently artificial formula rnv 2 .
In virtue of its motion it has the power of doing "work" :
if it were not for unavoidable friction the mass could
'get itself up-hill again through the height h by the use
of its motion, and thus sacrifice its kinetic energy in
favour of an equivalent amount of potential energy.
The exchange of potential and kinetic energy can be
seen going on in a high degree of perfection in a swinging
pendulum. At the top of the swing the energy is all
94 Glossary.
potential, at the bottom all kinetic. The swings get
gradually smaller because in every swing a little of the
energy is wasted in bending the cord or in overcoming
the resistance of the air.
What we get in return for the loss of energy in friction
is a little HEAT, and one of the great conclusions of
physical science in the middle of the nineteenth century
was to show that heat is also a form of energy but a very
special form, that is to say, its transformation is subject
to peculiar laws. Heat is often measured by rise of
temperature of water (or its equivalent in some other
substance). Calling this form of energy thermal energy
and measuring it by the product of the " water equivalent,"
M, and the rise of temperature AA^ produced therein,
we have three forms of energy all convertible under
certain laws, viz. :
Potential energy ... mgli
Kinetic energy ... ... \ mv"
Thermal energy M (A A Q )
We have mentioned only a lifted clock-weight as an
example of potential energy, but there are many others, a
coiled spring that will fly back when it is let go, com-
pressed gas in a cylinder that will drive an engine when
it is turned on, every combination, in fact, that is dormant
until it is set agoing and then becomes active.
From the dynamical point of view, the study of nature
is simply the study of transformations of energy.
In meteorology kinetic energy is represented by the
winds ; potential energy by the distribution of pressure at
any level, by the electrical potential of the air and by the
varying distribution of density in the atmosphere, causing
convection ; thermal energy by the changes of temperature
due to the effect of the sun or other causes. It is the
Entropy. 95
study of the interchange of these forms of energy which
constitutes the science of dynamical meteorology.
Entropy. A term introduced by R. Clausius to be
used with temperature to identify the thermal condition
of a substance with regard to a transformation of its heat
into some other form of energy. It involves one of the
most difficult conceptions in the theory of heat, about
which some confusion has arisen.
The transformation of heat into other forms of energy,
in other words, the use of heat to do work, is necessarily
connected with the expansion of the working substance
under its own pressure, as in the cylinder of a gas engine,
and the condition of a given quantity of the substance at
any stage of its operations is completely specified by its
volume and its pressure. Generally speaking (for example,
in the atmosphere) changes of volume and pressure go on
simultaneously, but for simplifying ideas and leading on
to calculation it is useful to suppose the stages to be kept
separate, so that when the substance is expanding the
pressure is maintained constant by supplying, in fact, the
necessary quantity of heat to keep it so, and, on the other
hand, when the pressure is being varied the volume is
kept constant ; this again by the addition or subtraction
of a suitable quantity of heat. While the change of
pressure is in progress, and generally, also, while the
change of volume is going on, the temperature is changing,
and heat is passing into or out of the substance. The
question arises whether the condition of the substance
cannot be specified by the amount of heat that it has in
store and the temperature that has been acquired just as
completely as by the pressure and volume.
To realise that idea it is necessary to regard the
processes of supplying or removing heat and changing the
temperature as separate and independent, and it is this
96 Glossary.
step that makes the conception useful and at the same
time difficult. .
For we are accustomed to associate the warming of a
substance, i.e. 9 the raising of its temperature, with supply-
ing it with heat. If we wish to warm anything we put it
near a fire and let it get warmer by taking in heat, but in
thermodynamics we separate the change of temperature
from the supply of heat altogether by supposing the
substance is "working." Thus, when heat is supplied
the temperature must not rise ; the substance must do a
suitable amount of work instead ; and if heat is to be
removed the temperature must be kept up by working
upon the substance. The temperature can thus be kept
constant while heat is supplied or removed. And, on the
other hand, if the temperature is to be changed it must
be changed dynamically not thermally, that is to say, by
work done or received, not by heat communicated or
removed.
So we get two aspects of the process of the transforma-
tion of heat into another form of energy by working,
first, alterations of pressure and volume, each independ-
ently, the adjustments being made by adding or removing
heat as may be required, and secondly, alterations of heat
and temperature independently, the adjustments being
made by work done or received. Both represent the
process of using heat to perform mechanical work or vice
versa.
In the mechanical aspect of the process, when we are
considering an alteration of volume at constant pressure,
p (vv ) is the work done, and in the thermal aspect of
the process HH is the amount of heat disposed of.
There is equality between the two.
But if we consider more closely what happens in this
97
case we shall see that quantities of heat ought also to be
regarded as a product, so that HH should be expressed
as 7 7 (0~(/) ) where T is the absolute temperature and the
entropy.
The reason for this will be clear if we consider what
happens if a substance works under adiabatic conditions,
as we may suppose an isolated mass of air to do if it rises
automatically in the atmosphere into regions of lower
pressure, or conversely if it sinks. In that case it neither
loses nor gains any heat by simple transference' across its
boundary ; but as it is working it is drawing upon its
store of heat, and its temperature. falls. If the process is
arrested at any stage, part of the store of heat will have
been lost through working, so in spite of the adiabatic
isolation part of the heat has gone all the same. From
the general thermodynamic properties of all substances, it
is shown that it is not H, the store of heat, that remains
the same in adiabatic changes, but EfT, the ratio of the
stcre of heat to the temperature at which it entered. We
call this ratio the entropy, and an adiabatic line which con-
ditions thermal isolation and therefore equality of entropy
is called an isentropic. If a new quantity of heat h is
added at a temperature T the entropy is increased by hj T.
If it is taken away again at a lower temperature T' the
entropy is reduced by hfT'.
In the technical language of thermodynamics the
mechanical work for an elementary cycle of changes is
dp. %v, and the element of heat c T. fy. The conversion
of heat into some other form of energy by working is
expressed by the equation
32\ 30 = Sp. Sv
when heat is measured in dynamical units.
13204 D
93 Glossary.
It is useful in meteorology to consider these aspects of
the science of heat although they may seem to be far
away from ordinary experience because, from certain
aspects, the problem of dynamical meteorology seems to
be more closely associated with these strange ideas than
those which we regard as common. For example, it may
seem natural to suppose that if we could succeed in com-
pletely churning the atmosphere up to, say, 10 kilo-
metres (6 miles) we should have got it uniform in
temperature or isothermal throughout. That seems
reasonable, because if we want to get a bath of liquid
uniform in temperature throughout we stir it up ; but
it is not true. In the case of the atmosphere there is the
difference in pressure to deal with, and, in consequence
of that, complete mixing up would result, not in equality,
but in a differrnce of temperature of about 100 C. between
top and bottom, supposing the whole atmosphere dry.
The resulting state would not, in fact, be isothermal ; the
temperature at any point would depend upon its level
and there would be a temperature difference of 1 0.
for every hundred metres. Bat it would be perfectly
isentropic. The entropy would be the same everywhere
throughout the whole mass. And its state would be
very peculiar, for if you increased the entropy of any
part of it by warming it slightly the warmed portion
would go right to the top of the isentropic mass. It
would find itself a little warmer, and therefore a little
lighter specifically than its environment, all the way up.
In this respect we may contrast the properties of an
isentropic and an isothermal atmosphere. In an isen-
tropic atmosphere each unit mass has the same entropy
at nil levels, but the temperatures are lower in the upper
levels. In an isothermal atmosphere the temperature
Entropy. W
the same at all levels, but the entropy is greater at the
higher levels.
An isothermal atmosphere represents great stability as
regards vertical movements, any portion which is carried
upward mechanically becomes colder than its surroundings
und must sink again to its own place, but an ISEN TROPIC
atmosphere is in the curious state of neutral equilibrium
which is culled " labile." So long as it is not warmed or
cooled it is immaterial to a particular specimen where it
finds itself, but if it is warmed, ever so little, it must go
to the top, or cooled, ever so little, to the bottom.
In the actual atmosphere above the level of ten kilo-
metres (more or less) the state is isothermal ; below that
level, in consequence of convection, it tends towards the
isentropic state, but stops short of reaching it by a variable
amount in different levels. The condition is completely
defined at any level by the statement of its entropy and
its temperature, together with its composition which
depends on the amount of water-vapour contained in it.
Speaking in general terms the entropy increases, but
only slightly, as we go upward from the surface through the
TROPOSPHERE until the STRATOSPHERE is reached, and
from the boundary upwards the entropy increases rapidly.
If the atmosphere were free from the complications
arising from the condensation of water- vapour the defi-
nition of the state of a sample of air at any time by its
temperature and entropy would be comparatively simple.
High entropy and high level go together ; stability
depends upon the air with the largest stock of entropy
having found its level. In so far as the atmosphere
approaches the isentropic state, results due to convection
may be expected, but in so far as it approaches the
isothermal state, and stability supervenes, convection
becomes unlikely.
13204 D 2
100 Glossary.
Equation of Time. The interval between two
successive transits of the sun over the meridian is the
true solar day, and the time based on this length is called
apparent time. The length of the true solar day varies
at different times of the year, and to avoid the inconveni-
ence of want of uniformity in the length of the day, an
imagiuary body called the mean sun may be supposed to
revolve uniformly round the Earth and complete each
revolution in a time equal to the average length of the
true solar day ; the time referred to this standard is
called mean time. To convert mean time into apparent
time, and vice versa, the correction known as the equation
of time must be applied. The equation of time varies at
different times of the year ; its value may be obtained
from the Nautical, or other Almanac. See also Observer's
Handbook.
Equator. " The line " of sailors. An imaginary line
on the earth's surface separating the northern hemisphere
from the southern hemisphere. The use of the word
" hemisphere " suggests that the earth is regarded as a
sphere, and on a spherical globe representing the earth
the equator is the line formed by the intersection with
the surface of a plane drawn at right angles to the polar
axis and bisecting it.
The position of the equatoiys identified by the vertical
or plumb line being at right angles to the polar axis, and
any complications introduced by the irregularity of the
figure of the earth have to work from that datum.
Latitude is measured from the equator northward through
90 to the North pole, southward through 90 to the
South pole.
By geodesic calculation it has been found that the
Equator..-. 101
diameter of the globe from a point of the equator to its
antipodes is 12,756,776 metres, whereas the polar axis
is 12,713,818 metres.*
Equatorial. Originally only an adjective derived
from equator ; thus, the equatorial regions are the regions
in the neighbourhood of " the line," but a meteorologist
thinks of them as regions of rather low atmospheric
pressure lying between the two belts of high pressure
which are found in either hemisphere just north or south
of the tropics. In this region the rotation of the earth
has little or 110 influence in adjusting the wind to balance
the distribution of pressure. The adjustment necessary
for persistence can only be reached by the curvature of
the air's path. It is perhaps for that reason that the
regions about ten degrees north and south of the Equator
are the regions in which tropical revolving storms originate.
The adjective has also come to be used with regard
to wind to mean a wind that is composed of air which
has come from lower latitudes, whatever may be its
direction at the time, as distinguished from a polar wind
which is composed of air that has travelled from higher
" latitudes. Typical equatorial winds are generally from
South- West or between South and West, and Polar winds
from North-East or between North and East. It is a
question of considerable meteorological interest to con-
sider in special cases whe-ther a South-East wind or a
North-West wind is actually equatorial or polar. Equa-
torial winds are generally warm, polar winds cold, but
north-easterly winds are sometimes very warm and some-
times very cold ; a north-wester almost always cold.
* U.S. Coast and Geodetic Survey.
102 Glossary.
Equilibrium.- Properly speaking, the balancing of
two or more forces in such a way that the combined effect
is the same as if there were no forces acting at all, so that
the body upon which they act, if at rest, remains at rest
and if in motion, it goes on moving without any alteration
of its velocity. In the case of a hammock slung by a
cord at each end, the forces acting along the cords and the
weight balance each other and the hammock with its load
is supported at rest. It is, therefore, not unusual to say
that the load is in equilibrium, but it is an unfortunate
use of the word because the state of rest is only a special
case. In meteorology we are very frequently concerned
with equilibrium of forces associated not with rest but
with the uniform motion of the body upon which they
act. For example, raindrops are all impelled downwardn
by their weight, and their motion is resisted by the air
through which they move, and after a very brief interval
from the start the resistance of the air balances the weight
and the drops fall with a uniform* speed (which depends
upon their size) as if there were 110 longer any gravity or
any air. The same is equally true of a falling bomb, but
the time required to reach the limiting velocity is much
greater. From the moment of its release it acquires
velocity from its \\ eight, but if the height is sufficiently
great it reaches a limiting velocity when the weight is
balanced by the friction of the air and no further increase
of velocity occurs.
So, on the other hand, a pilot balloon rises with a
uniform velocity as soon as the balloon is moving upwards
fast enough for the buoyancy of the balloon, the weight
* Mr. W. H. Dines points out that the speed is not strictly uniform
but diminishes wifih the increasing density of the air in the lower layers.
Equilibrium. 103
of the balloon and the friction of the air to get into
equilibrium.
So, also, in the case of a train running with uniform
speed along a level ; the rails push the train forward, the
resistance of the air holds it back ; there is equilibrium
which we recognise by the fact that the speed is uniform.
There are some other cases in which the word equili-
brium is 'used that are more difficult. It is sometimes
said, for example, that there is equilibrium between the
wind velocity and the barometric gradient, but it is a
peculiar kind of equilibrium : the wind is kept moving
without change of speed in a great circle of the earth but
not in a straight line through space. The balance of
forces in this case is the same as that which obtains when
we consider the motion of the moon round the earth, the
force of gravitation on the moon is " balanced " by the
moon's motion, a convenient form of expression but one
which requires some explanation before its meaning is
quite clear.
Equinox. The time of the year when the astronom-
ical day and night are equal, each lasting twelve hours,
At the equinox the sun is u on the Equator " or is " cross-
ing the line." It is on the horizon in the morning
exactly in the east, and exactly in the west in the
evening all over the world. Sunrise occurs at the
same time all along a meridian. The sun is visible by
refraction for a little longer than the duration of the
astronomical day.
There are two equinoxes. The spring or vernal equinox
about the 21st March, and the autumnal equinox about
the 22nd September. The currently accepted phrase
" equinoctial gales " indicates that the equinoxes are re-
garded in some quarters as the times of the year when gales
104 Glossary.
are specially frequent ; but on our coasts the equinoxes
mark the beginning and the end of the season of gales
rather than its culmination. Winter is really the season
for gales.
Erg. See under HEAT.
Error. In all the sciences dependent upon observation
of the size of things the word error has a numerical sense
which must not be confused with the ordinary trivial
sense of a mistake or fault. Errors in the latter sense
have to be avoided by skill and care, so they never occur,
or hardly ever ; but when everything possible in that
respect has been done there are always errors in the
technical sense, due to imperfections of the instrument,
or its adjustment, or to difficulty arising from changes in
the element while it is being measured. In this sense
error is difference between the reading of the assumed
measure and the true measure of the element. The size
of the residual error is a good indication of the degree of
nicety to which the measurement can be carried. Pressure
is the only meteorological element which is read to a
very high degree of accuracy, such as one hundredth per
cent. ; the temperature of the air can be read to the tenth
of a degree, or within less than one per cent., reckoning
on the absolute scale, as one must do for all calculations
of density, but the temperature of the air is not " known "
to that degree of accuracy, because it is subject to local and
temporary fluctuations. An accuracy within one per cent,
or even five per cent, is often acceptable.
What is aimed at is to improve the instruments and the
methods of reading, so that there is no systematic error,
that is to say, no error that is known always to be present
and to affect the measurement in the same wav,
Error. 105
When that stage has been reached, and we have no
good reason for thinking that the figure given by the
reading is in any way biassed, we have what is called
the residual error which has been made the subject of
prolonged study, and has led to the application of the
laws of mathematical probability. These have a real
practical ^application in all cases where we deal with very
large numbers of observations.
In that case the frequency of occurrence of errors of
given magnitude follows a well-known law called the
law of error, from which we are able to compute what
is called the "probable error" of an observation, a term
which frequently occurs. It only means that in any
particular case the actual error is no more likely to be
greater than the " probable error " than it is to be less, so
that the chances of the error being as great as the
probable errors are one in two.
The chances of an error being twice the probable error on either
side of the true value^are 10 in 57, for three times 10 in 238, while for
four times the probable error the chances are 1 in 147 and for five
times 1 in 1,388. If the chances of an error on one side only are
required the second of each pair of figures given above must be doubled.
The study of laws of error is of great practical im-
portance in all actuarial questions, and now forms part
of the science of statistics. There are various forms of
numerical error that call for notice. In dealing with
accurate timing, for example, by means of a clock or
chronometer, there is the index error, or clock error, and
the rate error, the amount by which the clock is gaining
or losing. Every instrument is liable to index error, on
account of the index being inaccurately set, and every
instrument is also liable to a scale error, on account of
the scale being imperfectly graduated. Before trusting
106 Glossary
implicitly to the readings of any instrument it is desirable
that the user of it should become acquainted with its
ways and habits in respect of error. This is particularly
the case with instruments used by aviators, namely,
altimeters, anemometers, aneroids, compasses, and so on.
Evaporation. The process of conversion of water
from the liquid to the gaseous form at the free surface of
water in the presence of air, or from the solid to the
gaseous form from the surface of ice. It expresses itself
by the gradual disappearance of drops of dew after sun-
rise, the drying of roofs and roads after rain, the loss of
water from cisterns and reservoirs in drought. There is
also copious evaporation from the stomata of the leaves of
plants.
The atmosphere is very rarely completely saturated, so
that evaporation is always going on when water surfaces
are freely exposed. The rate of evaporation depends upon
a number of conditions. One of the chief is the " drying
power" of the air and is represented by the difference
between the amount of water-vapour which would
saturate it and the amount which it holds at the time :
that again depends on the relative humidity and the
temperature. The drying power of air below the freezing
point is very small but, in spite of that, the disappearance
of snow by evaporation is surprisingly rapid.
The other important condition is the nature of the
surface of the water from which the evaporation takes
place. There are all stages of cleanliness of the surface
from the chemically pure water surface which is very
seldom realised in practice, to the complete superficial
film of oil which arrests evaporation altogether. Besides
Evaporation.
107
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108 Glossary.
these conditions, temperature and wind have to be con-
sidered, so that the measure of evaporation is the end of a
very intricate story. Still, in dry countries like Egypt,
South Africa and Australia it is a matter of serioun
economic importance. , It is generally given on the
analogy of rainfall as the depth of water evaporated.
Some results are given on page 107.
The differences shown in this table illustrate quite
forcibly the influence which evaporation may exercise
upon climate, but the figures must not be taken as strictly
comparable as the actual amount of evaporation depends
upon so many conditions. The measurements given for
Egypt are taken from a Wild's gauge which holds only a
small body of water and gives a high figure for evaporation.
See Keeling : Evaporation in Egypt and the Sudan (1909).
Craig : Cairo Scientific Journal, May (1912).
Expansion. The increase in the size of a sample of
material, which may be due to heat or to the release of
mechanical strain, or the absorption of moisture or some
other physical or chemical change.
The size may be taken as the length or volume, some-
times as the area. In the science of heat the fractional
increase of length or volume for one degree of tem-
perature is called the coefficient of thermal expansion.
Thus, the co-efficient of " linear " expansion with heat of
the brass used for barometer scales is 0*0000102 per degree
Fahrenheit, which means that for 1F. the length of the
scale increases by 10,2 ten-millionth parts of its length at
the standard temperature (62 F.). The co-efficient of
''cubical" expansion of mercury is '0001010, which
means that the volume of a quantity of mercury increases
by 1*01 ten-thousandth of its bulk at the standard tem-
perature (32 F.) for 1 F. The corresponding expansions
Expansion. J09
and coefficients for 1 C. or la. are larger in the ratio of
18 to 10.
Expansion of volume alters the density of a substance,
and changes of density are therefore numerically related
to expansion.
The expansion of a gas may be caused either by reduc-
tion of pressure or by increase of temperature. So in
order to see the effect of temperature alone we muse
keep the pressure constant. In these circumstances the
coefficient of expansion is '00366 for la. referred to 273a.
as standard, or *002 for 1 F. referred to 41 F. as standard.
Exposure. In meteorology, the method of presen-
tation of an instrument to that element which it is
destined to measure or record, or the situation of the
station with regard to the phenomenon or phenomena
there to be observed. If meteorological observations are
to be of much value attention must be paid to the manner
of the exposure of the instruments.
Details are to be found in the Observer's Handbook.
Uniformity of exposure is of the greatest importance and
for that reason the pattern of the thermometer-screen has
been standardized in most countries, while in these
Islands, a standard height above ground for the rain-
gauge has likewise been fixed. A SUNSHINE EECORDER
demands an entirely unobstructed horizon near sunrise
and sunset at all times of the year. The question of the
exposure of ANEMOMETERS is one of great difficulty.
The extent of the GUSTINESS of the wind as exhibited on
the trace of the tube-anemometer is a fair index of the
excellence of the exposure.
At Aberdeen Observatory, two anemometers are ex-
posed, one at an elevation of 30 feet above the other.
110 Glossary.
The results are noticeably different ; and the same state-
ment applies in an even more marked degree to the two
anemometers at Falmouth, one exposed upon the Obser-
vatory roof and one upon the tower of Pendennis Castle.
Extremes. Generally used with reference to temper-
ature or wind ; in the first case to mean the highest and
lowest temperatures recorded at an observing station in
a day, a month or a year. The maximum and minimum
temperatures are the extremes for the day ; in l'.U4 the
extremes for Greenwich for January were 55 F. and 20 F.,
and for July 92 F. and 45 F. When the observations
for a series of years are available we may find the normal
or average extremes and ABSOLUTE EXTREMES.
With regard to wind the highest wind recorded in a
gust shown on a tube - anemogram is the extreme
wind, and the highest wind-force on the Beaufort Scale
noted by an observer in the course of a gale is logged as
the extreme for that gale. The strongest gust for the
British Isles is given on p. 142, the highest hourly wind
velocity is 34*9 m/s (78 mi/hr) recorded at Fleetwood in
1894. '
Falirenlieit, Gabriel Daniel. The improver of the
thermometer and barometer, born 1686 at Dantzig. He
used mercury instead of spirit of wine for thermometers
and avoided negative temperatures by marking the
freezing point of water 32 ; the boiling point of water
Avas subsequently marked 212.
The Fahrenheit scale is still in common use in English-
speaking countries, and it has advantages because the size
of 'the degree is convenient and temperatures below F.
are of rare occurrence at the Earth's surface, except in
the polar (Arctic and Antarctic) regions and the con-
tinental countries bordering thereupon. In fact, the range
Fahrenheit. Ill
F. to 100 F. is a very serviceable range for the climates
of the temperate zone. But the investigation of the upper
air has necessitated the frequent use of temperatures on
the negative side of the Fahrenheit zero, temperatures as
much as 100 below zero occur, and to have the zero in the
middle of the working scale is very inconvenient. In the
physical laboratory too, temperatures approaching 5()0F.-
below the freezing point are realised in experiments on
the condensation of hydrogen and helium.
Probably the best scale for all purposes would be in
Fahrenheit degrees measured from 459 below the
Fahrenheit zero which is computed to be within half a
degree of the zero of absolute temperature. In that case
500 would correspond with 41 F. or 5 C. But the grow-
ing prevalence of the Centigrade or Celsius scale in
countries which do not speak English has led to the use
of temperatures measured in the centesimal degrees from
the zero of absolute temperature computed as 273 below
the freezing point of water.
For a table of conversion of the various scales in use
see page 355.
Fall. "The fall of the leaf" in common use with
American writers for Autumn.
Fluid. A substance which flows, to be distinguished
from a solid which will not flow. Some fluids are very
viscous, like pitch or treacle, and take a long time to flow,
others are mobile, like water or petrol, and take very little
time to flow until the surface becomes level. Gases are
included in the general term fluid, because they also will
flow through a pipe. Their peculiarity is that they can be
not only compressed by pressure but also expanded indefi-
nitely on the release of pressure. The density, i.e., the amount
112 Glossary .
that can be got into a limited space, is, in fact, almost
exactly proportional to the pressure.
So when we find a, large mass of gaseous fluid like the
atmosphere lying upon the earth's surface, it is dense in
the surface layer which has to carry the weight of all
there is above it ; and as the pressure gets less and less,
upward, the density gets less and less until space is
reached. We do not know what happens where the
atmosphere merges into space, but we are sure that the
earth, with the aid of gravity, carries its atmosphere
along without losing any appreciable amount into the
void.
Fog*. Obscurity of the atmosphere which impedes
navigation or locomotion. It may be due to a cloud of
water particles at the surface, as sea-fogs and valley-fogs
generally are, but an effective fog can be produced by
clouds of dust ; that is often the case off the West coast of
Africa during the season of the HARMATTAN. In towns
true water-fogs are generally rendered more opaque by
loading with smoke, and in some cases in towns obscurity
of the atmosphere that hardly amounts to fog may
be due to the condensation produced by the gaseous
products of combustion under the action of sunlight.
Sea-fog is apparently due most frequently to the passage
of air over sea water colder than itself ; there is first the
cooling of the air by the contact with the cold water, and
then the mixing up of the air near the surface by the eddy
motion resulting in the cooling of a considerable thickness
below the dewpoint. (See G. I. Taylor, Scientific Results
of the Voyage of the " Scotia" 1913.) Sea-fog is most
prevalent in spring and summer when the air is warming
rapidly. It does not often occur in the winter. See p. 121.
Fog. 113
Land-fog, on the other hand, is an autumn or winter
fog ; it is generally due to cold air passing over relatively
warm, moist ground. The process of cooling may be
either by radiation or by a change of wind, but again
eddy motion is necessary to mix the warm, moist air close
to the ground with the cold flood. Fogs of this kind are
not infrequent in the early mornings of summer, but they
persist sometimes through the day in autumn and winter.
Autumn is their special season.
Anticyclonic weather with light airs is very favourable
for land-fog, and the ending of a period of anticyclonic
weather is nearly always fog. Fog on our coasts is
generally included in the forecasts for the British Isles
when the wind changes from a Northerly or Easterly to a
Southerly point. For the monthly percentage-frequency
of fog and mist in the English Channel see p. 121.
The conditions for fog in London are set out in a Report
3f the Meteorological Office on fogs. (M.O. publication
No. 160.)
The frequency of fog at the observing hours, according
fco the returns for the past 20 years from British Stations
for the Daily Weather Report, is shewn in the following
table. It should be noted that up to the end of June,
1908, the morning and mid-day hours of observation were
<Sh. and 14h. instead of 7h. and 13h., and that at Oxford
the observing hours have been, and are still, 8h. and 20h.,
not 7h. and 18h. The following are the yearly frequencies
of observations of fog at Ih. (or 3h.) in the past two years:
Wick ...
10
Donaghadee
18
Spurn Head
10
Stornoway
Holyhead
13
Yarmouth
8
Malin Head
10
Pembroke
18
Eskdalemuir
3
Blacksod Point
9
Portland Bill
5
Benson ...
2
Valencia...
3
Dimgeness
15
London ...
3
Scilly ...
20
Tynemouth
i
114
Glossary.
AVERAGE NUMBEK OF OBSERVATIONS OF FOG IN A
iNur
ofY
of O
vatic
7h.
aber
ears
bser-
ns at
13h.
Station.
7h.
13h.
Sum-
mer.
Win-
ter.
Year.
Sum-
mer.
Win-
ter.
Year.
20
20
8
20
20
20
2O
20
20
2O
12
20
20
20
20
2O
20
2O
2O
20
20
20
1
20
17
2Q
12
2O
20
20
8
5
20
20
20
20
20
7
20
20
20
20
13
2O
17
20
20
20
20
20
13
4
43
3
i
&
Sum burgh Head...
Stornoway
Castle bay ,
Wick
Nairn
Aberdeen
Leith
IO
2
9
II
3
4
2
2
I
2
2
I
7
12
3
ii
12
5
5
9
o
3
8
2
I
I
I
2
4
IO
2
5
o
4
9
4
i
In "S
H 6
N. Shields
Spurn Head
Great Yarmouth...
Clacton-on-Sea ...
8
7
5
i
8
12
25
6
16
19
30
7
2
3
I
6
8
ii
2
s
03
0)
Malin Head
f Belmullet \
\ Blacksod Point J
Valencia ...
Roche's Point
Donaghadee
Liverpool (Bidston
Observatory).
Holyhead
Pembroke
8
3
2
8
7
i
16
ii
3
2
2
5
3
6
9
7
ii
5
4
13
10
7
25
18
4
i
o
3
2
9
6
I
I
1
{
5
2
5
4
13
9
6
3
09
Scilly (6t. Mary's)
Jersey (St. Aubin's)
j Hurst Castle \
| Portland Bill J
Dungeness
Dover
'!
7
3
6
5
5
10
7
20 7
10 , I
10 4
17 2
10
s
2
3
ii
3
6
7
6
2
1
3
London ... ...13 19
Oxford ... ... 4 24
/ Loughborough \
\ Nottingham J
Bath i 5
Birr Castle ... 4 i 6
22
28
25
6
10
o
o
7
6
2
NOTE. Summer (April to September) contains 183 days, and
Fog.
YEAR AT VARIOUS STATIONS IN THE BRITISH ISLES.
Sum-
mer.
18h.
Sum-
mer.
21h.
Win-
ter.
Station.
Number
of Years
of Obser-
' vations at
?:(*
Year.
18h.
2-lh.
6
I
7
7
^
o
i
o .
i
2
7 9 i 10
1000
9 30 3
8628
Sumburgh Head...
Stornoway
Castlebay
Wick
Nairn
Aberdeen
Leith
s
I
20
20
8
20
20
20
20
20
20
?
20
20
20
20
20
20
20
20
20
20
20
20
8 '
20
17
20
12
20
7
7
7
5
~
2
7
7
7
7
7
7
7
*8
7
7
5
7
J
i
4
7
9
2
7
10
10
2
4
I
6
6
8
i
10
10
?
N. Shields
Spurn Head
Great Yarmouth...
Clacton-on-Sea ...
ll
pq jo
6
2
I
4
2
O
9
6
3
2
o
3
2
I
7
5
9
4
i
7
4
i
16
ii
i
4
3
8
7
;
o
4
5
6
7
12
12
Malin Head
/ Belmullet \
( Blacksod Point /
Valencia
Roche's Point ...
Donaghadee
Liverpool (Bidston
Observatory).
Holy head
Pembroke
i
1
9 6
; 4
4 ^
3
2
15
7
6
5
2
10
2
j
5
i
'2
15
3
5
Scilly (St. Mary's)
Jersey (St. Aubin's
f Hurst Castle )
\ Portland Bill J
Dungeness
Dover
,p
1
o
cc
O
O
o
6
4
6
2
I
6
4
6
2
I
I
O
8
5
9
5
London ...
Oxford
/ Loughborough \
\ Nottingham /
Bath
Birr Castle
Inland.
Winter (October to March) 1*2 days, in leap year 183 davs.
116 Glossary.
Fog Bow. A white rainbow of about 40 radius seen
opposite the sun in fog. Its outer margin has a reddish,
and its inner a bluish tinge, but the middle of the band
is quite white. The bow is produced in the same way as
the ordinary rainbow but owing to the smallness of the
drops, under O025 mm., the colours are mixed and the
bow is nearly white.
Fohn. The name given to certain dry, warm, relaxing
winds of the valleys on the Northern side of the Alps.
The general direction of the Fohn winds is from the
South. The peculiar character of the air is accounted for
by supposing that it comes from over the plains on the
Southern side of the ridge. In its elevation it becomes
dynamically cooled, and if condensation occurs and rain
or snow is formed in it, the fall of temperature is so much
restricted on account of the latent heat of the vapour which
is condensed and left behind, that the air which forces
its way down into the valleys on the north side, being
dynamically warmed and dried, appears as a warm, dry
wind. Some of the details of the process are still obscure
because warm air does not naturally flow downhill, but
th'e main outline of the process is certainly established
and the subject has been studied in detail by Austrian
meteorologists.
The Chinook wind of the Western prairies of America
which comes down from the Rocky Mountains as a warm,
dry wind evaporating a good deal of the prairie snow in
winter is of similar character, and various other examples
of what is known in meteorology as the Fohn effect occur
from time to time on many hill sides. In regions like
Norway, Greenland and the Antarctic Continent it com-
plicates the temperature measurements very seriously.
Forecast. 117
Forecast. The name given by Admiral R. FitzRoy to
a statement of the weather to be anticipated in the near
future from a study of a synoptic chart or " weather
map." In the Meteorological Office the period of antici-
pation of a forecast does not exceed twenty-four hours,
but when conditions shown on the map are favourable a
more general statement of the probable weather for two
or three days is given in a form which is called " the
further outlook."
In practice a forecast includes
(1) A statement of the direction and force of the
surface-wind and the changes therein which are
expected within the period of the forecast.
(2) A statement of the state of the sky (as regards
clouds), precipitation (rain, hail, snow or sleet) and
temperature, whether it will be high or low for the
time of year, or higher or lower than at the time of
making the forecast.
(3) A note as to the probability of such occurrences
as night-frost, fog, or thunder.
For these statements the forecaster depends upon the
changes in the distribution of pressure which are indicated
on the map, although these changes are not described in
the forecast. They are. however, set out in a preliminary
statement called the " general inference," and for the
information of airmen the anticipated changes in the
pressure gradient over the several districts are formulated
and are expressed as an addition to the forecast giving
the " wind at 1,500 feet." That height is chosen because
the wind at that level is generally in close agreement
with that computed from the distribution of pressure at
the surface,
118 Glossary.
The direction and velocity quoted for 1,500 feet are
sufficiently applicable for heights up to 3,000 or 4,000 feet,
as changes in the wind above the level of 1,500 feet are
generally gradual. A Southerly, South-westerly, Westerly
or North-westerly wind generally gets gradually stronger
at higher levels, but an Easterly wind often falls lighter
and is replaced in the highest levels by a wind from a
Westerly quarter, though that is not always the case. The
motion of clouds, or the measurement of air-currents by
observations of a pilot balloon, are the only means available
at present for guidance as to the changes in the higher
level.
Freezing 1 . With reference to weather this word is
used when the temperature of the air is below the freez-
ing point of water 32 F., 273a., C.
American writers use the term " a freeze " where
we are accustomed to use " frost " to indicate freezing
conditions persistent for a sufficient time to characterise
the weather.
Frequency : The number of times that a particular
phenomenon of weather has happened in the course of a
given period of time, generally a number of years.
Here, for example, is a summary of the spells of wind
from the Easterly quarter, according to the direction of
the isobars, over S.E. England and Northern France in
nine years. Taking January, for instance, the nine years
supply a total of 279 days, of which 58 were days of East
wind. These consisted of one sequence of eight consecutive
days of East wind, one sequence of six days, three
sequences of four, two sequences of three and seven
sequences of two days, with finally twelve isolated days
of East wind.
frequency.
119
J
i
ti
:s~
^5
o 1 "7 I
^ >
r* c^
^
E.-S
!^SE
^s
J " :
r^J 'rtf.
>< ^
^ r
O W
M 00 COO M
O CO IO
o s -is. ro M
O IT) O CO M rf M
K H
6 co ro Th co
O l>x CO LO M
O ^ |>> M CO M H
s O M C1 CO
g
&
R
01
pj ^2
-P o
O O C
120
Glossary.
In this case the number of years over which the obser-
vations extend is given, quite an arbitrary number, and
thus the numbers for frequency of occurrence have to be
considered with reference to the number of years selected.
It is, however, usual to reduce frequency figures to a
yearly average.
Here, for example, is the average frequency of
GEOSTROPHIC winds from different quarters over the
South- East of England and Northern France obtained
from observations for the nine years, 1904-1912.
Frequency of winds (geostrophic) from different quarters.
Average Number of Days in the several months of
the year in which the Wind is from a specified quarter.
South-East of England and Northern France.
N.E
Fi
SE.
s
s.w.
w
N.W.
N
Calms.
Total.
January ...
3
2
2
2
9
6
2
I
4
31
February ..
3
I
I
4
7
6
3
2
I
28
March
4
2
I
3
7
5
2
2
5
31
April
5
2
I
2
6
5
2
4
3
30
May
4
2
2
2
5
4
2
2
6
29
June
5
I
I
3
6
4
3
4
3
30
July
4
I
I
i
6
5
3
4
6
3 1
August ...
2
I
I
2
10
6
3
2
4
3 1
September
4
3
2
2
5
4
3
4
o
J
3
October ...
3
3
2
5
7
3
3
2
4
^2
November
3
2
I
3
7
7
2
1
4
30
December..
i
2
2
7
10
4
2
I
2
31
The treatment of fractional parts of a day accounts for
the two discordant totals.
121
In view of the awkwardness of having to bear in mind
the possible number of occurrences, while considering the
actual or average number, it is convenient to use the
percentage frequency instead of the actual frequency.
This plan is often adopted for giving the results of
observations at sea, which are made six times a day, or
every four.hours.
For example," the percentage frequency of fog in the
English Channel is given by the figures in the following,
table :
Percentage Frequency of Fog and Mist in the English
Channel.
[Based on four-hourly observations from ships during the
15 years 1891-1905.]
Percentage of
Percentage of
Month*
Number of
observations.
whole number
of observations.
whole number
of observations.
Fog.
Mist.
January
1,187
2'5
15-8
February . . .
1,185
2'8
22*9
March
1,241
3'8
2 4*5
April
M 2 4
4-8
24-4
May
1,501
4*4
26*9
June
1,363
5'6
30-2
July
1,260
3*8
26*3
August
1,206
3' 2
I7-3
September ...
1,264
r,3
I7'0
October
i,454
i-6
16-5
November ... ' 1,284
I *I
15-0
December ...
1,294
i*5
18-2
122 Qlossanj.
Friction. A word used somewhat vaguely in meteor-
ological writings in dealing with the effect of the surface
of the sea or of the land, with its obstacles in the form of
irregularity of surface, hills, buildings, or trees upon the
flow of air in the lower layers of the atmosphere. The
effect of the irregularities of surface is to produce turbu-
lent motion in the lowest layer which gradually spreads
upwards, if the wind goes on blowing, and consists of
irregular eddies approaching to regularity in the case of
a cliff eddy which can be noticed when a strong wind
blows directly on to a cliff and produces an eddy with a
horizontal axis. An account of the eddy caused by the
Eastern face of the rock of Gibraltar is gi^en in the
Journal of the Aeronautical Society, Vol. 18, 1914, p. 184.
The general effect of this so-called friction is to reduce
the flow of air past an anemometer so that the recorded
wind velocity is below that which would be experienced
if the anemometer were high enough to be out of the
reach of the surface effect. Numerical values for this
effect are of great practical importance, because they are
concerned with the change of velocity in the immediate
neighbourhood of the ground. But it is not easy to
obtain them, because every exposure near land or sea is
more or less affected, and, therefore, no proper standard
of reference can be obtained by direct observation.
Recourse is, therefore, had to the computation of the
wind from the distribution of pressure, the so-called
" geostrophic " or GRADIENT WIND.
From the comparison of a long series of geostrophic
and observed winds we conclude that over the open sea,
or on an exposed spit of flat sand like Spurn Head, the
wind loses one-third of its velocity from "friction," and at
other well-exposed stations the loss is, on the average, as
Friction. 123
much as 60 per cent., but for any particular anemometer
it is different for winds from different quarters because
the exposure seaward or landward is different. Infor-
mation on this point for a number of Meteorological
Office stations is given in a memoir by Mr. J. Fairgrieve
(Geophysical Memoirs, Vol. 1, p. 189), and information
for other stations is in process of compilation at the
Meteorological Office.
The consequence of this 4 effect can sometimes be seen in
weather maps. On one occasion when the whole of the
British Isles was covered with parallel isobars running
nearly West and East, all the stations on the Western
side gave the wind as force 8 (42 mi/hr) while those on
the Eastern side gave force 5 (21 mi/hr), so that the
velocity was reduced by one-half in consequence of the
"friction" of the land. If the velocity at the exposed
Western stations be taken at two-thirds the velocity of
the wind free from friction, we get the following interest-
ing result which is probably correct enough for practical
use : One-third of the velocity is lost by the sea friction
on the Western side, and one-third more by. the land
friction of the country between West and East.
Frost. According to British meteorological practice
frost occurs when the temperature of the air is below
the freezing point of water (see FREEZING) ; it may be
either local, as a ground-frost, a spring-frost or a night-
frost often is, or general, such as a frost which gives
bearing ice in the course of three or four days. But the
word would hardly be used unless there were water or
plants or something else to be frozen, so that its use is
generally restricted to the lowest levels of the atmosphere.
We should hardly speak of a frost in relation to the cold
of the upper air, or even of a mountain top.
124 Glossary.
Meteorologically, the difference between the conditions
for a general frost and those for a local frost is so great
that different words are needed. The American meteor-
ologists have some reason, therefore, in speaking of a
general meteorological frost as " a freeze."
The British Isles are accessible for a freeze or general
frost in two ways, first by Northerly winds bringing cold
air from the Arctic regions over the North Atlantic,
round a high pressure lying over the Greenland-Iceland
region, secondly by Easterly or South-Easterly winds
coming round a high pressure over Scandinavia and
Northern Europe, which in the winter is persistently
cold. The Northerly wind has to cross a considerable
stretch of the north-eastern extension of the Gulf -stream
water, so that it has to travel quickly to avoid being
warmed. Consequently, Northerly " freezes " are generally
short and sharp. The more prolonged frosts are generally
caused by the Easterly winds which have only a short
stretch of sea to cross. A long freeze may begin with a
Northerly wind, and snow, followed by a persistent
Easterly wind.
The short frosts, or night frosts, may occur with very
light winds from any quarter except between South and
West ; they are characteristic of clear nights, with great
loss of heat from the ground by radiation to the clear
sky. The conditions are set out in detail in a pamphlet
prepared in the Meteorological Office and reprinted in
" Forecasting Weather" Chapter XII.
Low temperatures are often quoted as degrees of frost,
meaning thereby the nu&iber of degrees below the
freezing point of water.
Gale. Wind with an hourly velocity of more than
17m/s, or 39mi/hr. The figure is selected as the lower
Gale. 125
limit of force 8 on the Beaufort Scale. A wind estimated
as force 8 or more is counted a gale.
The relation of the estimation to the measured hourly
velocity is subject to some uncertainty on account of the
incessant fluctuations of velocity in a strong wind which
are known as GUSTINESS. They are not shown in the
records of the cup anemometer which were used for com-
puting the equivalents.
The number of gales recorded for any locality depends
largely on the exposure of the anemometer, as the table on
the next page shows.
Judging by this table, anyone who is unacquainted
with the practical difficulties of anemometry would be
tempted to draw the conclusion that the localities
represented by Kew, Falmouth, Aberdeen, Valencia and
Yarmouth are immune from gales, or nearly so.
For any purpose of aerial navigation such a conclusion
would be egregiously untrue. It is true of the anemo-
meters, but not of the free air above them. The records,
which in these particular cases go back to 1868, are good
enough when we are concerned only with comparing the
wind of to-day with that of yesterday, or any other day
in the last forty-seven years, and of determining normals
for reference, the diurnal and seasonal variation, and so
on ; but when we want to compare one locality with
another we must face the problem of making allowance
for the exposure of the anemometer.
To meet this requirement we propose to give the basic
characteristics of the localities under our observation as
regards wind in terms of GRADIENT WIND, or more
strictly geostrophic wind. It is a very voluminous
inquiry, but is now nearly completed, and some of the
results will be included in this volume.
126
Glossary.
,3
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G
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c
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Fleetwood
Holyhead
Scilly
Falmouth
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Deerness
Aberdeen
Yarmouth
CD
M
Gale.
127
The seasonal variation may be expressed as follows :
O
fl
OD
0nMoooc*0Mp>oovoo>-
1
42
real
C^
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-o^o^^^o^cr^
h
rH
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& JS 2sl
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The " favourit
21 to 19 against.
128
For special localities tables of local statistics must be
consulted. Some guidance may be obtained from the
diagrams given under the heading wind.
Gale- Warning*. Notice of threatening atmospherical
disturbances on or near the coasts of the British Islands
are issued by telegraph from the Meteorological Office to
a number of ports and fishery -stations. The issue of a
warning indicates that an atmospheric disturbance is in
existence which will probably cause a GALE (Force 8 by
BEAUFORT SCALE) within a distance of (say) 50 miles of
the place to which the warning is sent. The place itself
may be comparatively sheltered, and the wind may not
attain the force of a gale there. The meaning of the
warning is simply " Look out. Bad weather of such and
'mch a character is probably approaching you."
The fact that such a notice has been received is made
known* by hoisting in a conspicuous position a black
canvas cone (gale-cone) 3 feet high and 3 feet wide at
the base, which has the appearance of a triangle when
hoisted.
The " South cone " (point downwards) is hoisted in
anticipation of gales and strong winds
From S.E. veering to S.W., W., or N.W.
S.W. W. or N.W.
W N W
vv . ,, J.-N . vv .
The " North cone " (point upwards) is hoisted in antici-
pation of gales and strong winds
From S.E., E. or N.E., backing to N.
N.W. veering to N., N.E., or E.
N. N.E. or E.
. N.E. E.
* The display of cones and issue of notices to the general public
has been suspended during the war.
Gas. 129
The warning is intended to continue from the time the
telegram leaves the Meteorological Office until 8 p.m. the
following day.
The gale-warning service of the British Isles was
established under the direction of the late Admiral
FitzRoy in 1861, and has been maintained in operation
ever since, with a slight interruption in 1867.
Gas. The name used for any kind of fluid which has
unlimited capacity for expansion under diminishing
pressure. It is to be distinguished from a liquid which
has only a limited capacity for expansion under reduced
pressure.
A liquid may occupy only the lower part of a vessel
like a bottle ; it will flow to the bottom of the vessel and
leave a "free" surface. But a gas cannot be located in
that way ; its volume is determined not by the amount
of material but by the size of the vessel which contains it
and by the pressure upon its boundaries.
There are many different kinds of gas, such as nitrogen,
hydrogen, carbonic acid, coal-gas, marsh-gas, and so on ;
but the word is often used when coal-gas is meant, and
recently it has been used for heavy poisonous gas of
unspecified composition. In scientific practice gas means
any substance which obeys approximately the gaseous
laws ; these laws are two, viz. :
1. When the temperature is kept constant the pressure of a given
mass of gas is inversely proportional to the volume which it occupies,
or the density is directly proportional to the pressure.
2. When the volume is kept constant the pressure is proportional to
the absolute temperature, or when the pressure is kept constant the.
volume is proportional to the absolute temperature.
GeostropMc.- See GRADIENT WIND.
13204 E
130 Glossary.
Glazed Frost. When rain falls with the air-tempera-
ture below the freezing point a layer of smooth ice, which
may attain considerable thickness, is formed upon all
objects exposed to it. This is known as glazed frost.
The accumulation of ice is frequently sufficient to bring
down telegraph wires. In these islands the phenomenon
is one of comparative rarity.
It must be distinguished from SILVER THAW, which
occurs when a warm, damp wind supervenes upon severe
cold, the moisture condensing on still-freezing surfaces
and thus producing a coat of ice, similar in appearance to
glazed frost. Super-cooled water-drops are said to be the
cause of glazed frost.
Glory. The system of coloured rings surrounding the
shadow of the observer's head on a bank of cloud or fog
or even of dew. It is a diffraction-effect due to the
bending of rays of light round small obstacles, water-
drops in this case. As in all diffraction effects the violet
ring is nearest the centre, followed outwards by blue,
green, orange, and red on the outside; the blue and
violet are seldom seen. A Glory may be seen surrounding
the shadow of an aeroplane on a cloud.
Gradient. A convenient word rather overworked in
modern meteorology. We use it in pressure gradient,
temperature gradient, potential gradient, to denote
different ideas. In pressure gradient for any locality we
imagine the distribution of sea-level pressure to be
mapped out by isobars ; take a line through the locality
at right angles to the isobars nearest to it on either side
and measure the step of barometric pressure which
corresponds with a measured distance along the line
from high pressure to low. This use of gradient was
Gradient. 131
introduced by Thomas Stevenson, C.E., of the Board of
Northern Lights. It corresponds with an engineer's use
of the word gradient in specifying a slope from a map of
contours, but to get the pressure-gradient we have first to
determine the line along which the slope is steepest, so
that pressure-gradient has a definite direction. There is
a convention that the distance to be taken is 15 nautical
miles and the step of pressure is to be given in hundredths
of an inch. The gradient will work out at practically
the same figure if the distance is a geographical degree
and the step of pressure is given in millimetres. To get
the same figure for the gradient with the step of pressure
in millibars the distance would have to be taken as
45 nautical miles. But numerical values of the gradient
are very seldom quoted.
Temperature gradient may be based on the same idea
and give the rate of change of temperature, along the
horizontal through a locality, at right angles to the
isotherms, as obtained from a chart of isotherms properly
corrected for height. But it is much more frequently
used to indicate the step of temperature for a kilometre
step of vertical height. Used in this sense temperature
gradient may be positive or negative, and by international
agreement the temperature gradient is positive when the
step is towards lower temperature for increasing height,
because temperature generally decreases aloft ; but it
does not always do so. The change from positive to
negative temperature gradient is called an INVERSION of
temperature gradient or simply an u inversion ", and so
an " inversion " comes to mean a region where tempera-
ture increases with height.
Potential gradient is used for the change of atmospheric
13204 E 2
132 Glossary.
electrical potential in the vertical, and for that alone. It
is generally given in volts per metre. That also may be
positive or negative and is taken to be positive when the
potential increases with height.
The word LAPSE (q.v.) has been adopted as a better
name than gradient for these rates of change in the
vertical.
The pressure gradient is the one which comes most
frequently into practical consideration, as it is closely
related to the direction and force of the wind, so that the
idea of pressure gradient should always be present in the
mind of a student of weather maps, though the gradient
may seldom be evaluated in figures. On looking over a
map the localities where the gradient is steep will always
be noticeable by the closeness of the isobars. The deter-
mination of the pressure gradient is comparatively easy
when the isobars in the locality are free from local
irregularity and nearly parallel. There is then no
difficulty in identifying the direction of the gradient,
because the line drawn at right angles to successive
isobars is approximately straight for a sufficient distance
on the map. Experience is required to make a workable
estimate of the gradient when the isobars are irregular.
In practice tho gradient is not taken by setting out a
length of 15 or 60 or 45 nautical miles, but by scaling the
distance apart of consecutive isobars. It is most con-
venient to express this distance in nautical miles, because
60 nautical miles make up a degree of latitude, and every
map made for meteorological purposes is scaled according
to latitude. If, for example, isobars are drawn for steps
of JSQ' millibars, and the shortest line drawn to bridge two
Gradient.
133
isobars across a station scales out at 75 nautical miles ;
the gradient is 1 millibar (0*03 m.) for 15 nautical miles,
or 3 on the conventional scale of pressure gradients. For
calculation in C.G.S. units it is convenient to have the
gradient expressed in terms of millibars for 100 kilo-
metres.
It is best to use a large scale map for obtaining pressure
gradients so that intermediate isobars can be inserted by
estimation when those drawn for the ordinary steps are
not regular, but with the best maps the estimation of the
gradient is sometimes uncertain on account of local
irregularities of pressure which may be indicated on la
barogram but cannot be allowed for in a map based on
telegraphic reports from stations 100 miles (160 kilometres)
apart.
Some of the steepest authentic gradients that have been
noted on British weather maps are :
Date and
Place.
Gradient.
Inter-
Millibars
national
meaeure.
per 100
kilometre.
1912, August 26, East Anglia
(Norwich floods).
1907, February 20, between Ice-
land and Faroe.
II'O
I0'0
13*2
12 'O
1912, November 26
land.
, West of Scot-
9*7
II'7
134 Glossary.
Gradient Wind. The flow of air which is necessary
to balance the pressure-gradient. The direction of the
gradient wind is along the isobars, and the velocity is so
adjusted that there is equilibrium between the force
pressing the air inwards, towards the low pressure, and
the centrifugal action to which the moving air is subject
in consequence of its motion.
In the case of the atmosphere the centrifugal action
may be due to two separate causes ; the first is the
tendency of moving air to deviate from a GREAT CIRCLE
in consequence of the rotation of the earth ; the deviation
is towards the right of the air as it moves in the Northern
hemisphere, and towards the left in the Southern. The
second is the centrifugal force of rotation in a circle
round a central point according to the well known
formula for any spinning body. In this case we regard
the air as spinning round an axis through the centre of
its path. This part of the centrifugal action is due to
the curvature of the path on the earth's surface. Both
components of the centrifugal action are in the line of
the pressure gradient : the part due to the rotation of
the earth is always tending to the right in the Northern
hemisphere, the part due to the curvature of the path
goes against the gradient from low to high when the
curvature is cyclonic, and with the gradient when it is
ancicyclonic, so that in the one case we have the gradient
balancing the sum of the components due to the earth's
rotation and the spin, and in the other case the gradient
and the spin-component balance the action due to the
earth's rotation.
The formal reasoning which leads up to this result is
gi /en at the end of this article. The method used therein
Gradient Wind. 135
for calculating the effect of the rotation of the earth was
suggested to the writer in 1904 by Sir John Eliot, F.R.S.,
Director of the Indian Meteorological Service.
For the sake of brevity in reference to these two
components it is very convenient to have separate names
for them. . Let us call the one due to the rotation of the
earth the geostrophic component,* and the one due to the
curvature of the path the cyclostrophic component.
Consider the relative magnitude of these components
under different conditions. It will be noticed that the
geostrophic component depends upon latitude, the cyclo-
strophic component does not, so, other things being equal,
their relative importance will depend upon the latitude ;
so we will take three cases, one near the equator at
latitude 10 within the equatorial belt of low pressure,
one near the pole latitude 80 of undetermined mete-
orological character, and one, half - way, between, in
latitude 45, a region of highs and lows travelling
Eastward.
Using V to denote the wind-velocity, when the radius
of the path is 120 nautical miles the cyclostrophic com-
ponent is equal to the geostrophic
in latitude 10 when V is 5*6 metres per second ;
in latitude 45 when Y is 22*9 metres per second ;
in latitude 80 when V is 31-9 metres per second.
It will be seen that in the equatorial region the cyclo-
strophic component is dominant as soon as the wind
reaches a very moderate velocity.
* A table to find the geostrophic component is given on pp. 172, 173.
136 Glossary.
EQUATION FOR GTEOSTEOPHIC WIND.
2 he Relation between the Earttts Rotation and the Pressure Distribution
for Great- Circle-Motion of Air.
The rotation co of the earth about the polar axis can be resolved
into co sin about the vertical at the place where latitude is and
co cos <j> about a line through the earth's centre parallel to the tan-
gent line.
The latter produces no effect in deviating an air current any more
than the polar rotation does on a current at the equator.
The former corresponds with the rotation of the earth's surface
counterclockwise in the Northern Hemisphere and clockwise in the
Southern Hemisphere under the moving air with an angular velocity
co sin 0. We therefore regard the surface over which the wind is
moving as a flat disc rotating with an angular velocity co sin 0.
By the end of an interval t the air will have travelled Vt y where
V is the " wind-velocity," and the earth underneath its new position
will be at a distance Vt X co t sin 0, measured along a small circle,
from its position at the beginning of the time t.
Taking it to be at right angles to the path, in the limit when t is
small, the distance the air will appear to have become displaced to the
right over the earth is Fco -sin 0.
This displacement on the "igt 2 " law (since initially there was no
transverse velocity) is what would be produced by a transverse
acceleration
2 co Fsin 0.
. . the effect of the earth's rotation is equivalent to an acceleration
2 co Fsin 9, at right angles to the path directed to the right in the
Northern Hemisphere, and to the left in the Southern Hemisphere.
In order to keep the air on the great circle, a force corresponding
with an equal but oppositely directed acceleration is necessary. This
force is supplied by the pressure distribution.
Gradient Wind.
137
EQUATION FOB CYCLOSTROPHIC WIND.
Force necessary to balance the acceleration of air moving uniformly in a
small circle, assuming the earth is not rotating.
Let A be the pole of circle PRQ. Join PQ, cutting the radius OA in N.
Acceleration of particle moving uniformly along the small circle with
V' 2 V 2
velocity Fis -^ along PN = =-. - where R = radius of earth ;
Jri\ jj, sin p
and o is the angular radius of the small circle representing the
path.
The horizontal component of this acceleration, that is, the component
F 2 cosp F 2
along the tangent at P, is p . = -- cot p.
GENERAL EQUATION CONNECTING PRESSURE GRADIENT, EARTH'S
ROTATION, CURVATURE OF PATH OF AIR AND WIND VELOCITY.
I. Cyclonic motion. The force required to keep the air moving on a
great circle in spite of the rotation of the earth must be such as to
give an acceleration 2w V sin directed over the path to the left in the
Northern Hemisphere. It must also compensate an acceleration due to
the curvature of the path, F- cot p JR, by a force directed towards the
low pressure side of the isobar.
For steady motion these two combined are equivalent to the
acceleration due to the gradient of pressure, i.e., *- where I) is the
138
Glossary.
density of the air, and y the pressure gradient, directed towards the
low pressure side.
y F 2
.*. = 2 (j Fsin + cot p.
1
II. Anticyclomc motion. In this case 2 w F sin and ^ are directed
outwards from the region of high pressure, and the equation becomes
y F 2
-/ = 2 u> Fsin cot p.
D
R
Gramme. The unit of mass on the C.G.S. system.
It is one- thousandth part of the standard kilogramme,
Gramme. 139
which was originally constructed to represent the weight
of a litre (cubic decimetre) of water.
A gramme is equivalent to 15*4 grains, or rather more
than one-thirtieth of an ounce.
A pound is equivalent to 454 grammes. For GRAMME-
CALORIE see p. 301.
Grass-temperature. For estimating the effect of
RADIATION from the Earth's surface at night a minimum
thermometer is exposed just above the surface of short
grass, so that the bulb does not actually touch the grass.
Abroad the thermometer is sometimes laid on the grass
itself.
Gravity. See p. 308.
Great Circle. A line on the earth's surface which
lies in a plane through the centre of* the earth's figure.
All meridian lines are great circles so is also the equator,
but all lines of latitude, with the exception of the equator,
are small circles since their planes do not pass through
the earth's centre. The visible horizon is a small circle.
The great circle which passes through two points on
the earth's surface is made up of the shortest and the
longest track between the two points. The shortest track
is less than a semicircle, the longest greater than a
semicircle.
Gulf Stream. A warm ocean current that flows out
of the Gulf of Mexico along the coast of Florida. It is
ascribed to ttie action of the TRADE WINDS which cause
a mass of water to flow into the Gulf from the East. The
current near Florida is strengthened by water which
branches from the main trade wind current and flows
outside the Antilles. The Gulf Stream flows Northward
into the region of prevailing Westerly winds, which
140 Glossary.
cause a current to flow slowly Eastward across the
Atlantic ; this current, also called the Gulf Stream,
carries water from the Gulf Stream proper to the coasts
of Europe. The air no doubt has its temperature slightly
raised by the warm current, but our temperate climate is
due to prevailing Westerly and South Westerly winds,
which are also the cause of the Eastward extension of the
Gulf Stream.
Gust. A " coup de vent." The word was used
originally for any transient blast of wind, but is now
limited to the comparatively rapid fluctuations in the
strength of the wind which are specially characteristic of
winds near the surface of the earth, and are probably
due to the turbulent or eddy motion arising from the
FRICTION offered^ by the ground to the flow of the
current of air.
The subject of gusts, as indicated by a tube-anemo-
graph, has been investigated for the Advisory Com-
mittee for Aeronautics by the Meteorological Office,
and the results are contained in four reports on Wind
Structure published in the annual reports of the Com-
mittee. The number and extent of the fluctuations are
very irregular ; they have been counted as seventeen in
the minute, but another count would probably give a
different figure. If the wind be regarded as fluctuating
between a gust and a lull, the range between gusts and
lulls is dependent on the one hand on the mean velocity
of the wind, and on the other hand upon the nature of
the exposure of the anemometer. Expressing the fluc-
tuations as a percentage of the mean velocity we get the
following results for various anemometers. (Report of
the Advisory Committee, 1910.)
Gust.
141
Anemometer.
Range of Fluctuation
as a Percentage of the
Mean Velocity.
Southport (Marshside)
Scilly (St. Mary's)
Shoeburyness, E.N.E. wind
W. wind
Holy head (Salt Island)
Falmouth (Pendennis), S. wind ...
W. wind...
Aberdeen
30 per cent.
50
30
80
50
25
50
IOO
Alnwick
80
Kew
IOO
In this table a fluctuation of 100 por cent, means that a
wind with a mean velocity of 30 miles per hour fluctuates
over a range of 30 miles per hour, between 15 miles an
hour and 45 miles an hour, in consequence of the
gustiness.
The most gusty exposure within the experience of the
Meteorological Office is at Dyce, in Aberdeenshire, where,
for the purpose of inquiry, an anemometer was installed
by Dr. J. E. Crombie, with its head projecting 15 feet
above the tree-tops of a small wood.
Gusts are to be distinguished from squalls. A squall
is a blast of wind occurring suddenly, lasting for some
minutes at least, and dying away as suddenly. A squall
is attributable to meteorological causes, whereas gusts are
the result of mechanical interference with the steady flow
of air.
m/s.
1905.
Pendennis,
46-0
1906.
Scilly,
38-4
1907.
Southport,
36-2
1908.
Scilly,
37'6
1909.
Scilly,
40*2
1910.
Pendennis,
38-9
1911.
Eskdalemuir,
40*2
1912.
Pendennis,
43'8
IQI3-
Southport,
38-4
1914-
Quilty,
4I*O
^S-
Pendennis,
4O'O
1916.
r\
Pendennis,
40-8
142 Glossary.
The strongest gusts recorded on anemometers of the
Meteorological Office in recent years are :
mi/hr.
103
86
81
84
90
87
90
98
86
92
89
91
Gustiness. The name given to the factor which is
used to define the range of the gusts shown on the record
of an anemometer. The gustdness of an interval is the
factor, (maximum velocity minimum velocity) -f- mean
velocity.
The figures given for the fluctuations of wind in the
records of various anemometers given a^ove may be
called the " percentage gustiness " of the winds. To
obtain an estimate of the relative gustiness of the winds
in the upper air, Mr J. S. Dines used the pull of a kite
wire defining the gustiness as
(maximum pull minimum pull) -f- mean pull.
Using this method it appears that gustiness falls off
rapidly in the first 500 feet of ascent, and thereafter it is
irregular. (Second Report on Wind Structure, p. 10.)
Hail. Usually described as frozen raindrops, though
hailstones are often very much larger than any raindrop
an possibly be. Hail is formed in the columns of rapidly
Hail. 143
ascending air that are part of the mechanical process of a
rain-storm or thunderstorm. They are associated \vi h
the cumulo-nimbus type of cl>ud. The convection
currents which begin with insta' ility in the atmosphere
result, first in heavy cloud, and then in raindrops still
carried upward in air which is automatically becoming
colder in consequence of the diminished pressure. So the
drops may freeze, and then any further upward journey
may result in condensation in the form of ice on the
already formed hailstone.
To maintain a mass of water or ice in the air a very
vigorous ascending current is required. If a raindrop
reaches a certain size it is broken up into smaller drops
by the current which is necessary to keep it from falling,
but when the hailstone is once formed there is no limita-
tion of that kind upon its growth.
From their structure, which is often very composite, it is
clear that hailstones have a long history, and from their
size, which may be large enough to give measurements,
it is said, of three or four inches in diameter, a pound
or more in weight, they must have required ascending
currents of great velocity to support them.
There is, however, evidence to show that some of the
strongest winds of the earth are katabatic winds, that is,
they are due to falling air, so it requires only a special
adjustment of the temperature of the environment to give
rise to currents of rising air, anabatic winds, of the most
violent character. (SOFT HAIL, see p. 343.)
Halo. The term halo is an inclusive one applied to
all the optical phenomena produced by regular REFRAC-
TION, with or without accompanying reflection, of the
144 Glossary.
rays of the sun or moon in clouds consisting of ice-
crystals (see CLOTD, Cirro-Nebula). The most common
halo is a luminous ring of 22 radius surrounding the sun
or moon, the space within it appearing less bright than
the rest of the sky. The ring, if faint, is white if more
strongly developed its inner edge is a pure red, while
yellow and green follow, more faintly. Next in order of
frequency of occurrence is a similar but larger ring of
46 radius. MOCK SUNS are simply more brilliant
patches occurring at certain definite points in a halo
system. There is a great variety of minor and rarer
halo phenomena. (For some of these see Observer's
Handbook, p. 57.) In polar regions, where ice crystals
extend much lowe^ in the atmosphere, halo systems attain
great brilliance and complexity.
Halos are very varied in form, they are produced by
the REFRACTION of the sun's rays, or the moon's rays,
through a cloud of ice crystals forming what is called
cirro-nebula or cirrus-haze, one of the highest forms of
cloud. They are of great interest from the point of view
of the physics of the atmosphere, but they have no^
meteorological significance. In weather lore they are'
often spoken of as presaging storms and it is possible that
the ice cloud is one of the earliest results of the fall of
pressure with which the storm is associated ; but the
formation of a halo is not by any means a necessary step
in the preparation of a storm ; many storms arrive
without announcing their coming in that way. Moreover,
the appearance of a halo at the end of a spell of dirty
weather is said to be a sign of clearing. We may perhaps
conclude that cirro-nebula, with no other clouds in the
sky to interfere (the condition for seeing a halo), may be
found at the beginning or the end of a depression.
Harmattan. 145
Harmattan. A very dry wind which is prevalent
in Western Africa during the dry season (November to
March). During these months, (the winter of the
Northern Hemisphere) the air over the desert of Sahara
cools rapidly, owing to its clearness and lack of moisture,
so that it tends to flow outwards to the coast, especially
south-westwards to the Gulf of Guinea, and replace the
lighter air there. Being here both dry and relatively
cool, it forms a welcome relief from the steady damp
heat of the tropics, and from its health-giving powers
it is known locally as " The Doctor," in spite of the fact
that it carries with it from the desert great quantities of
impalpable dust, which penetrates into houses by every
crack. This dust is often carried in sufficient quantity
to form a thick haze, which impedes navigation on the
rivers.
Harmonic Analysis. See p. 311.
Haze. Obscurity of the atmosphere which may occur
in dry weather and may be due to dust or smoke, or
merely to irregularities of density and consequent
irregular refraction of the light by which distant objects
are seen. During HARMATTAN winds off the West Coast
of Africa, dust haze is thick enough to be classed as fog.
At sea the weather is often classed as hazy when there is
no distant horizon, and yet no visible mist or fog. The
obscurity may, however, be due to water particles. It
would therefore be desirable to limit the use of the word
haze to occasions when the air is not very damp, that is
when there is a noticeable difference between readings
of the wet and the dry buib thermometers.
Heat. The name used for the immediate cause of the
sensation of warmth, a primary sensation which is easily
recognised and needs no explanation. As used in relation
146 Glossary.
to the weather, heat and cold are familiar words for
opposite extremes of temperature of the air. What the
American writers call a heat-wave is a spell of hot wenther
in which the maximum temperatures reach 90 or 100 F.
(above 305a.), and a cold-wave is a spell of the opposite
character during which temperatures in the neighbour-
hood of the Fahrenheit zero, or 32 degrees of frost, may be
experienced. In continental climates, during the passage
of severe cyclonic depressions, the transitions from heat
to cold are sometimes extremely abrupt and far-reaching ;
a difference of temperature of 50 F. in a few hours is
not unknown. We have visitations of similar character
in this country, but they are less intense. A few days in
succession with a temperature over 80 F. would suffice
for a heat-wave, and a few days with 10 of frost would
certainly be called a cold-wave. One of the most
noticeable features of our climate is the succession of
cold spells which interrupt the genial weather of late
spring and early summer. They are not very intense,
but a drop in the mean temperature of the day from
55 F. to 45 F., which roughly defines them, produces a
very distinct impression,
As used in connexion with the study of the atmosphere
heat has another sense which must not be overlooked.
It denotes the physical quantity, the reception of which
makes things warmer, and its departure makes them colder.
If you wish to make water hot, you supply heat to it
from a fire or a gas-burner or, in modern days, by an
electric heater, a very convenient contrivance for getting
heat exactly where you want it. On the other hand, if you
want water to become cooler, you leave it where its heat
can escape, by CONDUCTION, aided by CONVECTION or
by RADIATION. You can also warm water by adding
Heat. 147
some hot water to it, or cool it by adding cold water to it.
Either process suggests the idea of having the same
quantity of heat to deal with altogether, but distributing
it, or diluting it, by mixing.
The idea of having a definite quantity of heat to deal
with, and passing it from one body to another is so easily
appreciated and so generally applicable, that the older
philosophers used to talk confidently of heat as a substance
which they called Caloric, and which might be transferred
from one body to another without losing its identity.
They measured heat, as we do still, by noting by how
much it would raise the temperature of a measured
quantity of water. For students of physics the unit of
heat is still a gramme-calorie, the heat which will raise a
gramme of water through one degree centigrade. To
raise m grammes from ^C to t 2 C, m( 2 -i) gramme
calories are required. The amount can be recovered, if
none has been lost meanwhile, by cooling the water. If
we wish to be very precise, a small correction is required
on account of the variation in what is called the capacity
for heat of water at different temperatures, but that need
not detain us.
For students of engineering the unit, called the British
Thermal Unit, is a pound-Fahrenheit unit instead of the
gramme-centigrade unit, and the heat required to raise m
pounds of water from ^F to 2 F is m (t 2 - tj B.T.U.
It is in many ways a misfortune that students of Physics
and Engineering do not use the same unit. It is no
doubt a good mental exercise to learn to use either
indiscriminately without confusion, but it takes time.
From measurements of heat we get the idea that with
different substances the same change of temperature
requires different quantities of heat ; the substances have
148 Glossary.
different capacities for heat. We define capacity for heat
as the heat required to raise a unit of the substance
(1 gramme or 1 Ib.) through 1 degree.
It is a remarkable fact that of all common substances
water has the greatest capacity for heat. It take* one unit
to raise the temperature of a unit mass of water one
degree, it takes less than a unit, sometimes only a small
fraction of a unit, to raise the temperature of the same
amount of another substance through one degree. We
give the name specific heat to the ratio of the capacity for
heat of any substance to the capacity for heat of water.
Numerically, specific heat is the same as the capacity for
heat in thermal units.
The specific heat of water is 1, the specific heat of any
other common substance is less than 1. The specific heat
of copper is only 1/11. So the heat which will raise the
temperature of a pound of copper 1 will only raise the
temperature of a pound of water 1/11, or the heat which
will raise the temperature of a mass of water 1 will raise
the temperature of the same mass of copper 11.
This peculiar property of water makes it very useful
for storing heat and carrying it about. From that point
of view it is the best of all substances for cooling the
condenser of an engine, for distributing heat at a moderate
temperature in a circulating system, and for many other
economic purposes.
In meteorology its influence is very wide. Large
masses of water, of which the ocean is a magnificent
example, are huge store houses which take up immense
quantities of heat from the air when it is warm and give
it out again when the air is cold, with very little change
in its own temperature, so that a large lake, and still more
the ocean, has a great influence in reducing the extremes
HeaL 149
of temperature of summer and winter, and of day and
night, in the countries which border it.
There is another remarkable storage of heat in which
water takes a predominant share that is dealt with in
physical science under the name of latent heat.
Water at 288a. (59 F.) cannot be evaporated into
water-vapour unless every gramme of it is supplied with
589 calories of heat, which produce no effect at all upon
the temperature. -.The water is at 288a. to start with,
and the water- vapour is at exactly the same temperature
and yet 589 calories of heat have gone. They are latent
in the water- vapour but produce no effect on the thermo-
meter. You can get them back again easily enough if
you condense the vapour back again into water, but you
must manage somehow to take away the heat while the
condensation is taking place. The separation of the
u waters that are above the firmament from the waters
that are below the firmament," or in modern language,
the evaporation of water from the sea or a lake or the wet
earth and its condensation in the form of clouds and rain,
implies the transference of enormous quantities of heat
from the surface to the upper air, the dynamical effect of
which belongs to another chapter of the romantic story
of heat which deserves more than the few words which
we can afford for it. Readers can find an interesting
account in Tyndall's Heat a Mode of Motion.
The idea of heat as an indestructible substance, caloric,
which could be transferred from one body to another
without loss, became untenable when it was found that
when air was allowed to expand iri a cylinder it cooled
spontaneously to an extent that corresponded exactly,
so far as could be ascertained, with the means then
available, with the amount of mechanical work that the
150 Glossary.
cylinder was allowed to do. It was the last step in the
process of reasoning by which men had come to the con-
clusion that, when mechanical work was devoted to
churning water or some other frictional process, heat was
actually produced, not brought from some other substance
but created by the frictional process.
It took many years for men to reconcile themselves
to so novel an idea, and a good deal of ingenuity was
devoted to trying to evade it, but it has now become the
foundation stone of physical science. Heat is not an
unalterable indestructible substance but a form of energy.
It can do mechanical work in a steam-engine or a gas
engine or an oil-engine, but for every foot-pound* of
work that is done a corresponding amount of heat must
disappear, and in place of it a corresponding amount of
some other form of energy is produced. A good deal of
heat, besides, may be wasted in the process so far as
practical purposes are concerned. In a steam engine, of
the whole amount of heat used, only one tenth may be
transformed, the rest wasted, as we have said ; but it is
still there raising the temperature of the water of the
condenser or performing some other unproductive but
necessary duty.
There is, therefore, a numerical equivalent between
heat and other forms of energy.
We give the relation :
1 B.T.U. is equivalent to 777 foot-pounds of energy.
1 gramme-calorie = 42,640 gramme-centimetres.
= 41,830,000 ergs.
* Afoot-pound of work is the work done in lifting one pound through.
a distance of one foot.
A gramme-centimetre is the work done lifting one gramme through
one centimetre.
An erg is the absolute unit of work on the C.G.S. system ; 1 gramme
centimetre = 981 ergs.
Heat. 151
We have led up to this statement in order to point out
how extraordinarily powerful heat can be in producing
mechanical energy.
If, in the operations of nature, one single cubic metre
of air gets its temperature reduced by l c 0. in such a way
that the heat is converted into work by being made to
move air, the equivalent of energy would be a cubic
metre of air moving with a velocity of nearly 45 m/s.
(101 mi/hr.).
So familiar have we become with heat as a form of
energy that we measure the heat of sunlight in joules*
and the intensity of sunshine in watts per square centi-
metre, i.e.. the number of joules falling on one square
centimetre per second.
THE SPECIFIC HEAT OF AIR.
The foregoing statement is necessary to lead up to a
matter of fundamental importance in the physics of the
atmosphere, namely the heat that is required or used to
alter the temperature of air in the processes of weather ;
in technical language this is the capacity for heat of air
or the specific heat of air.
We have explained that when air is allowed to do work
on its environment, in expanding, heat disappears, or more
strictly is transformed. So the amount of heat required
to warm air through a certain number of degrees depends
upon how much expansion is allowed during the process.
The most economical way of warming air from the
* A joule is a more convenient unit than the small unit, the erg ;
one joule = ten million ergs (10 7 ergs) and one calorie = 4*18 joules.
The icatt is a unit of power, that is, rate of doing work ; a power of
one watt does one joule of work per second.
152 Glossary,
thermal point of view is to prevent its expanding
altogether ; it then has " constant volume " and its specific
heat is 0*1715 calories per gramme per degree at 273 a.
It is remarkable, but true, that if you have a bottle full
of air, it will take more heat to raise the temperature of
each gramme of it by a degree if you take the stopper out
while the warming is going on, than if you keep it tight.
The difference between warming a bottle of air with the
stopper in and with it out, simple as it may seem, has got
in it the whole principle of heat as a form of energy.
The effect of leaving the stopper out is that the pressure
of the air inside the bottle is the atmospheric pressure
for the time being and is therefore practically constant
throughout the brief operation. So we get the specific
heat of air at constant pressure 0*2417 gramme-calories
per gramme per degree, or 1*010 joules. The specific heat
of air at constant volume is 0*72 joules. The difference of
the two represents the heat equivalent of the work used
in expanding unit mass of the gas against atmospheric
pressure.
High. Sometimes used as a contraction for high
barometric pressure. The technical term anticyclone was
coined by Sir F. Galton for the purpose, but, whether for
the sake of brevity or for some 'other reason, a " high " is
often spoken of.
Hoar Frost. A feathery deposit of ice formed upon
leaves and twigs in the same way as DEW (q.v.) by the
.cooling of exposed objects through the radiation of their
heat to the clear sky.
Horizontal in the plane of the horizon. The surface
of still water is horizontal. In dynamics and physics a
Horizontal.
153
horizontal line is a line at right-angles to the direction
of the force of gravity which is vertical and identified by
the plumb line.
The Visible Horizon, or Distance of Visibilitl| for objects of given height
1,000
900
800
700
600
500
400
500
200
100
10.000
9.000
8000
7.000
6,000
5.000
4000
3.000
2.000
1.000
\
/
\
/
/
/
=f
I
/
/
o
|
c
ft
7T/
y
5
1
I
/
<^
^
/
S
I/
^
/
/
</
/
^
Curve
I rf/ti
r. to L
*// Hf
nil Ht
ight- *
(Lf^le.
X
/
.
-r
TL
ght >
MMes. 10 20 30 40 50 60 70 80 90 100 110 120 130 Miles
Diagram showing the relation between the height of an observation
point in feet and the distance of the Visible Horizon in miles
{neglecting refraction), or the height in feet of a cloud or other
distant object and the distance in miles at which it is visible on the
horizon.
The " sensible or visible horizon " which is visible
from a ship at sea, the line where sea and sky apparently
join, is a circle surrounding the observer a little below
the plane of the horizon in consequence of the level
of the earth's surface being curved and not flat. The
depth of the " sensible horizon " below the " rational
horizon " or horizontal plane is approximately the same
as the elevation of the point from which the " sensible
154 Glossary.
horizon " is viewed. Apart from any influence of the
atmosphere the distance of the visible horizon for an
elevation of 100 feet (30 metres) is about 12 miles. The
actual distance is about 2 miles greater on account of
refraction. It varies as the square root of the height, so
that it would require a height of 400 feet to give a
horizon 24 miles off. A level canopy of clouds 10,000 feet
high is visible from a point on the earth's surface for
a distance of about 125 miles, or the visible canopy has a
width of 250 miles.
Horse Latitudes. The belts of calms, light winds
and fine, clear weather between the TRADE WIND
belts and the prevailing Westerly winds of higher
latitudes. The belts move North and South after the
Sun in a similar way to the DOLDRUMS q.v.
Humidity, in a general sense means dampness, but in
meteorology it is used for RELATIVE HUMIDITY and
means the ratio of the actual amount of aqueous vapour
in a measured volume of air to the amount which the
volume would contain if the air were saturated. (See
AQUEOUS VAPOUR.) In practice, at climatological stations,
the humidity of air is determined from the readings of
the dry and wet bulbs with the aid of tables prepared for
the purpose and called humidity tables or psychrometric
tables. But humidity is the most variable of the ordinary
meteorological elements, as it depends not only on the
sample of air under observation but also on its
temperature. Hence the record of a self-recording hair-
hygrometer which can be obtained in a form not much
different from an ordinary barograph gives a most instruc-
tive record. In the spring and summer it sometimes
Humidity. 155
shows very high humidity in the night and early morning,
approaching or actually reaching saturation, and very
great dryness, perhaps only from 15 to 20 per cent,
humidity, in the sunny part of the day, with very rapid
changes soon after sunrise and towards sunset.* These
are the changes which correspond with the characteristic
changes in the feeling of the air at the beginning and end
of the day.
Hurricane in French, ouragan, in German, Danish
and Swedish, orkan. " A name [of Spanish or Portuguese
origin] given primarily to the violent wind-storms of the
West Indies which are cyclones of diameter of from 50 to
1,000 miles, wherein the air moves with a velocity of
from 80 to 130 miles an hour round a central calm space
which, with the whole system, advances in a straight or
curved track ; hence any storm or tempest in which the
wind blows with terrific violence " (New English
Dictionary), The hurricanes of the Western Pacific
Ocean are called typhoons in China, and .baguios in the
Philippine Islands. Those of the Indian Ocean, which
are experienced in India, are called by the Indian meteoro-
logists cyclones of the Arabian Sea or of the Bay of
Bengal ; while the hurricanes of the South Indian Ocean
which visit Mauritius are also called cyclones.
Shakespeare uses the word hurricano for a water-spout.
Overleaf is a reproduction of a barogram showing the
variation of pressure during a cyclone which passed over
Cocos Island, Sumatra, in 1909, November 27th. It is
interesting to notice that in spite of the rapid fall of
pressure with the onset of the cyclone the diurnal varia-
tion of the barometer is still apparent and it reappears
before the normal level is recovered.
156
Glossary.
Coco s Island ttarogram November, 1909
The occurrence of hurricanes shows a marked seasonal
variation. The following table is taken from the Barometer
Manual for the Use of Seamen.
Variation of the Barometer during a Hurricane. 157
lO
10
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CN vO
to Tt" M \O tO '
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158 Glossary.
The force of the wind which is experienced in hurri-
canes is equalled, if not surpassed, in the tornadoes which
occur on the American Continent, but the area affected by
a tornado is generally a narrow strip a few miles at most
in width.
In the Beaufort Scale of wind force the name hurricane
is given to a wind of force 12, and its velocity equivalent
is set at an hourly velocity exceeding 34 m/s, or 75 mi/hr,
but from what has been said under GUST it must be under-
stood that at all ordinary exposures a wind with an hourly
velocity of 75 miles an hour will include gusts of con-
siderably higher velocity, reaching a hundred miles an
hour or more. The strongest recorded gust in the British
Isles marked 103 mi/hr on the anemometer at Pendennis
Castle on March 14, 1905.
Hydrometer. An instrument for measuring the
density or specific gravity of sea- water. (See Marine
Observer's Handbook, M.O. Publication 218.)
Hydrosphere. The name given to the layer of water
of irregular shape and depth lying on the earth's surface,
between the geosphere, or the solid earth below, and the
atmosphere, the gaseous envelope above.
HyetOgraph.. A self-recording RAIN-GAUGE, an
instrument for recording automatically and graphically
the fall of rain. (See Observer's Handbook.)
Hygrograph.* A self-recording HYGROMETER, an
instrument for recording automatically the humidity of
the atmosphere. Some form of hair-hygrometer is
generally employed for the purpose.
Hygrometer. 159
Hygrometer. An instrument for determining the
humidity of the atmosphere. Almost all materials exposed
to the weather are affected by the humidity of the air, so
that it is easy to form a rough estimate of whether the air
is damp or dry. Many different materials such as hair
catgut, the awm or beard of the wild oat, flannel, have
been used in instruments to give an indication of the
state of the atmosphere in this respect. But for the
purposes of meteorology there are three well-known forms
of hygrometer : the hair-hygrometer, the indications of
which depend upon the length of a hair or a bundle of hairs
exposed to air of different states of moisture ; the dew-point
hygrometer, in which a polished surface is artificially
cooled until a deposit of dew is produced and the DEW-
POINT determined ; and the PSYCHROMETER, or wet and
dry bulb hygrometer, in which the temperatures of a bulb
covered with moistened muslin and of a dry bulb close to it
are read and the humidity determined by tables.
The psychrometer is in almost universal use at meteoro-
logical stations, as it is the least dependent upon the skill
of the observer ; but a few hair-hygrometers are also em-
ployed for eye observations, and for automatic records
either at the surface or in soundings with kites or BALLONS-
SONDES the hair-hygrometer is generally used.
HygTOSCOpe. An instrument for showing whether
the air is dry or damp. If its indications are sufficiently
regular to permit of graduations, it can be made into a
HYGROMETER. Any substance which is hygroscopic, that is
to say, which is affected in shape, size, or appearance by
the variations of moisture in the air can be used as a
hygroscope. A bundle of seaweed is sometimes used,
(the hygroscopic substance in that case is the salt) ; the
160
Glossary.
ordinary " Jacky and Jenny " in a toy house with a catgut
support is another example.
Hypsometer. The word is derived from hupsos, and
means an instrument for measuring height, but it is
employed exclusively for apparatus for determining very
precisely the temperature of the boiling point of water.
That amounts to the same thing as measuring the pressure
at which the water is boiled, because the boiling point
depends upon the pressure of the atmosphere, and a table
of the relation between the two makes the reading of the
temperature equivalent to a reading of the barometer.
Table of the boiling point of water under pressures
occurring in the atmosphere up to about 8000 feet.
Pressure.
Boiling Point.
Millimetres of mercury
at 0C. sea level, lat. 45.
Millibars.
a.
mm.
mb.
374
787-67
1050* 12
373
760 oo
1013-23
372
733*i6
977'45
3/1
707-13
942-74
37o
681-88
909 * 08
3 6 9
657-40
876-44
368
633-66
844-79
3 6 7
610-64
814- 10
366
588-33
784-36
365
566-71
755-54
364
545-77
727-62
Hypsometer. 161
From the pressure (with a corresponding reading lower
down, which may, should, or must be assumed) the
height can be computed.
The hypsometer has advantages for measuring heights
as a substitute for a mercury barometer, which is a
troublesome instrument to carry on a journey of ex-
ploration. With a pair of thermometers that have all
modern improvements, and with careful manipulation,
the temperature can be measured to one-thousandth of a
degree, corresponding approximately with. '0^1 inch, or
03 millibar, that is to say, the pressure can be determine 1
to the equivalent of one foot of height. That, however,
is not to be attained by the inexpert traveller in a hurry.
Ice. See p. 321.
Iceberg. A large mass of ice that breaks away from
the tongue of a glacier running into the sea and floats
away. An accoun t of the subject is given in the Keamaris
Handbook. Icebergs drift with favourable winds and
currents into latitudes of forty or fifty de^r^es. The final
period of their life history is not very well understood ;
there seems to be a sudden ending that is not accounted
for. Nobody has apparently "stood by" an iceberg on
the track of Atlantic steamers until the end came. Nor
do we know through how "many seasons, for example, a
North Atlantic iceberg floats, or lies aground, between its
" calving " and its dissolution. It probably weathers one
season but collapses in the second.
Incandescence. The spontaneous glow of a substance
in consequence of its temperature. The word is now quite
familiar in consequence of the incandescent or glow-lamp
which is luminous on account of the temperature to which
the carbon or metallic filament is raised by the electric
13204 F
162 (Glossary.
current. Every substance becomes incandescent when
heated to a sufficiently high temperature : thus lightning
is presumably incandescent air, the sun incandescent
vapour. The temperature at which incandescence begins
is different with different substances, so the following
figures are only roughly approximate : Red hot covers a
wide range beginning with 800a. Dull red is about lOOOa.
Cherry red 1200a. Orange 1400a. White hot 1500a. The
sun GUOOa. Carbon melts at 4300a, platinum at 2000a.
Index, the pointer which moves on the scale of an
instrument and by which the reading is taken. Two
indexes, the two hands or fingers, are required to tell the
time by a watch, but only one index is required in read-
ing the barometer. The index in this case is the top of
the mercury column, so also in the ordinary thermometer
the end of the mercury is the index. In a maximum
thermometer the outer end of the detached thread of
mercury is the index. In a minimum thermometer a
special index is introduced into the spirit, which is trans-
parent. In both these cases the index has to be set after
one reading to make it ready for another.
In like manner every measuring instrument has its
index.
Index-error. See ERROR.
Insolation. Originally exposure to sunshine ; solar-
isation is also used in the same sense.. It is now applied
to the solar radiation received by terrestrial or planetary
objects (Willis Moore).
The amount of solar-radiation which reaches any
particular part of the earth's surface in any one day
depends upon (1) the constant of solar-radiation, (2) the
area of the intercepting surface and its inclination to the
Insolation.
163
sun's rays, (3) the transparency of the atmosphere, (4) the
position of the earth in its orbit. The following table
quoted from Angot is taken from Willis Moore's Descrip-
tive Meteorology.
Calculated Insolation Reaching Earth, assuming the mean
coefficient of transparency of the atmosphere to be 0'6.
(Angot).
1
*> >;
b
1
1
1
^3 f_*
,4
43
a
9
o
a
a
49
p
o
&
9
be
5
CS
0>
ft
p
o>
H
1-3
^
s
"-3
-5
<1
r jj
fc
p
5*
N.
90
O'O
O'O
O'O
1-4
6-7
9'9
7*9
2-4
O'l
O'O
O'O
O'O
28-4
80
O'O
O'O
0'2
2-7
7*5
10-3
8'5
3*8
0-5
O'O
o-o
O'O
33*5
60
O'l
I'O
3*9
8-2
I2'oi3'8
I2'6
9-2
4*9
i'5
O'2
O'O
67*4
40
3*3
5*7
9*4
12-9
15*316-2
15-6
13*5
I0'2
6-6
3*8
2-7
115-2
20
9-0 II-2
13*6
15-2
15-815-9
15-8
15*3
H'c
11-7
9*4
8'2
I55*i
Equator
s.
14-0 14-9
15*3
14-6
13*5
I2'8
13*1
14-2
15-0
14-2
13-6
170-2
20 16-815-9
13*9
II'2
8-8
7*7
8'3
10-5
13*1
is;3
i6'6
17-0
155-1
4
16-6 13-9
9*9
6-0 3-4
2'4J 3*0
5*2
8-8
15-9
17*3
115*2
60
I3'4 9*2
4*4
1-3! o-i
o-o, o-i 0-8
3*4
7*8
12-3
14-6
67*4
8o c
90
8'8
3*5
2' I
0-4
o-o
O'O O'O
O'O O'O
O'O
O'O
O'OJ O'O O'l
O'O O'O O'O
2-3
I'O
TA
II'O
10-5
33-5
28-4
The unit is the amount of energy that would be received
on unit area at the equator in one day, at the equinox,
with the sun at mean distance if the atmosphere were
completely transparent. It is 458'4 times the solar
constant, or in gramme calories per minute 885, taking
the solar constant to be 1-93.
13204
Glossary.
Inversion. An abbreviation for u inversion of
temperature-gradient" (see GRADIENT). The tempera-
ture of the air generally gets lower with increasing height
but occasionally the reverse is the case, and when the
temperature increases with height there is said to be an
" inversion ".
There is an inversion at the top of a fog-layer, and
generally at the top of other clouds of the stratus type.
Inversions are shown in the diagram of variation of
temperature with height in the upper air, p. 38, by the
slope of the lines upwards towards the right instead of
towards the left, which is the usual slope. In the
troposphere inversions do not generally extend over any
great range of height ; the fall of temperature recovers
its march until the lower boundary of the stratosphere is
reached. At that layer there is generally a slight inversion
beyond which the region is isothermal, so far as height is
concerned. For that reason the lower boundary of the
stratosphere is often called the fc< upper inversion ".
In some soundings with ballons-sondes from Batavia
the inversion has been found to extend upwards for
several kilometres from the commencement of the strato-
sphere.
It is important also to note that frequently in anti-
cyclonic weather, and especially cold anticyclonic weather,
there is often an inversion at the surface ; the temperature
increases upwards instead of decreasing.
Ion. The name selected by Faraday for the com-
ponent parts into which a chemical molecule is resolved
in a solution by the electrolytic action of an electric
current. Of the two component ions one is always elec-
tro-positive, and the other electro-negative. The electro-
Ion. 165
negative ions consist of atoms of oxygen, chlorine or some
other corresponding element or radicle, and the electro-
positive ions consist of atoms of hydrogen, potassium, or
some other metallic element or radicle. Each electro-
positive ion is called a cation. It is charged with a
definite quantity of positive electricity and travels with
the electric current to the cathode, the conductor by
which the current leaves the solution, while the electro-
negative ion, called the anion, is charged with an equal
quantity of negative electricity and travels against the
electric current to the anode, the conductor by which the
current enters the solution.
It is supposed that a solution which will conduct an
electric current is ionised by the spontaneous dissociation
of the components of its molecules and the consequent
formation of free ions carrying their appropriate electric
charges. In a solution, recombination and dissociation
are constantly going on and the electric current causes
the free ions charged with positive electricity to move
slowly with the current and those charged with negative
electricity to move against the current.
Similarly, a gas may conduct electricity to a less extent,
but in the same way, as a solution when it contains
free ions, which may be produced by the action of radio-
active agents, ultra-violet light, very hot bodies, the
combustion of flame and in other ways. The conduction
of electricity through the atmosphere is now, therefore,
attributed to the free ions which exist in it, and its
capacity for conducting electricity is attributed to its
ionisation.
The ions in the air may be atoms of hydrogen or
oxygen, or they may be aggregates of those atoms witfe
some other material.
] 66 Glossary.
A certain number of the ions in atmospheric air doubt-
less arise from the radio-active materials in the soil.
These materials give rise to an emanation, as it is called,
which must gradually reach the surface through the pores
of the soil, The supply will naturally depend on- the
state of the soil, whether damp or dry, frozen or covered
with snow, and presumably also on variations of the baro-
metric pressure which promote or check the escape of
emanation. Other ions must be produced by light from
the sun. These will naturally chiefly arise at considerable
heights above the ground, where sunlight is stronger and
relatively richer in ultra-violet light than near the
ground. In addition there seems to be some other
powerful source at high altitudes, possibly some form of
electrical radiation from the sun.
lonisation. See p. 322.
Iridescence or Irisation, words formed from Iris,
the rainbow, to indicate the rainbow- like colours which
are sometimes seen on the edges of clouds ; tinted patches,
generally of a delicate red and green, sometimes blue and
yellow, occasionally seen on cirrus and cirro-cumuius
clouds up to about 25 from the sun. They may be also
seen at times on the edges of fracto-cumuius or strato-
cumulus clouds. The boundary between the two tints is
not a circle with the sun as centre, as in a CORONA, but
rather tends to follow the outline of the cloud. They are
probably not due to the refraction of light by water drops,
which produces the colours of the rainbow, but to the
diffraction of light scattered by the very small water
drops, and are to be classed like the corona with the
iridescence of the opal and the mother-of-pearl.
Diffraction-colours formed in artificial clouds in the
Isanomalies. 167
Bame way as in the corona, become more brilliant as the
cloud gets older and the drops more uniform in size.
Hence it seems probable that an iridescent cloud is an old
cloud that has been drifting for some time.
Isabnormals. See ISANOMALIES.
Isanomalies. This word is a combination of the
prefix iSo- and the word ANOMALY, which, like the
more common adjective anomalous, signifies departure or
deviation from normal. The normals used for reference
are obtained on various plans, Normals of temperature
have been obtained by taking the general mean of the
observed temperatures of successive parallels of latitude
and thus assigning a normal temperature to each latitude ;
isanomalies of temperature are then the departures of
mean temperature for any place from the normal for its
latitude ; places that are relatively warm for their latitude
have a positive anomaly, and places that are relatively
cold for their latitude a negative one. Isanomalies are
then lines on a map showing equality of departure of the
average temperature of any place from, the normal for
its latitude.
There are, however, not many meteorological elements
which can be said to have a normal value for latitude,
and it is usual to employ as normals for any place the
average or mean value for a long period of years.
In that case departures, or differences from the normal
for the corresponding period, of the value for any one
period, say a month or a year, are called ABNORMALS or
abnormalities, and a chart showing equality of departure
from the normal a chart of ISABNORMALS. Isabnorinal
is an objectionable compound because it is made up of a
Greek prefix and a Latin body ; if the departures are to
be called abnormals the lines of equal departure ought to
168 Glossary.
be called equi-abnorrnals, so that the tendency is to use
ISANOMALIJSS for lines of equal departure of a value from
its long period average.
Isentropic. Without change of ENTROPY (q.v.) gen-
erally equivalent in meaning to ADIABATIC.
ISO. The prefix ISO- is the Greek equivalent of the
Latin EQUI- and implies the setting out of lines on a chart
or diagram to show the distribution of set values of some
meteorological element. The words with this prefix can
generally be interpreted 'by the reader on this basis :
some examples are set out under the separate headings
below.
Thus :
ISOBARS, from baros, are lines on a chart showing
equal barometric pressure.
ISOHELS, from helios, are lines showing equal dura-
tion of sunshine.
ISOHYETS, from huetos, are lines showing equal
amounts of rainfall.
ISOPLETHS, from plethos, lines showing equal
amounts of a meteorological element.
The word isogram was recommended for this pur-
pose by Sir Francis Galton.
ISOTHERMS, from therme, are lines showing equal
temperatures.
Isobars. If some of the air were removed from a
room the pressure inside would be reduced and the
pressure outside would force air through the windows
and doors till the room was again filled with the normal
amount of air. If over any area some of the air were by
any means removed the pressure over that area would be
reduced, and the pressure of the air in the surrounding
Isobars. 169
districts would tend to force air into the region of deficient
pressure. Such areas of deficient pressure are found to
exist, but for the following reason the air does not actually
{low into them. Anything moving above the surface of
the earth will continue to move in a straight line if no
force acts on it ; but the earth in its rotation turns
under the. moving body ; the moving body is therefore
apparently deflected to the right in the Northern Hemi-
sphere. This is true of a cannon-ball, and it is true of a
moving current of air. Hence the wind does not actually
blow into the area of low pressure ; air from the North is
deflected to the West side of the area, air from the West to
the South side, and so on ; the wind therefore instead of
blowing straight in from all sides blows round an area of
low pressure counterclockwise in the Northern Hemisphere.
We thus have the apparent paradox of a force tending to
push the air into the centre of the low pressure area while
the air is actually moving round the centre at right-angles
to the force that is acting on it. There are many examples
of similar things in nature ; the Earth's motion round the
sun for instance ; or the water in a basin when there is
a hole in the centre from which the plug has been taken
out ; the slightest circular motion sets up a swirl, and the
water moves round the basin at right-angles to the force
of gravity which is tending to force it towards the hole.
In the case of an area of high pressure the air blows
out from the high pressure, but it is deflected to the right
as in the former case, with the result that the wind blows
round the area of high pressure in a clockwise direction.
In either case if you stand with your back to the wind
the low pressure will be on your left hand, this is BUYS
BALLOT'S LAW. In the Southern Hemisphere the reverse
is the case.
170 Glossary.
If the observed heights of the BAROMETER (reduced to
sea level) from a number of places are put on to a map
and arrows are put in to represent the direction of the
wind, we have a weather map which at first sight looks
like a disordered collection of figures ; we may make it
clearer, however, by drawing lines through places where
the barometer stands at the same height ; thus we may
draw one line through all places where the barometer
stands at 1,015 mb., another through all places where it
stands at 1,010, and so on. Such lines are called ISOBARS.
It may happen that we cannot find any station where the
barometer stands at say 1,015 mb., but if we find one where
it stands at 1,016 and another where it stands at 1,014 we
take it that the 1,015 line passes midway between the
two stations. When the isobars are drawn in we can
readily see the shapes of the areas of . low and high
pressure, and we see also that the wind blows in accord-
ance with Buys Ballot's law. The areas of low pressure
are called CYCLONES, DEPRESSIONS, or simply LOWS ; the
areas of high pressure ANTICYCLONE?, or HiGHS.
The isobars are analogous to contour lines on an ordinary
map, the high pressures" corresponding to the hills, the
low pressures to the valleys. The moving air does not go
straight from the highs to the lows, but it blows round
the highs in a clockwise, and round the lows in a counter-
clockwise direction. On the contours of the earth we may
descend from a height of say 1000 feet to 500 feet by a gentle
slope many miles in length, or in another place we may
descend by a precipitous scarp ; in the former case a stream
will run down sluggishly, in the second it will be a swift
torrent full of rapids and waterfalls. So with the pressure ;
we may travel a Jong way between a place where the baro-
meter reads, say, 10^0 millibars to another where it reads
Isobars. 171
1015 millibars, or we may have to go only a comparatively
short distance. The steepness of the gradient on a map is
measured by the distance between the contour lines ; the
steepness of the barometric gradient is measured by the
nearness of the isobars. The strength of the wind
depends on the steepness of the barometric gradient, just
as the velocity of the stream depends on the steepness of
the slope, but the analogy is not quite perfect for the
stream runs down the slope across the contour lines,
whereas the wind blows nearly along the isobars with a
slight inward curvature towards the low pressure. In
the case of the wind close to the surface if the five-
millibar isobars are 400 miles apart the barometric
gradient is sJight, and the wind will be about 10 miles per
hour ; if the distance apart is 60 miles the gradient is
steep, and the wind will be about 70 miles per hour. The
wind calculated from the barometric gradient is called the
GRADIENT WIND or, if no allowance is made for the cur-
vature of the path of the air, the GEOSTROPH1C WIND ;
it is in most cases the wind met with at or about 1500. feet ;
nearer the surface, owing to the friction, the actual wind
is less than the gradient wind. The gradient wind as
stated depends principally on the distance apart of the
isobars ; it is modified, however, to a small extent by the
variations of density of the moving air and therefore by
the height of the barometer and by the temperature ; a
table is given on pp. 172-173 showing the geostrophic
wind for various pressures and temperatures according to
the formula of p. 136. The values are dependent upon
the latitude and are given in the table for two latitudes
52 and 40.
Further tables are given in the Computer's Handbook
MX). 223. Section II.
172
Glossary.
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174 Glossary : Lithoplate XI.
If we put down on the weather map besides the baro-
meter read ings and the wind arrows, the state of the weather
we shall see that in anticyclones it is usually fine, while
in the depressions it is rainy and cloudy on th^ East side,
and finer on the West side of each depression. The
weather in fact is intimately connected with the shape of
the isobars. As this is a most important fact in meteoro-
logy it will be well to consider a little more closely the
areas of high and low pressure.
A DEPRESSION is a part of the atmosphere where the baro-
meter is lower than in the surrounding parts. (See PI.
XI). The isobars round such an area are more or
less circular or oval, though there are often irregularities.
The size of a depression may vary enormously ; one may
cover only a part of an English county, another may fill the
whole space between our islands and the Arctic circle. Some
are much deeper than others ; a deep depression is one where
the barometer is very low near the centre ; a shallow de-
pression is one where the barometer, though low near the
centre, is not very much lower than in the surrounding
districts. North of the Equator the wind blows round
the depression in a counter-clockwise direction, and the
steeper the barometric gradient, that is the deeper the
depression, the stronger is the wind. The sky is dull and
overcast on the east side of the depression, with rain near
the centre, especially heavy on the north-east side. Near
the centre in the region where there is an abrupt change
of wind direction from south on the east side, to north on
the west side of the depression, there is heavy rain and
often squalls. On the west side, in the region of northerly
or north-westerly winds, the cloud sheet is broken up into
detached clouds which get further apart, fewer, and less
rainy the further one goes from the centre.
f* ANTICYCLONE.
DISTRIBUTION OF WEATHER, WIND, *AND PRESSURE,
7 A,M, 17th NOVEMBER, 1915.
oV
2CV
ISOBARS are drawn for intervals of five i
bars. .
WIND. D^ection ia shown bj arrowi flying
with the wind.
Force, on the oala (M2, ii Incji-
oated bj the number of fther.
Calm x*"\
ATHIR, Shown by tho following symbol*
) clear sky. Q iky Clouded.
) iky * clouded.
) overcaft ky
snow A halt
) iky | clouded,
i raiii falling
Scfotf.
s oait. T thunder. "K thundarstorm.
COL.
DISTRIBUTION OF WEATHER, WIND, AND PRESSURE,
7 A.M. 1ft MAY, 1015.
ISOBAR! are drawn for interval! of ten milH- WEATHER. -Shown by the following symbols -
_ . bH f * Q clear iky. f) ky J clouded.
WIND. Direction if shown by arrow* flying S< . \Z ,
with the wind. (ft) tk J * ded. ^J) aky j clouded.
Force, on the acale 0-12, it indi- jtv overeat iky A rain falling
number of feathers W 8now j^ ^^^ s 1^
s*mist. T thunder IT thunderstorm
Ps. 1130, 26779. 464, 6000 1/18-
o
Wy. & S., M.O. Press, S. W. 7.
Isobars: Lithoplate XII. 175
If we note a depression on a weather map for one
day we shall usually find that on the map for the follow-
ing day the depression has moved in an easterly direction ;
a depression seldom remains long in one place, and its
drift is usually towards some point between north-east and
south-east,, though there are exceptions.
Since the air in front of a depression is coming from
the south, and in the rear from the north, there will often
be a great difference of temperature between the two sides.
This is particularly noticeable in winter when the
approach of a depression is heralded by warm, and its
passing away by culd weather.
As the depression moves it carries its weather and wind
system with it, so that an observer situated on its track would
have the following sequence of weather : The barometer
begins to fall, the wind becomes southerly, the sky
becomes overcast and the weather muggy, and in winter
the temperature is well above the normal ; the clouds get
thicker, the wind stronger, rain begins to fall ; as the
centre approaches the barometer gets lower, the wind gets
stronger and the rain heavier ; as the centre passes over
there are often gusts of wind or squalls, with heavy rain,
" clearing showers"; the barometer ceases to fall and
commences to rise ; the clouds show signs of breaking,
and the wind changes round to the north or north-west
and of tea blows more strongly than it did before the
centre passed ; as the centre moves away the wind lessens,
the rain ceases, or only occurs in showers, the sky clears.
The rate at which these changes take place depends on the
size of the depression and its rate of travel ; 24 hours is
an ordinary time for such changes to be gone through,
but it may be longer and it may be shorter. The above
sequence of weather is a typical one, but there are many
176 Glossary: Lithoplate XIII.
differences in individual depressions ; some are rainy
without much wind, others bring much wind, but not
much rain.
The above typical sequence of weather will only bo
experienced by an observer who is on the track of the
centre of the depression. One further south will have
south-westerly winds at first, with dull weather, becoming
rainy ; the wind will gradually veer to the west when the
centre is passing to the north, the barometer will begin to
rise and the wind will veer further to the north-west as
the centre passes away. An observer a long way from
the track of the centre will perhaps only experience a
slight fall of the barometer, with cloudy weather.
An observer north of the track will have south-easterly
or easterly winds at first, with probably much rain ; the
wind will back to the north-east or north as the centre
passes to the south. The winds on the north side of a
depression are usually less strong than those on the south
side, the isobars on the polar side being less crowded
together than those on the equatorial side. Thus a
depression passing on the south gives less wind, but
probably more persistent rain, than one passing on the
north. Gloomy days with an east or north-east wind,
and rain all day, are usually due to depressions passing to
the south of the observer. The easterly current on the
north side of a depression frequently brings snow in
winter.
If high clouds are visible they will frequently be
seen to be moving away from the centre of low pressure;
thus a south wind on the surface, with high clouds
moving from the west, is a sure sign of the existence of a
depression to the west.
X/ fl^Y''\ DEPRESSION.
DISTRIBUTION OF WEATHER, WIND, AND PRESSURE,
6 P.M. 13th FEBRUARY, 1915.
ISOBARS are drawn for jinterrals of ten mtlH- WEATHER. Shown by the following symbols .
bars. r\ clear sky. O**^ i clouded.
WIND.- Direction is shown by arrows flying >c , , , ;: i.
with the wiftd. 8k * i doa ded - Iky * clouded -
Force, on- the scale 0-12, is indi- |flv overcast sky. A rain falling
cated by number of feathers ^ gnow ^ hai ]^ s fog
Calm
/^S
=mist, T thunder. 1C thunderstorm
Plate XIV.
V-SHAPED DEPRESSION,
DISTRIBUTION OF WEATHER, WIND, AND PRESSURE,
7 A.M. 8th OCTOBER, 1916.
ISOBARS are drawn for intervals of ten mill!- WEATHER. Shown by thefol!owtogyrnboji
bars.
WIND. Direction is shown by arrows flying
with the wind.
Force, on the scale 0-12, is indi-
cated by the number of feathers.
clear iky. Q ky i clouded.
sky \ clouded. ^ sky f clouded.
overcwt sky. f rain falling
snow. A hail. = tog.
Tthnnder. TCthunderiVor
Isobars : Lithoplate XIV. Ill
At times, especially in summer, very small depressions
are apt to form ; several such depressions are sometimes
seen on the weather map for the same day ; they follow
the same laws as large depressions, but being shallow
they do not usually occasion much wind; they bring
heavy rains and thunderstorms in the summer.
An ANTJ CYCLONE,^ or high-pressure system, is the
contrary to a depression ; here the barometer is high in
the centre, the isobars are usually more or less circular
or oval, they are also usually further apart than is the
case in a depression, therefore the winds in an anticyclone
are usually lighter. An anticyclone is not so small as
the smaller depressions, though the large ones may equal
it in size. It often coyers a large area. An anticyclone
moves but slowly and irregularly ; it may remain in the
same position for many days, or even weeks, at a time.
The weather in an anticyclone is usually fine and bright,
though extensive cloud sheets may form, and fogs are
prevalent in winter ; rain seldom falls, and persistent
rain never.
The depression and the anticyclone are the main
arrangements of isobars, but there are five other shapes
each of which has its characteristic weather.
The SECONDARY DEPRESSION. Sometimes in the
neighbourhood of a depression, usually on its southern
side, the isobars take a slight bend outwards, marking the
position of a small centre of low pressure ; such a
depression usually travels forward in the same direction
as the main depression, and may even outstrip it in rate
of travel. It usually produces much rain, and sometimes
much wind. In the summer secondary depressions are
* Itttrstraifttms ~oT~ the various types of isobars are given under the
respective heading's ANTICYCLONE, SECONDARY. &c.
17.8 Glossary : Lithoplate XV.
often very shallow, and resemble the shallow depressions
already noticed ; like them they occasion heavy rain and
thunderstorms. When they are secondary to deep
depressions they often cause a crowding together of the
isobars on their southern side, and thus occasion strong
winds.
The V-SHAPED DEPRESSION. This is a further
extension of the Secondarj T ; the isobars, instead of
bulging out slightly, extend out a long way in the form
of the letter V. The wind on the east side is from a
southerly point, that on the west side from a northerly
point, in accordance with Buys Ballot's law ; the east side
is a region of cloud and rain, often heavy driving rain ;
over the central line there is an abrupt change, very fine
weather being experienced on the west side. As the
Y-shaped depression moves over any place the observer
experiences southerly winds and driving rain, both wind
and rain becoming stronger as the central line approaches ;
as it passes over there is a sudden change of wind to a
northerly point ; the rain stops, and the sky rapidly clears ;
the central line is often a region of heavy squalls ; there
is a marked fall of temperature with the passage of the
central line. See Plate XV.
WEDGE OF HIGH PRESSURE. Between two depressions
there is oiten a region of high pressure where the isobars
are shaped like an inverted V ; the high-pressure wedge
usually extends from an anticyclone poleward between
two depressions that are skirting the northern edge of the
high pressure. The wedge moves forward in an easterly
direction between the two depressions ; it is in fact merely
the relatively high pressure between the depressions.
The front of the wedge is often a region of extremely fine
weather with northerly winds, rapidly becoming light as
Plate
Xl/
SECONDARY DEPRESSION.
DISTRIBUTION OF WEATHER, WIND, AND PRESSURE,
7 A.M. 22nd FEBRUARY, 1015.
ISOBARS are drawn for internals of ten
tan,
WIND. Direction ii shown by arrows flyiaf
with the wind.
Force, on the scale 0-12. is indi-
cated by the number of feathers
/^N
W1AT1IW. -Shown by thetollowlBf symbols -
clear iky. Q iky $ clouded.
jn sky | clouded. <JJ) sky | clouded.
T^ overcast slry. rain falling
jT snow. A hail. = f off.
=mi8t, T thunder. ~K thunderstorm
Plate XVI.
WEDGE.
DISTRIBUTION OF WEATHER, WIND, AND PRESSURE,
7 A.M. 9th DECEMBER, 1915.
ISOBARS are drawn for intervals of five milli- WEATHER. Shown by the following symbol a .
8 ah ft O clear iky. (Qtky $ clouded.
*) tky J clouded. ^Jj) sky f clouded
D overcArt sky rain failing
r< A half. = fo$r.
T thunder. "K thunderstorm
with the wind.
Force, on the scale 0-12, is indi-
cated by the number of feathers,
Calra/^S
Isobars : Lithoplate XVI. 179
the central line approaches ; on the central line there is a
calm ; after its passage the wind backs to the south-west,
clonds begin to come up, and rain usually follows rapidly
as the new depression approaches. After the passage of a
depression, if the weather clears up very rapidly and the
wind falls quickly, it is usually the sign of the approach
of a wedge ; in such a case very fine weather may be
forecasted for a few hours, followed again by bad weather.
" It has cleared up too quickly to last."
The COL. A col is a region between two anticyclones,
and may be likened to a mountain pass between two
higher peaks. Since the wind is blowing round the two
anticyclones in a clockwise direction the col is a region
where light airs from very different directions are brought
into close proximity. This gives conditions for fog in
cold weather, and for thunderstorms in hot weather.
STRAIGHT ISOBARS. Occasionally the isobars run
straight over a very considerable area. In these latitudes
straight isobars usually have the lowest pressure to the
North, and thus in accordance with Buys Ballot's law the
winds are westerly. There may be a great diversity of
weather in a region of straight isobars, for it must be
remembered that the northern side extends to a low-pres-
sure region and the southern side to an anticyclone ; there-
fore in the Northern region we get much cloud and some
rain, in the Southern clear skies and fine weather.
Major Gold, D.S.O., Commandant of the Meteorological
Section, G.H.Q., makes the following comments, which
fairly illustrate the difficulty of making positive state-
ments about the relation of weather to isobars :
Straight Isobars. If the anticyclone is a warm one, the Southern
side, also, gets cloudy skies and rainy weather, see January 6th, 1916,
180 Glossary.
January 4th, 5th, 1914. January 2nd, 10th, llth, 12th, 17th, 18th, 1910,
Perhaps there is a seasonal variation in the weather in straight
isobars. In January they nearly always seem to gret rain right to the
edge of the anticyclone. Straight W. to E. isobars usually mran cool
or moderate temperature in Summer and rather mild in Winter. One
gets also straight isobars running from S. to N. (See January, 1913 :
not much rain but some clond and mist). Thunderstorm weather
when it is very warm to the South.
Also N. to S. straight isobars (January 2nd, 12th. 1911 ; December,
1913: squally, sno * , hail and sleet weather in winter. September
29th. 30th, October 2nd. 3rd, 4th, 1915 ; showery ; thunder in Flanders).
Wedge : when the dominant anticyclone is to the North, it is more
stable than the wedge proper (which is very unstable as a rule) and
gives X. winds on the E. side and E. winds on the W. side.
It is the business of the forecaster who has a weather
map before him to note the arrangements of the isobars, and
the positions of high and low pressure ; he must note
whether the low-pressure systems are main depressions,
secondaries, or V-shaped depressions ; he has to judge
from the map the directions in which the disturbances
are likely to travel, and, knowing the weather which each
kind brings, to warn different districts what wind and
what kind of weather they are to expect. In judging the
direction of travel the meteorologist is guided by certain
rules and by past experience ; a depression usually travels
from, some point between south-west and north-west to
some point between north-east and south-east ; they
frequently skirt the Western seaboard of Europe ; they do
not pass through anticyclones ; when an anticyclone is
situated to the West of these islands depressions do not
come in from the Atlantic, but there is then a tendency
for depressions to pass from North to South down our
Eastern coasts. The meteorologist must also forecast
what temperatures are likely ; in the region of Easterly
winds on the North, side of a depression cold weather is
Isobars. 181
likely, with snow in winter; the approach of a depression
from the west during a frost in winter is sure to bring
about a thaw on the southern side of its path. The endless
varieties of weather must, as far as possible, be foreseen
by the meteorologist with the map before him.
The solitary observer who has no means of making
a map may however recognise some of the signs of the
approach of certain types of weather. Remembering
BUYS BALLOT'S LAW, and watching his barometer, he may
recognise the approach of a depression, and may even on
many occasions roughly plot out its track ; he may often
tell whether the fine weather is of an anticyclonic type, or
whether it is the result of a wedge and therefore only
transitory. In short, if he has a knowledge of the
principles disclosed by weather maps and has a barometer
he will be in a much better position than his neighbours
to forecast the weather from local manifestations.
Isothermal of equal temperature. An isothermal
line is a line of equal temperature, and, therefore, is the
same as isotherm.
Isothermal is frequently used in meteorological writings
on the upper air for the so-called " isothermal layer " by
which is meant the layer indicated in the records of all
ballons-sondes, of sufficient altitude, by the sudden
cessation of fall of temperature with height and generally
by a slight INVERSION (see also GRADIENT) followed by
practical uniformity of temperature. The layer is not
really isothermal. Its temperature on the occasions when
simultaneous soundings have been secured at different
places shows a temperature-gradient in the stricter sense
of difference of temperature at the same level, and an in-
spection of the diagram reproduced under BALLON-SONDE,
182 Glossary.
representing the results of a large number of soundings
in the British Isles, shows that the range of temperature
at the highest layer is greater than the range at the surface.
But it is also clear from the diagram that in each single
sounding the balloon reaches a region where the thermo-
meter ceases to fall. To avoid the misconception which
the use of the word isothermal for this region would imply,
M. Teisserenc de Bort, who was largely instrumental
in its discovery, coined the word stratosphere, while he
gave the name of troposphere to the region below. These
names have -now been generally adopted. The strato-
sphere has also been called the advective region, in
contradistinction with the convective region below it.
We are still without any effective explanation of the
origin of differences of temperature which are found in
the stratosphere ; they must probably be classed among
the most fundamental characteristics of the general
circulation of the atmosphere and among the primary
causes of the changes of weather, but hardly any light has
been thrown on the mechanism of the process.
Katabatic. Referring to the downward motion of air
due to convection. A local cold wind is called katabatic if
it is caused by the gravitation of cold air off high ground ;
such a wind may have no relation to the distribution of
atmospheric pressure. See BORA and BREEZE.
Khamsin. A hot, dry wind which passes over the
Egyptian plain from the southward, forming the front of
depressions passing eastward along the Eastern Medi-
terranean, while there is an area of high pressure to the
of the Kile in Middle Egypt.
Kilometre. 183
Kilometre. A length of one thousand metres,
approximately five-eighths of a mile.
Lake. The water that collects in a hollow or depres-
sion in the land's surface. In meteorology a lake serves
the purpose of a huge rain-gauge, and, subject to some
allowance -for lag and evaporation, indicates the variations
in the collective rainfall of the area which it drains. For
example, the Victoria Nyanza under the equator, apart
from gradual fluctuations of level which in the last
twenty years have followed closely the variations in the
SUNSPOT-NUMBERS (q<v.\ has a seasonal variation which
is connected with the seasonal rainfall of the spring and
early summer and of the late autumn in equatorial Africa.
(see CLIMATIC TABLES).
Land-Breeze. A light wind passing across a coast
line from the land, sea\\ard. It generally begins with
the setting in of coolness in the evening and disappears
with the advance of temperature over the land in the day
time, or is replaced by a sea-breeze, and it is therefore
regarded as a katabatic wind due to convection between
the colder layer over the earth and a warmer layer over
the sea. In sunny weather there is a large diurnal change
of temperature in the land and hardly any in the sea, and
the direction of the wind is regarded as alternating with
the direction of the temperature-gradient along the level.
Lapse, from the Latin lapsus, a slip, a word suggested
for use instead of gradient (which is from gradus a step)
to denote the loss of temperature or pressure of the
atmosphere with height. So that lapse-rate, or lapse-ratio,
for temperature will be the fall of temperature per kilo-
metre of height. A lapse-line will be a line representing
184 Glossary.
the change of temperature with height. The word is
connected with the word labile which means liable to
slip, and applies technically to the peculiar state of equili-
brium of an isentropic, or thoroughly churned atmos-
phere (see ENTROPY). The equilibrium is neither stable
nor unstable, that is to say, if it is disturbed by slow
mechanical process it will, when left to itself, neither go
back to its original state nor go forward, but remain
indifferent, in its displaced condition.
In these circumstances the air will have the greatest
possible lapse-rate of temperature short of instability.
The lapse rate of temperature may change its sign and
indicate an increase of temperature with height. This
apparently always happens in and above a layer of fog,
and not infrequently in and above other forms of cloud.
In that case the lapse-line showing the change of temper-
ature with height will have a slope to higher temperature
upward, opposite from and therefore easily distinguished
from the ordinary slope to lower temperature upward.
We refer to that state as a recovery, instead of using the
term " inversion of temperature-gradient" which is used at
present and is often shortened to " inversion." Generally
speaking, the recovery is only temporary in the journey
upward and is followed by a relapse with perhaps a
different lapse-rate, unless the point has been reached at
which the fall of temperature with height ceases. That
is the boundary between the stratosphere and the tropo-
sphere. We may call that point on the lapse-line the
lapse-limit or TROPOPAUSE.
The shape of a lapse-line is a very important index of
the condition of the upper air ; it has often been deter-
mined by the results obtained with a ballon-sonde. The
normal average shape has a lapse rather less than that of
Lapse. 185
the isentropic atmosphere of saturated air, or almost one
half of the " adiabatic gradient " for dry air.
The investigation of the air near the sea surface during
fog off the Banks, carried out on the u Scotia," has shown
that the effect of the mixing of the air of the surface over
the cold water is to replace the normal lapse-line at the
lower end by' a line which shows a gradual recovery of
temperature from the cold surface to the undisturbed
condition at a kilometre more or less in height. In the
lower, or colder, part the water- vapour is condensed in fog.
It would appear that in a region where convection is
going on over an extended area there must be an isen-
tropic lapse-rate, that the process of the gradual ascent of
warmed air is a gradual formation of a thicker isentropic
layer. An isentropic lapse-rate seems also to be indicated
when mixing takes place in the surface layers owing to
turbulence over water which is not less warm than the
air in contact with it.
Lenticular In shape like a lens or lentil. The
word is used to identify a cloud of characteristic shape
formed by a large mass of clustered cloudlets which is
apparently disposed horizontally, has well-defined edges,
a pointed end and broad middle or base. Sometimes the
cloud becomes thin in the broad part and gives one the
impression of a horizontal bow or horse-shoe of cloud,
foreshortened by being seen from a distant point under-
neath it.
Level. A surface is level if it is everywhere at right*
angles to the force of gravity which is indicated by the
plumb-line. When a table is not level the force of gravity
makes things roll towards the lower edge, but when we
are considering areas so large that the curvature of the
186 Glossary.
earth has to be allowed for, the words higher or lower
have no meaning unless we can refer the heights to some
" level " surface accepted as a datum. The accurate com-
parison of levels in different regions of the earth is a
problem of the greatest refinement and delicacy. Part of
the problem is to determine whether the level of the sea,
apart from any disturbance due to waves, is the same all
over the world or not. For example, it used to be a
debatable question if a cut were made across the Isthmus
of Panama from the Atlantic to the Pacific, whether the
water would flow from the Atlantic to the Pacific or
vice versa. That question has doubtless been solved by
the accurate levelling of the engineers of the Panama
Canal. On land, levels can be set out with great accuracy
by means of a spirit-level, but allowance has to be made
for the curvature of the earth. Land-levels in Great
Britain are referred to the ordnance datum which is the
assumed mean level of the'sea at Liverpool, and is O650 ft.
below the mean level of the sea of the British Isles, and
in Ireland to low water of spring tides in Dublin Bay,
which is 21 ft. below a mark on the base of Poolbeg light-
house. The datum in mariners' charts is usually " low
water ordinary spring tides."
Tidal and river levels in Great Britain are usually
referred to Trinity High Water (T.H.W.) 12'47 ft. above
Ordnance Datum.
Lightning*. The flash of a discharge of electricity
between two clouds or between a cloud and the earth.
A distinction is drawn between " forked " lightning, in
which the path of the actual discharge is visible, and
" sheet " lightning, in which all that is seen is the flash of
illuminated clouds and which is attributed to the light of
a discharge of which the actual path is not visible.
Lightning. 187
Since the introduction of photography many photo-
graphs of lightning have been obtained, and in general
character they cannot be distinguished from photographs
of electric discharges of six inches or more in length
which are obtained in a laboratory, but the varieties of form
of lightning discharges are very numerous. Frequently
a flash shows many branches, especially the upper part
of a flash 'bet ween the clouds and the earth. Among a
collection of photographs thrown upon a lantern screen,
Dr. W. J. S. Lockyer once interpolated a photograph of the
River Amazon and its tributaries, taken from a map, and
the photograph was accepted without comment as a picture
of lightning.
No satisfactory evidence has yet been produced as to
what limits or defines the portion of the atmosphere which
is freed from electric stress by a discharge of lightning,
nor how the path of the discharge is selected.
Lightning-conductors, which are metal rods leading
from the salient points of buildings to conductors buried
in moist earth, have been used since the time of Benjamin
Franklin to protect buildings from damage by lightning.
A good deal of attention was devoted to the method of
operation of lightning-conductors, especially by Sir Oliver
Lodge, whose lectures before the Society of Arts are the
best source of information on the subject.
The chief use of conductors is supposed to be the relief
of stress in the immediate neighbourhood of a building
by the so-called silent or brush-discharges from its exposed
points. These brush-discharges are often visible in snow
storms as discharges from the yards and points of ships
or from an ice-axe and other projecting points in high
mountains. The phenomenon is known as Corposants, or
St. Elmo's fire. For PROTECTION AGAINST LIGHTNING
see p. 325.
188 Glossary.
Line-Squall. A squall of wind, accompanied by rain
or hail, associated \vith a sudden drop of temperature and
the passing of a long line or arch of dark cloud. The
sequence of events as represented on recording meteoro-
logical instruments is one of the most clearly defined and
easily recognised of all types of weather. There is a
sudden rise of the mercury in the barometer by about
2 mb. (less than *1 in.), a veer of wind through about
8 points, a simultaneous fall of temperature as much as
5 to 10C., or 10 to 20F., a sudden squall of wind, some-
times of great violence, lasting for a few minutes.
The sequence of phenomena is represented by the
sketches made by Mr. G. A. Clarke, of Aberdeen
Observatory, with the accompanying records, which are
reproduced in figure 1. The four sketches of the line of
dark cloud are made at intervals of 2 minutes, and the
set represent 6 minutes in the life-history of the cloud.
The line of the squall, which is marked locally by the
line or arch of the cloud, often extends across the country
for hundreds of miles and represents the sudden transition
from a southerly wind to a westerly wind, or from a
southerly type of weather to a westerly type. The cloud
appears to be due to convection between the cold westerly
current and the warmer southerly current ; the squall is
therefore probably katabatic in its origin, and its violence
on the actual passing of the cloud accounted foi in that
way ; it represents the dash forward of a breaking wave,
or more strictly speaking, of the water of a broken wave.
The phenomenon has been studied in the Meteorological
Office during the past 10 years. A short account of the
results of the investigations is given in Shaw's Fore-
casting Weather.
To face p. 188.
LINE SQUALL : FIGURE 1.
LINE-SQUALL AT ABERDEEN! OCT H. i
M* CLARKE.
Line Squall. 189
Line squalls frequently occur at the time of the passage
of the TROUGH of a deep depression when the transition
from southerly wind to westerly wind takes place
suddenly. They also occur as a preliminary to a thunder-
storm, and in such cases the wind of the squall is
sometimes very destructive. They form the most serious
danger to aircraft ; at the same time, their characteristics
lend themselves to forecasting with unusual precision
provided their existence is once identified, because the
line travels across the country with a very definite
velocity.
Special arrangements are therefore made to obtain
notifications of the passage of a line squall over the
stations at the outside edge of our area.
Liquid. The name given to a class of fluids. The
peculiar property of a liquid is that a limited quantity
poured into a sufficiently large vessel forms a definite and
permanent layer with a free surface. Gases can also be
poured from one vessel to another, but unlike liquids the
boundary between the heavy gas at the bottom and the
lighter gas above it is obliterated in time and a complete
mixture of gases results ; with a liquid the well defined
surface of separation remains. Liquids are of all degrees
of mobility from pitch which moves only inches in a
month, through the stages of treacle, and glycerine, which
visibly move, but take time, to water or ether which move
at once on tilting and can be " shaken up."
Low, used to denote a region of low pressure, in the
same way as HIGH is used for the region of high
pressure : a depression. See also ISOBARS.
Lunar dependent upon lima, the moon ; thus a
190 Glossary.
lunar rainbow is a rainbow formed by the rays of the
moon, a lunar cycle a cycle dependent upon the moon's
motion. A month is really, from its name, a lunar cycle,
but the introduction of a calendar month makes it
necessary to draw a distinction between it and the lunar
month, which is the period from new moon to new moon.
In astronomy it is called the synodic month, and is equal
to 29*5306 days. The endeavour to bring the month or
the revolution of the moon round the earth into relation
with the year or the revolution of the earth round the
sun, has given rise to the differences of calendar which
have been or are in use.
Mackerel Sky. A sky covered with cirro-cumulus
clouds arranged in a somewhat regular pattern, and show-
ing blue sky in the gaps. See CIRRO-CUMULUS and
CLOUDS.
Magnetic Needle. A strip of steel permanently
magnetised and provided with an agate cup for balancing
on a point, like the needle of a compass. See COMPASS.
MammatO - cumulus. When low clouds have
rounded projections, or pap-like protuberances, from their
under surface the term mamma to-cumulus is applied to
them. They are appropriate to the disturbed atmospheric
conditions which accompany the close of a thunderstorm.
An example is given in the accompanying illustration,
obtained by Captain Cave at Ditcham Park. They do not
occur often in England and never persist for long. Their
relation to cumulus clouds in the ordinary sense is not
GLOSSARY.
To face p. 190.
Mammato Cumulus (Festoon Cloud) after thunderstorm
in August, 1915.
This photograph is reproduced in illustration of the commotion
which occurs in the atmosphere in the various stages of a thunderstorm.
It may be taken as the sequel to the illustration of the Cumulo-Nimbus
cloud shown in figure 1 reproduced under CLOUD. The cumulo-nimbus
is a thundercloud approaching, the mammato cumulus a thunderstorm
receding.
Mamma to -cumulus. 191
apparent. Both are bulging clouds, but in these the
bulging is downward, while in ordinary cumulus it is
upward.
Mares' tails. A popular term used to describe cirrus
cloud, in which the thread-like filaments are arranged in
the form vof fans or plumes. See CIRRUS.
Maximum. The highest reading of an instrument
during a given period. The context generally shows the
period to which reference is made. An instrument, like
a maximum thermometer, is often designed for the
purpose of giving the highest reading that has occurred
since it was last read. The term " absolute maximum " is
also used, the meaning of which is generally clear from
the context, but see ABSOLUTE EXTREMES.
Mean. The mean value of a set of values is the
number formed by adding all the individual values
together and dividing the sum by the number of values.
In some cases there is an ambiguity unless the context
makes it clear how the values are classified. For example,
the mean temperature of the atmosphere lying over a
certain place might indicate the arithmetical mean of the
temperatures taken at equal intervals of height, or at
equal intervals of pressure, going upwards. See also
AVERAGE.
Meniscus. The curved upper surface of liquid in a
tube. If the tube is of narrow bore the curvature is
pronounced, and in estimating the height of the liquid
column, allowance must be made for it. In the case of
192 Glossary.
water, the meniscus, when viewed horizontally against
the light, appears as a dark belt. The upper edge
represents the highest point to which the water is drawn
up against the glass, and the lower edge the lower part
of the surface out in the middle of the tube. When the
tube is broad like the measuring glass of a raingauge, the
bottom edge should be used in reading ; but in narrow
tubes the mid-point would be more suitable. Mercury
has a convex upper surface and in the case of the baro-
meter the index is adjusted to the top of the meniscus.
Mercury. Mercury is a metallic element of great
value in the construction of meteorological instruments.
In the mercurial barometer its great density enables the
length of the instrument to be made moderate, while the
low pressure of its vapour at ordinary temperatures makes
possible a nearly perfect vacuum in the space above the
top of the barometric column. In the mercury thermo-
meter there is no risk of condensation in the upper end
of the stem, as in the case of the spirit thermometer.
Specific gravity = 13 '5955 at 273a.
Specific heat = 0'0335 at 273a,
Freezing point = 234 *2a.
Meteor. A meteor, or shooting star, is a fragment of
solid material entering the upper regions of the atmo-
sphere from outer space and visible by its own luminosity.
The luminosity is attributed to incandescence due to the
compression of the air in front of the meteor. (See
ADIABATIC.) A larse meteor may leave a luminous trail
that persists for half-an-hour or longer.
Meteor. 193
Accurate determinations of the track of the meteor by
reference to the constellations and of the different posi-
tions of the trail, by observers in different parts of the
country, may enable the height of the streak and the
velocity of the lofty air currents containing it to be
determined. From the results it has been conjectured
that the height to which the atmosphere extends with
sufficient 'density to retard the speed of meteors is
300 k. (188 miles).
Meteorograph. A self-recording instrument which
gives an automatic record of two or more of the ordinary
meteorological elements. Of late the term has been more
generally applied to the instruments that are attached to
kites or small balloons and sent up to ascertain the pres-
sure, temperature and humidity of the upper atmosphere.
Meteorology. The science of the atmosphere. The
word "meteor" from which the name is derived has now
acquired a restricted meaning. It can be, and sometimes
is, used for any atmospheric phenomenon.
Metre. The unit of length in the metric system.
1 metre = 39 '37 inches = 3 '281 feet.
MicrobarogTaph.. An instrument designed for
recording small and rapid variations of atmospheric
pressure. It consists of an airtight reservoir of ample
size containing air, and the difference of the external
atmospheric pressure and the internal pressure in the
reservoir is made to leave a record on a drum driven
by clockwork. The reservoir is well protected from
13204 G
194 Glossary.
changes of temperature by a thick covering of felt or
other non conducting material, and it is also provided
with a small leak, the magnitude of which can be ad-
justed. If the external pressure changes slowly the leak
allows the internal pressure to follow it closely, but as the
leak is small, the internal pressure cannot adjust itself
rapidly to any sudden changes in the external pressure,
and consequently a record of such changes is obtained.
Millibar. The thousandth part of a BAR, which is
the meteorological unit of atmospheric pressure on the
C.G.8. system. Since the " bar " is equal to a pressure of
one megadyne per square centimetre, i.e., to 1,000,000
dynes per square centimetre, a millibar is equivalent to
1,000 dynes per square centimetre.* The millibar has
been in general use in the Meteorological Office since
May 1st, 1914. The principal advantage of using a unit
of this type is that a statement of atmospheric pressure as
a certain number of millibars is perfectly definite. Accord-
ing to the older practice that a separate unit had to be used
for length in reading the height of the mercury in the
barometer, generally the inch or the millimetre, but this
* It should be explained here that a megadyne is a measure of
force. 'Ihe dyne is the unit force of the C.G.S. system of units, and
stands for the force which produces unit acceleration in one gramme.
As the force of gravity is the mo*t familiarly known of all forces, we
may say tha the force of one dyne differs but 'ittle from the weight
of a milligramme, and a megadyne stands in the same relation to the
we : ght of a kilogramme. The precise numerical relation is dependent
upon locality, because the weight of a body, that is, the force which
gravity exerts on it, depends upon latitude and the distance from the
Earth's centre. At sea-level, in latitude 45, the gramme weighs
'6 dynes, the kilogramme 0-9806 megadynes,
Millibar. 195
length is not a measure of the atmospheric pressure until
the density of the mercury, the temperature of the scale
and the value of gravity at the place are allowed for.
The " millibar " on the other hand can only be used for
pressure. If a barometer graduated in the C.G.S. system
is set up at any place, there is a definite temperature called
the fiducial temperature at which the scale reading of the
mercury column gives the pressure of the air in millibars ;
a correction must be applied to the reading when the
temperature of the instrument is not the fiducial tempera-
ture. (See Observer's Handbook.)
1,000 millibars are equivalent to the pressure of a
column of mercury 750*1 millimetres (29*531 inches)
high at C. (273a.) in latitude 45.
Millimetre. The thousandth part of a metre,
25-4 mm. = 1 inch.
Minimum. The opposite of maximum. See MAXI-
MUM.
Mirage. The image of an object which is seen dis-
placed, upwards or downwards, usually vertically, by the
REFRACTION of the rays of light in their passage through
layers of air of different densities near the ground.
Where the density of the layers of air decreases, from the
ground upwards, more rapidly than the normal rate, as it
does when the ground is covered with a layer of very
cold air, the rays of light are bent towards the earth and
the image is therefore seen raised above the object, which
may even be below the horizon at the time.
L3204
196 Glossary.
If the density increases rapidly upwards at the ground,
as it does over highly heated deserts, the rays are bent
upwards and the image is formed below the object.
In its commonest form Mirage has the appearance of a
sheet of water, often surrounded by banks, reeds and
other objects. In this case what appears to be a sheet
of water is the image of the sky behind the object at
which the observer is looking, the rays of light being
totally reflected from the layer of heated air which is
in contact with the ground. The banks, reeds, &c., are
the images of various objects, repeated with more or less
distortion by being viewed through layers of air of
different and varying density, so that a dark stone
appears as though it were an upright stake, or plant, and
so on. Hills situated at a short distance away may appear
as detached masses floating on this lake-like surface, their
lower portions being invisible under the conditions pre-
vailing. In the same way dark stones or gravel capping
a gentle roll of the ground in the desert may present the
appearance of a distant vertical cliff of considerable
height. Mirages may often be seen over smooth road
surfaces on calm hot days in England, especially over
tarred roads. They simulate pools of water on the road-
way in which surrounding objects are reflected.
Mist. Cloud at the level of the ground, consisting of
minute drops of water suspended in the air. Mist occurs
most frequently in the British Isles in the autiynn or
winter, especially in still weather. A calm autumn or
winter night that commences by being clear will usually
become misty towards morning if the air is damp, because
the nocturnal cooling lowers the temperature of the air
below its dew point. At such times hill tops may have
Mist. 197
clear weather, while valleys only a few hundred feet
below are covered with a dense blanket of mist.* In wet
weather, on the other hand, the clouds may be so low as to
cover the hills and produce mist on them while the plains
below experience clear weather. (SCOTCH MIST see
p. 340.)
Mistral. A strong dry, cold wind that is experienced
on the Mediterranean coast of France. It blows from the
north-west.
Mock Sun. An image of the sun, sometimes very
brilliant, that occurs most frequently at a distance from
the sun equal to the radius of the ordinary HALO, i.e., 22.
Mock Sun Ring. A colourless HALO passing through
the sun parallel to the horizon, hence it is also called the
Horizontal Circle. On it are situated most of the MOCK
SUNS.
Monsoon. The term is applied to certain winds
which blow with great persistence and regularity in
opposite directions at different seasons of the year. The
monsoon winds are confined to tropical regions, and are
most marked on the shores of India and China, where
there is a south-west monsoon in the summer months
and a north-east monsoon in the winter months. The
term is also applied to the rainy season of India which
sets in with, and is governed by the monsoon wind that
blows from the south-west or west in the summer on the
south and west coasts of India.
Moon. The only satellite revolving round the earth.
The possibility of its influencing the weather has often
* Se reproduction of a photograph of Valley-fog under CLOUDS.
198 Glossary.
been advanced, but never demonstrated by means of
statistical evidence to the satisfaction of meteorologists.
The brilliance of the moon is due solely to the sunlight
falling upon it. Telescopes show a rugged and clear cut
landscape very different from what would be visible if
the surface were hot enough to radiate a considerable
quantity of heat across a quarter of a million miles to the
earth. One of the many fallacies connected with the
moon is that its rays are injurious to plants, but no doubt
this arose simply because nights of ground frost, harmful
to vegetation, are almost always clear ; and so it happens
that with the moon suitably placed, the damage is done
on the occasions when it is visible and not when it is
hidden by cloud. An explanation of the moon's apparent
influence in scattering clouds is -given in the Quarterly
Journal of the Royal Meteorological Society, vol. 28,
under the title of " La lune mange les nuages," and in
Shaw's Forecasting Weather, p. 175. For a note on the
supposed connexion of the weather with the moon, see
PHASES OF THE MOON.
Nadir. See ZENITH.
NepllOSCOpe. An instrument for measuring the
motion of clouds. A description of the different form^
and methods in use is given in the Observers Handbook.
A Camera Obscura is a very useful form of nephoscope.
Nimbus. Ragged clouds of indefinite shape from
which rain or snow is falling. See CLOUD.
Normal. The name given to the averages of any
meteorological element such as pressure, mean tempera-
ture, maximum temperature, minimum temperature,
duration of sunshine, velocity of wind, taken for a
Normal. 199
sufficient number of cases to form a satisfactory basis of
reference, and thus obtain the difference from normal
which is the excess or defect of a particular example
above or below the normal.
Thirty-five years form a very good period for satis-
factory normals, but shorter periods have to be used if the
figures for 35 years are not available.
The formation of a set of normals for all the stations in
its region is the first duty of a National Meteorological
Institute in respect of climate, and in this respect the
Russian and Indian Governments have set a laudable
example in the Climatological Atlases which they pub-
lished almost simultaneously. In this country we
have published successive editions of normals of instru-
mental observations for 30 Telegraphic Reporting Stations
and for about 150 Climatological Stations (Temperature
and Rainfall), many of which date back for 40 years, and
about 80 Sunshine Stations with records for 30 years.
Monthly maps showing those Climatological normals have
also been published as an appendix to the Weekly Weather
Report, M.O. 214A, Appendix 4. A selection of these
maps is given in The Weather Map.
The non-instrumental observations, as wind, fog, snow,
&c., can also be usefully summarised in the form of
FREQUENCY normals but this is less often done.
Observatories with self-recording instruments furnish
material for an elaborate series of normals which are
most effectively represented by isopleths, of which a
number are given in The Weather Map, pp. 72 to 87.
As an example of normals expressed in figures, we give
the hourly normals for wind velocity at Kew Observatory,
and the monthly normals for a number of stations in
England and France.
200
Glossary.
Table of Normal Hourly Velocities of the Wind in
month at Kew Observatory
Before noon.
Hour.
i
2
3 4
5
6
7
8
9
10
ii
12
Metres per second.
Jan. ...
3'3 3'3
3'3
3'3
3*4
3'4
3*3
3-4
3'5
3'8 !4'2
4'3
Feb. ...
3*3 3*3
3*3
3'3
3*3
3'3
3*3
3'4
3'8
4*i
47
4'9
March ..
3'i 3*i
3-0
3*i
3'i
3'i,
3'3
3'6
4*3
47
5'i
5' 2
April ...
2"? j 2"]
2'5
2'6 \2'6
2'8
3*3
3'8
4*3
4*7 5'o
SV2
May ...
2'3
2'3
2'2
2'2
2'2
2'6
3-2
3-6
4-0
4'3 47
47
June ...
2'1
2'0 . 2'0
I'9
2'I
2-5
3-0
3'3
3-6
3'8 4-2
4-2
July ...
1-9
r8
r8
1-8 r8
2'2
2-6
3-0 j 3-4
37 ; 3'9
4-0
August.
2'0
1-9
i'9
1-9
I'9
2'I
2-5
3'i
3'5
3*8 ,4-1
4-2
Sept. ...
rS
r8
1-9
1-9
r8
I'9
2'I
2-6
3'i
3-5
3'9
3'9
Oct. ...
2-4.
2'4
2- 4
2'4
24 2-5
2-6
27
3-2
3-6
4-2
4'3
Nov. ...
3*o
3'0 3*0
3-0 ! 3*O 2*9
2-9
3-0
3*3
3*4
4-0
4-2
Dec. ...
3-4 s 3-4 ; 3*3
3-4 ; 3-4 3-4
3-4
3*5
3-6
37 4' i
4'3
1
This table was prepared to furnish a reply to a question as to the
best time of year for learning to fly on a machine of small power.
The reply given Dy the figures is that, on the average, the wind is
strongest from 11 a.m. to 4 p.m. in March and April, lightest from
midnight to dawn in June, July, August and September. September,
Normal.
201
metres per second for each hour of the day, for each
(averages for 30 years 1881-1910).
After noon.
24
Day
13
!
14 15 J 16
17
.
18
19
20
21
22
23
Metres per
second.
4'3
4'3
4'9
4'i | 3*8
47 l4'4
4-0
37
3'8
37
37
3-6
3-6
3 ; 4
3*4
3'4
3'4
3'4
3'3
37
Jan.
Feb.
5'2
5'2
5'i 4'9
4' 5
3'9
37
3'5
3*5
3*3
3-2
3'i
3'9
March
5-2
5'2
5'2 5-r
4*8
4*3
3*4
3'3
3-0
2-9
27
3'8
April
4*8
47
47 47
4*5
4'i
3'6
3'i
2-9
2-6
2*5
2'4
3'5
May.
4'2
4'3
4'5 4'5
4*2
3'9
3'4
2-9
27
2-5
2-3
2'I
3'i
June
4'*
4*2
4-1 4-1
3'9
3-6
3*2
27
2'4 2'2
2T
2'0
2-9
July.
4'3
4*3
4*3 4'2
4-0
3*6
3-0
2'6
2-5
2-3
2'2
2'I
3-0
August
4-0
4-i
3'9 37
3'4
2'8
2-5
2'4
2-3
2'2
2'0
i'9
27
Sept.
4'3
4'2,
3'9 3'5
3-i
2-9
27
2'6
2'6
2-6
2'5
2-4
3-o
Oct.
4'3
4-2
3'9 3'5
3'4
3'3
3*3
3'2
3-2
3*i
3-0
3 -0
3*3 Nov.
4'3 4' 2
3'9 37
3-6
3-6
3-6
3'5
3-6
3*5
3'4
3'5
3-6 Dec.
on the whole, is the best month, July the next best. March the worst.
But in view of the seasonal frequency of strong winds shown in the
diagrams on pp. 281 to 285, the answer does not seem complete. For
such questions FREQUENCIES give better answers than NORMALS. A
similar table giving the values for the top of the Eiffel Tower, Paris
(300 metres high), will be found on p. 287.
202
Glossary.
aq^ joj A^IO
"1 9 A UIB8 I\[
C^ M O
CO ^
co O M o
oo vn xn co
xn
O
oo ^ m co
co
CO
"J8qin809(j
xn rt- *^-
5
Cs C^ vn
O
CO
C^ 'Tj- xr> co
CO
"rh
uaqraaAO \^
Tj- O HH
co vb xn
y- p
r^ vo ON
00
o
0> rf- xn w
<N
CO
SKKXPO
W vb : ^t-
p oo
co "O O W
^
a>
^ "^~ xn co
M
w
jaqina^dag
r~ r*" P
in oo
oo rh in ci
xn
wnSny
r-
c<j O
O oo xn g^
oo co in w
s*
g
co m TJ-
vO w
Ar
IJj&lJS;
^ P
it
oo
CO
CO
O vr; O
c
<O O "^ N
t^ ^d" xn co
CO
CO
vp
CO
co xn rf
vo w
AH
r 1 " r* r
co vb ^j~
vb S
co t-> "O vo
OO TJ- xn co
CO
CO
OJO
CO
jTidy
r^ -^- oo
co vb rh
vb w
00 Tj- in rj-
CO
c
'qoj'Bj^
OO P M
co ^o \n
-^- xn
S 2> vb 5-
xn
CO
t
xwJuqe*
i^ m -ri-
co b */">
1C I?
op HH oo xn
CO
"5-
^CjBnur?j
w n p CM
O xn b co
Cl
5
co vb xn
i-* f-1
Averages
for
o o o
000
Q\ o^ Q>
t-- OO
a a
O^i t^ CO O
O O O O
oo
00
8
1 1 !
OO 00 00
OO 00 00
i i
xn o^i
O 00
00 00
i c^ J,
O^ O^ O^> oo
00 00 OO OO
1
oo
1
oo
oo
: s v ,
g : ~ -i
^S^
fe fn *! x
o t>o ** a
FJ 1 1
pL| f_^ <5 J
1 M
^.3
49
i
4
: fl g
' ^ |
1 1
M Q F^
c3
3 ^ 2-
PQ *= o
^ ^fl^g
CJ 8D 33 v^J
I |
1-5 PM
Observer. 203
Observer, in meteorology, is a person who undertakes,
in co-operation with others at a reseau of stations, to make
regular simultaneous records of the weather upon an
organised plan. Good observing requires punctuality and
accuracy, and therefore skill, in reading and setting in-
struments, and intelligence in noting occurrences which
are worth recording, though they are not in the prescribed
routine. The best observer is one who is personally
interested in scientific work on a co-operative basis. The
work of the observers for the Weather Map is partly
represented by the four maps of the gale of December
27-28, 1915, which face Weather Map, p. 88, and which
show also the work of the compiler and map-maker at
the Central Office. The work of the telegraphist is also
necessary, but if that is perfect it makes no show at all
on the maps. If it is not perfect the map at once bears
evidence of the imperfection.
Ombrometer. Another name for RAINGAUGB.
Orientation from Or lens (Lat.), the rising of the sun
the East. The direction of an object referred to the points
of the compass. The meaning is much the same as the
meaning of ' azimuth ' except that the azimuth is more
particularly referred to the meridian line.
The exact orientation of a position is generally best
made out by the aid of an ordnance-map, identifying the
position and the bearings of some prominent landmarks
visible from it. Churches in England are usually oriented
East and West, but the orientation is not always very exact.
Orographic Rain is produced by the forced ascend-
ing currents due to mountains. A horizontal air current
striking a mountain slope is deflected upwards, and the
204 Glossary.
consequent DYNAMICAL COOLING produces rain if the air
contains much aqueous vapour. The dynamics of the
process is not altogether free from difficulty, as the lifting
of the air requires a certain amount of energy. If the
mountains extended to the boundary of the troposphere the
air would presumably go round, and not over the moun-
tain. On the analogy of flowing water we might expect
the air to go round any mountainous obstacle instead of
over it. It does sometimes, but not always.
Ozone is an allotropic form of oxygen for which the
chemical symbol is 3 . It is produced by passing elec-
trical sparks through oxygen, or by the action of cathode
or ultra-violet rays. There is generally at least a trace of
it in pure atmospheric air. The quantity is usually esti-
mated by the depth of the colour of so-called ozone papers
exposed for a given time. Some observers have described
a powerful influence exerted by ozone in increasing the
transfer of electricity between the atmosphere and the
ground.
Pampero. A name given in the Argentine and
Uruguay to a severe storm of wind, with rain, thunder and
lightning. It is a LINE SQUALL, with the typical arched
cloud along its front. It heralds a cool South-Westerly
wind in the rear of a DEPRESSION ; there is a great drop
of temperature as the storm passes.
Parantlielion. A mock sun (see HALO) appearing
on the MOCK SUN RING at about 60 from the ANTHELION.
Paraselenae Mock moons, i.e., images of the moon,
occurring most often at certain points on the ordinary halo
of 22 radius. Like parhelia or mock suns they are
probably formed by the reflection of light from the
surfaces of the snow-crystals in cirro-nebula.
20f>
Parhelia or mock suns. Images of the sun occurring
in connexion with solar halos. See MOCK SUN.
Pentad a period of five days. Five-day means are
used in meteorological work, as five days form an exact
sub-division ( T V rc O of the ordinary year, an advantage not
possessed by the week.
Periodical. Recurring at regular intervals. Periodi-
cal variations of meteorological elements generally have a
period of one day, or one year, corresponding with the
rotation of the Earth and its annual progress round the
sun. Many attempts have been made to identify the
variations of rainfall and other meteorological elements
with a period .of years. Thus the period of the frequency
of sunspots, about 11 years, has been regarded with some
favour, as a meteorological period. A period of 19 years
has been suggested with regard to the climatic elements
of Australia and the wandering of the anticyclones of the
southern tropical belt. Thirty-five years make up the
period which seems to fit in best with the variations of
climate suggested by Briickner in his examination of the
records of rainfall, lake levels, floods, droughts, and other
experiences going back in time as far as possible.
A period of three or four years is indicated for a recog-
nisable oscillation in barometric pressure.
The fluctuations of the yield of the wheat harvest
between 1885 and 1905 wero shown in the Meteorological
Office to be represented with curious fidelity by a combi-
nation of oscillations with a common point of mean value,
so that the crops appeared to repeat themselves numeri-
cally after 11 years.
Professor Turner has suggested that these periods may
possibly be fractional periods of the period of revolution
of the swarm of leonid meteors, which is about 33^ years.
206 Glossary.
Up to now there has not been sufficient material for a
proper critical examination of these various suggestions.
Moreover it is probable that the question cannot be dealt
with separately for a small part of the earth's surface when
the primary causes are external to the earth's atmosphere.
Some method must be found for determining the change
in the atmosphere as a whole, and then perhaps some
particular feature like the trade winds may be found to
form a sort of " pulse " of the circulation and thus act as
an indicator of the atmospheric changes. If so, the
process of examining for periodic changes will be so much
simplified as to offer a very promising field of meteoro-
logical inquiry.
Persistence, a term used by Hon. R. Abercromby, the
repetition of meteorological conditions or what may be
called moods of weather. For several months the distri-
bution of pressure may be of the same general type, with
temporary interruptions. In N.W. Europe cyclones
generally arrive from the Atlantic and pass eastwards,
alternating with relatively high pressure in connexion
with a persistent anticyclone to the south. If the path
along which the centres of the lows travel remains the
same, the alternation of cyclonic and anticyclonic weather
persists. If, on the other hand, the Scandinavian high
pressure develops in area and .intensity, the path of
depressions may be to the southward of our islands, and
easterly weather becomes persistent.
On the other hand, the development of high pressure
over Greenland, extending southward over the Atlantic,
gives a northerly type of weather which is sometimes
persistent for a season.
Persistent rain. For some reason which cannot be
explicitly stated rain generally lasts for only a few hours ;
Persistent rain.
207
looking over the published results for the observatories
of the Meteorological Office for the year 1912 we find the
longest sequences of hours with rain are as follows :
Longest Periods of consecutive hours of rainfall 1912.
Valencia.
Kew.
Eskdalemuir.
Number of
hours.
Number of
hours.
Number of
hours.
January
February
March
April ..;
May .
22 .
19
15
16
T-2
9
6
5
6
24
ii
16'
6
Q
June ...
IQ
9
II
July
12
y
IO
August
September
October
November
December
14
28
9
8
14
9
15
8
6
13
19
29
II
19
At Kew Observatory in the past ten years rain extending
over 25 consecutive hours was recorded three times,
viz., 1906, November 7th-8th ; 1914, March 8th-9th, and
1915, May 13th-14th.
The table does not fully represent all that the inspec-
tion of the published figures suggests, for example, in
August, at Valencia, only one fine hour inttrvened
between two spells of eleven and twelve hours respec-
tively, and at Valencia, too, though April had a longer
spell of rain than March, there were 228 rain-hours in
208 Glossary.
March as compared with April's 47. At Eskdalemuir, a
moorland station in Dumfriesshire, in the last three days
of March only 28 hours out of the 72 were free from rain.
A twelve-hour rain is exceptional at an inland station
like Kew, on the eastern side of Britain, but it does occur
sometimes. In June, 1903, there was a run of thirty hours,
separated by only one rainless hour from a preceding run
of twenty hours, and that again by two rainless hours
from another run of seven hours, so that there was nearly
continuous rain for sixty hours at a stretch. In the same
month Valencia had 110 spell of more than eleven con-
secutive hours of rainfall, though two separate fair hours
broke up a spell of twenty-five hours' rain. The persist-
ence of rain in particular localities on special occasions in
a subject of great interest which is not at all understood.
In it lies the explanation of local floods which are some-
times of the most extensive character, such as those of
East Anglia in August, 1912, and in Eastern Ireland in
August, 1905. Snowstorms are often similarly localised.
In a similar way there is considerable difference in the
succession of days of rain. Sometimes it rains with very
little meteorological provocation, and, on the other hand,
the meteorological conditions which are recognised as
favourable for rain are sometimes productive of very
little. The longest spell of consecutive days of rain at
Kew in the 45 years 1871 1915 was one of 36 days which
occurred in 1892 from September 16th to October 21st.
Two spells in 1891 were nearly as long, viz., one of
31 days from September 26th to October 26th, and one of
34 days from November 7th to December 10th. See
DURATION OP RAINFALL p. 303.
Phases of the Moon : The appearances of the moon
by custom restricted to the particular phases of new
Phases of the Moon. 209
moon, when nothing is visible, first quarter, when a
semicircle is visible, with the bow on the West, full
moon, when a full circle is visible, and last quarter, when
a semicircle is visible with the bow on the East. These
changes of phase are due to the fact that the moon in its
monthly course round the earth, is at one time between
us and the sun, while at another time we are between it
and the sun. In the first case the half of the moon
illuminated by the sun is turned directly away from us,
and it is the period of new moon, in the second it is
directed towards us, and the moon is full. At first quarter
half the side of the moon facing us is lit up, and the
remaining part will gradually become so ; at last quarter
the appearance is similar, but the bright portion is
diminishing.
It is a common practice of immemorial antiquity to
associate changes in the weather with the phases of the
moon ; but it must be remembered that, before the days of
the cheap press and the daily newspaper, the phases of the
moon were the shepherd's calendar, and the only means
of marking intervals of time greater than a day and less than
a year. There are about twelve and a half lunar months
in a year, and the adjustment of the time-keeping of the
moon to the daily and annual periods of the earth is a
scientific question of great complexity and long history.
For conditions of weather that are of too long duration
to be associated with a day, and too short to fill a year,
the phases of the moon afford the only natural method
of time-keeping, so the primary classification of such
events as spells of weather must necessarily count in
phases, or weeks.
Although we have now progressed beyond that stage
and can use decimals in our reckoning, we are not
216 Glossary.
altogether free from ancestral habits. If the sun happens
to be shining we are accustomed to refer to the occurrence
as '*a fine day." It would be regarded as pedantic to
speak of a fine hour or a fine minute, but the fine day is
sometimes over in an hour, and is replaced by a wet day.
A well-known meteorological authority asserts, from
his experience as a sailor, that weather changes set in
with the "turn of the tide." It is possible that there
may be also in this case some association with a single
epoch of events which are really distributed over some
hours, but no statistical inquiry has been made into the
matter.
Phenology. The study of the sequence of seasonal
changes in nature. All natural phenomena are included,
seed-times, harvests, flowering, ripening, migration, and
so on, but often in practice the observations are limited
to the times on which certain trees and flowering plants
come into leaf and flower each year, and to the dates
of the first and last appearance of birds and insects.
A phenological report is published each year by the
Royal Meteorological Society.
Pilot balloon. A small free balloon, the motion of
which gives information concerning the wind currents
aloft. Toy balloons, having a diameter of about J8 inches
when inflated, are often used, but a rather larger size is
preferable ; they are filled with hydrogen and released,
and their progress measured by means of a specially
designed theodolite. If two theodolites are used at some
distance apart the trajectory of the balloon can be com-
pletely determined, but one theodolite may be used if
the rate of ascent of the balloon is known, assuming this
to be uniform. Pilot balloons have shown that the wind,
Pilot-balloon. 211
at a height of 1,500 feet, is usually in close agreement
with the GRADIENT WIND. An East wind is often
shallow, and there is a REVERSAL of wind direction
on many occasions, the upper wind being Westerly ; on
some occasions, however, the East wind is maintained up
to 'great heights, 'though it seldom increases in velocity
above 3,000 feet, at which height an East wind is usually
at its maximum. Winds from other than Easterly direc-
tions may increase up to 30,000 feet or so, but still higher,
when the STRATOSPHERE is reached, there is a decrease
in velocity. In the first 3,000 feet there is usually a
VEERING of the wind with height, whatever the direc-
tion of the wind. Sometimes great changes in direction
occur at various heights ; these are usually veerings.
When this is the case the velocity falls off near the level
of the change, and when the direction is reversed there
is generally a region of calm between the opposite cur-
rents. During the approach of depressions from the
West a Southerly surface-wind changes to a Westerly
wind in the upper air. In anticyclonic weather there is
sometimes very little wind up to the greatest heights
reached, and what little there is varies in direction from
one level to another. Ordinary pilot balloons are some-
times followed to heights of four or five miles ; to reach
greater heights larger balloons must be used, such as those
used for sending up recording instruments (BALLONS-
SONDES).
Instructions for observations with pilot balloons and
for calculating the results are given in the Computer's
Handbook.
Pluviograpll. A self-recording rain-gauge ; the rise
of the water in the gauge is recorded by means of a pen
212 Glossary.
attached to a float. Some form of device by which the
gauge automatically empties itself when the water reaches
a certain height is often employed.
Pluviometer. -A rain-gauge (q.v.).
Pocky Cloud. Cumulus cloud, .with a festooned
appearance on the under side. See MAMMATO-CuMULtrs.
Polar. Occurring in the regions of the north and
south pole. The meteorology of these regions is of a
special type caused by the continuous presence of the sun
above the horizon during a long period of the year, and
its absence during an equal period. This makes the
diurnal range of temperature small. The seasonal change
on the other hand is large, extremely low temperatures
during the long night being followed by less severe
cold in the time of continuous daylight, when the thermo-
meter often rises above the freezing point. The snowfall
has always been found to be moderate ; the great quantity
on the ground represents the accumulated fall of many
years because there is no loss except by evaporation and
that is very slight at the very low temperature. Aurorae,
as well as optical effects due to the presence of ice crystals
in the air, are of common occurrence. In the south polar-
regions a spot has been found where the normal velocity
of the wind is beyond the limit of gale. Both poles are
separated from the equatorial regions by a great circum-
polar whirl of prevailing westerly winds, and regions of
relatively low pressure. These features are especially
marked in the southern hemisphere.
Pole. The geographical poles lie at the extremities of
the axis of rotation of the earth. The magnetic poles are
at some considerable distance from the geographical poles.
Potential. 2i;>
Potential as applied to energy indicates the energy
which is due to the position of a body. In considering
the total amount of energy available, in any case we must
consider not only the position but the quantity of work-
ing substance that is collected there. If we wish to
consider the influence of the position alone we must
limit our ideas to a particular amount of the working
substance. We naturally choose the unit measure as the
amount for this purpose, and the potential energy of unit
quantity is called the potential at the point. Thus, the
electrical potential at any point in the atmosphere is the
amount of energy which one unit of electricity possesses
in virtue of its position at the point. Similarly, the gravita-
tional potential or geo-potential at any point above the
earth's surface is the potential energy of a unit quantity
of material, a gramme or a pound, placed there.
Potential temperature. The temperature which
a specimen of air would acquire if it were brought down
from the position to mean sea-level under ADIABATIC
conditions.
Precipitation. See p. 329.
Pressure. Force per unit of area exerted against a
surface by the liquid or gas in contact with it. The
pressure of the atmosphere, which is measured by means
of the barometer, is produced by the weight of the over-
lying air. The pressure exerted by the wind is -generally
very small in comparison. That due to a wind of force
6 is approximately one-thousandth part of the pressure of
the atmosphere.
Prevailing: Winds. When a station experiences
wind more often from a certain direction than from
others, that wind is termed the prevailing wind, The
,'214 Glossary.
best example is the trade wind, which blows from the
N.E. at many places between the equator and 30N. Lat.
with great regularity, and from the S.E. in the correspond-
ing belt south of the equator. In latitudes 40 to 60,
north or south of the equator, westerly winds are very
common and form circumpolar whirls. These are best
developed in the southern hemisphere, where there is less
land. For England and the neighbouring portion of
Europe the best guide to the prevailing winds in different
parts is the average distribution of pressure shown in the
maps of Mean Pressure in the Monthly Weather Report
and its Annual Summary. The prevailing direction is
between S.W. and W. : there is more southing in the
western districts than in the eastern. In monsoonal
regions the prevailing winds are in opposite directions at
different seasons, and in others there may not be a
favoured direction. In all cases a long series of observa-
tions is required to make sure of the normal conditions.
Probability. When the occurrence of an event is
apparently doubtful, but under similar circumstances has
happened before, more often than not, we speak of it as
" probable." Mathematically, the probability is repre-
sented by the fraction obtained by dividing the number
of times that the event has happened by the total number
of times that the circumstances have arisen. It is a
quantity that must lie between and 1, and is usually
denoted by the letter p. It should be clear from the
definition that the probability of the event failing to
happen, when added to the probability that it will happen,
must make 1. For example, we may consider the prob-
ability that to-morrow will be fine if to-day is wet. Out
of a number N of occasions of wet days we count the
Probability. 215
number n when the following day was fine, and the
probability of a fine day to-morrow is njN. It may be
called the random probability because the occasions are
simply chosen at random without any guiding principle.
It is, of course, necessary to deal with a large number of
observations before a reliable value for the probability is
obtained.
It will -be noticed that in this sense probability is
directly determined by the frequency of occurrence, so
that the facts represented in tables of frequency can be
equally well represented by tables of probability.
When we have no guide as to the expectation of an
event except the number of times that a similar event has
occurred previously, and we express the probability as a
fraction 1/n with 1 as numerator, we may say that the
random chance of the occurrence is one in n or that the
odds against the occurrence are n-\ to one.
Prognostics. Signs of coming weather. Some of
them are dealt with under SHEPHERD OF BANBURY and
WEATHER MAXIMS. There is a widespread belief that
certain animals are in some way aware of the approach
of wet weather, and behave in some special manner in
consequence, but this seems unlikely. There is no reason
for supposing that they can feel anything beyond the
changes of temperature, moisture, wind, &c., in the air
surrounding them, and changes in these are followed by
a great variety of weather. Perhaps the most valuable
instrument for pi ognostication is the barometer. Regions
of high and low pressure have fairly definite weather-
associated with them, and mostly move across our islands
from the west or south-wes"t. A southerly wind, there-
fore, with a pronounced fall of the barometer, as an
216 Glossary.
example, is a fairly reliable indication of an advancing
depression, and therefore of rain. A gradual dimming of
the clear blue of the sky, and the formation of a thickening
sheet of high cloud, which often forms halos round the
sun and moon, shows that the stormy area is not far off.
By the application of BUYS BALLOT'S LAW (q.v.), it can
be seen that the wind will generally veer towards west if
the low pressure is going to pass on the north side of the
obseryer, but will back if it is going to cross on the
south side. In fine weather the wind often drops at
night on the ground, while continuing to blow a few
hundred feet up. It follows, therefore, that brisk motion
of the lower clouds on a still sunny morning indicates a
wind which may be expected down below during the
middle of the day.
PsychrOHieter, the name given to the dry and wet
bulbs as forming an instrument for measuring coolness ;
the combination of a thermometer having its bulb coated
with wet muslin and an ordinary thermometer used for
estimating the dampness of the air, by observing the
difference between the readings of the tw 7 o thermometers.
In dry air evaporation takes place freely and cools
the wet bulb. The cooling for a given temperature of
the dry bulb depends principally upon the dryness of the
air, or its absorbing power for moisture. In the aspiration
psychrometer a fan is used so as to produce a draught of
definite speed past the instrument.
Pumping*. Unsteadiness of the mercury in the
barometer caused by fluctuations of the air pressure
produced by a gusty wind, or due to the oscillation of a
ship.
Purple Light. 217
Purple Light. A parabolic glow of colour varying
from pink to violet appearing vertex upwards in the western
sky at a considerable elevation above the point of sunset after
the sun has passed below the horizon. It is a DIFFRACTION
glow similar to the white glow round the sun during the
day, which is caused by the interference of light scattered
by particles of many sizes ; as the sun sets, its light
reaches only the more uniform particles of the upper
atmosphere, and the coloration becomes purer, culminating
in the coloration of the margin. In very clear weather a
second, fainter purple light may follow the first. (See
also BLUE OF THE SKY and TWILIGHT).
PyrheliO meter. An instrument for measuring the
radiant heat received from the sun. In the form of
instrument devised by Angstrom there are two metal
strips, one of which receives the solar heat, while the
other is warmed by means of an electric current.
The current required to give equal heating of the two
strips depends upon the intensity of the sun's rays, and
when measured gives the amount of heat received.
Radiation. See p. 330.
Rain is produced by the condensation of the aqueous
vapour in the atmosphere. Each cubic foot, or cubic metre,
of air is capable of holding a certain definite amount of
water in the form of vapour ; the amount depends greatly
. upon the temperature, being large when the temperature
is high, and small when it is low. The ^ater vapour is
mixed with the air in varying proportions, and when the
temperature of the mixture falls sufficiently a point is
reached where the vapour is condensed into fine particles
218 Glossary.
of water, and a cloud is formed. As the cooling continues
more water is condensed to form larger drops which
fall as rain. The cooling which produces rain is probably
dynamical. See ADIABATIC and PERSISTENT RAIN : also
DURATION OF RAINFALL, p. 303, and RAINDROPS, p. 334.
Rainband. A dark band in the solar spectrum on the
red side of the Sodium D lines, due to absorption by
water vapour in the Earth's atmosphere. It may be best
seen when the spectroscope is pointed at the sky rather
than directly at the sun. The band is strengthened with
increase of water-vapour, and also when the altitude of
the sun is low, and his light has to shine through a greater
thickness of air. It is of doubtful value as a PROGNOSTIC
of rain.
Rainbow. A rainbow is seen when the sun shines
upon raindrops, or indeed upon spherical drops of water
produced by a waterfall or by any other means. The
drops may be at any distance from the observer, but the
centre will always be exactly opposite the sun, and the
angular diameter of the circle for each colour is invariable.
When sunlight falls upon a drop of water it is reflected
and refracted in all directions, but there are certain
directions in which the light is much more intense thail
in others. An observer therefore looking at the drops in
general will see some of them much better than others,
and those drops which show up will lie in that particular
direction in which they reflect the greatest amount of
light. But the particular direction is different for each
colour, and hence the rainbow consists of a series of rings
of different colours. A similar explanation applies to the
secondary bows. In the primary bow the red is outside.
A rainbow is usually circular, the head of the observer
Rainbow. 219
being the centre of the circular arch, but a horizontal rain-
bow can be seen when the sun's rays from behind the
observer fall on drops of dew on the grass, or gossamer
threads of a meadow. In that case the bow is not circular.
Rain-day. A day on which more than a certain
specified amount of rain has fallen. It has been usual in
the past to measure rainfall in hundredths of an inch, and
01 inch has been the specified quantity, a day on which
005 inch, or more, fell, counting as a rain-day.
Rainfall water which falls from the atmosphere.
The term is very commonly taken to include snow and
hail. Precipitation is the proper inclusive term.
The measurement of a definite amount of rain, say,
fifteen millimetres, 15 mm., means that if all the water
had remained where it fell and not soaked in or run off,
the depth of water on the ground would be 15 mm.
An inch of rain is equivalent to 101 tons per acre ; a
millimetre to a kilogramme per square metre, Or one
thousand metric tons per square kilometre.
Raingauge. An instrument for measuring the rain-
fall. All the rain which falls on a definite area, generally
a circle of either five or eight inches in diameter, is
collected into a glass vessel, which is graduated to give
the amount of rain.
Rain-spell* According to the definition of the
British Rainfall Organization, a rain-spell is a period of
more than fourteen consecutive days, every one of which
is a rain-day. On a general average, one or two such
periods fall to the lot of most stations in the British Isles
within the year.
Reaumur Rene Antoine Ferchault de, d. 1757,
220 Glossary.
whose name is given to a scale of temperature now almost
obsolete. On it the freezing point of water is zero, and
the boiling point 80.
Reduction, as applied to meteorological observations,
generally means the substitution for the values directly
observed of others which are computed therefrom and
which place the results upon a comparable basis. Thus
reduction to sea-level in the case of barometer readings,
means estimation according to certain rules of the value
which the pressure would have at a fixed level lower than
that of the place of observation, and the reduction of a set
of mean values extending over a regular series of years
to a uniform or normal period indicates a similar procedure
based upon comparison with neighbouring stations.
Reduction to Sea Level. Both temperature and
pressure are " reduced to sea level " before they are
plotted on charts. To reduce mean temperature to sea
level 1 F. is added for each 300 feet in the elevation of
the station ; la for 165 metres ; other rates are used for
maximum temperature and minimum temperature (see
Computer } s Handbook, Introduction, p. 1 L). This reduc-
tion is regarded as necessary in forming maps of
ISOTHERMS of regions with a considerable range of level,
otherwise the isotherms simply reproduce the contours ;
but it reduces the practical utility of the maps because
the addition of ten or twelve degrees to the temperature
actually observed gives an entirely false idea of the actual
state of things in the locality represented.
The same objection does not apply to the reduction of
pressure to sea level because the human organism has
To face p. 221.
DIAGRAM for obtaining HEIGHT DIFFERENCFS Iron pRtssuwr-
PifTEKtNCES lor different TEMPERATURE ol tt,* an -column from tbe
formula
H H. =. 0674 7 log* ?
where H is the height m kilometres r is the temperature on the
absolute scale, and p is the pressure measured in am units, but pre-
ferably in millibars H. and f>, aie corresponding values of H and /> at
ny definite height, usually ground level
The process ol um the diagram u z> follow* -
C 1
Tbere are two pcotncton lot temperature
ttkmebe o lie^ht u reprewntod by cm (X o
1Z-S cat (ft of tbe luge d
For urotractoi A I kilomrtrr is represented by 2 large division:
<ENCEb for protractor B I kilometre is represented by 6 large divisions
Reduction to Sea Level. 221
no such separate perception of pressure as it has of
temperature.
The reduction of pressure to sea level is carried out in
accordance with the general rule for the relation of
difference of pressure to difference of height. This goes
according to the equation
h - h, = KT (Iog 10 Po - Iog 10 p)
where //, }>, h , p Q are corresponding values of height and
pressure, T is the absolute temperature, arid k, a constant
which is numerically equal to 67*4 when the height is
to be given in metres, or to 221*1 when the height is
to be given in feet. This equation is derived from the
direct expression of the relation of pressure and height
g p dh = dp.
The best way of working the equation is to use what is
called semi-logarithmic paper, that is squared paper which
is ruled in one direction in equidistant lines representing
equal steps of height, and in the other direction according
to the logarithm of the numbers indicated on it, like the
graduation of a slide-rule. The relation between height
and pressure for any one temperature is represented by a
straight line that has a slope that can be calculated when
k is known. The regular course is to find the proper
point for the height and pressure of the starting point,
and travel along the line of proper slope for the tempera-
ture so long as that temperature can be accepted, say for
half a kilometre. When the half kilometre is reached
adjust the slope for the mean temperature of the next half
kilometre, and so on until the observed difference of
pressure has been traversed. If the line reaches the
boundary of the ruled paper, the vertical line or the
222 Glossary.
horizontal line as the case may be, begin again on the
other side of the ruling.
It will be seen that to make an accurate determination
of the height the temperature of each stage must be
known. If there are no actual measurements for the
occasion an approximation may be made by using mean
values either for the initial temperature or the lapse of
temperature with height.
To simplify the process of obtaining the height differ-
ences from pressure differences semi-logarithmic paper is
provided at the Meteorological Office ruled with the slope
lines for given values of the temperature, so that with
a parallel ruler the composite line for any particular
determination can be easily drawn. A reduced copy of
the form is shown in the figure.
The humidity of the air makes very little difference
to the computation of height in our latitudes where
temperature and moisture do not reach tropical figures.
The best way for allowing approximately for humidity,
which diminishes the density under standard conditions,
is to regard the temperature as increased by one tenth of
a degree for each millibar of water- vapour-pressure in the
atmospheie.
Refraction. The name applied to the bending to which
rays of light are subjected in passing from one medium to
another of different optical density. It plays an important
part in many optical phenomena in the atmosphere ;
MIRAGE, HALOS, and RAINBOWS are refraction pheno-
mena, the colours of the two latter being due to the fact
that rays of different colours suffer a different amount of
bending. Another refraction effect is that the apparent
ALTITUDE of a heavenly body is greater than its real
Refraction. 223
altitude because the rays of light entering the atmosphere
are passing from a less dense to a more dense medium,
and their final direction is nearer the vertical than their
original direction.
Registering balloon. A small free balloon carrying
with it a light meteorograph and sent up to ascertain ihe
temperature, humidity, &c., of the air. See BALLON
SONDB.
Regression Equation. See p. 339.
Relative Humidity. All natural air, unless it is
artificially dried, contains more or less water in the form
of vapour. For each temperature there is a fixed and definite
limit to the amount of water in a definite volume of air, such
as a cubic foot or a cubic metre. Air which contains this
full amount is called saturated air. The actual amount that
can be present in a given volume depends on the tem-
perature, and increases rapidly as the temperature rises.
The relative humidity is the ratio of the amount that is
present to the maximum amount that could possibly be
present. This ratio is expressed as a percentage, so that
saturated air, at whatsoever temperature it may be, always
has a relative humidity of 100. Thus a relative humidity
of, say, 75 means that a certain volume of air is holding
in the form of vapour 75 grammes or ounces of water,
whereas it is capable of holding 100 grammes or ounces.
When saturated air is cooled by any means it ceases to be
able to hold all the water in an invisible form, and fine
water drops appear forming a fog or cloud.
From an analysis of upwards of 100,000 hourly readings
at the observatories of the Meteorological Office during
the three years 1907, 1908, 1909, it appears that on the
average out of one thousand observations at every hour of
Glossary.
the day or night throughout the year the frequencies of
specified values of relative humidity are as follows :
TABLE OP FREQUENCIES OP OCCURRENCE OF SPECIFIED
VALUES OF RELATIVE HUMIDITY REFERRED TO A
TOTAL OF ONE THOUSAND HOURLY OBSERVATIONS.
Relative Humidity.
Aberdeen.
Valencia.
Falmoutk.
Kew.
Frequency.
IOO ...
i
14
0'2
I
95 to 99
45
186
104
76
90 to 94 ...
150
191
213
171
80 to 89
378
335
3^8
3*1
70 to 79
270
217
^34
206
60 to 69 ...
119
5i
I O2
135
50*059
33
5
18
71
40 to 49
4
i
I
17
30*039
O'2
o
O
2J
Total
IOOO
IOOO
IOOO
IOOO
See also ABSOLUTE HUMIDITY, p. 290.
Reversal. A large change (more than 90) in direction
between the surface current and the wind in the upper air.
Reversals are most common with Easterly surface winds, and
least common with Westerly. A reversal may take place
quite close to the ground, or anywhere up to 15,000 feet or
more. The most permanent case of a reversal is over the
TRADE WIND, where the North-Easterly surface current is
replaced by a South- Westerly current in the upper air.
Reversal. 225
When an eruption of the Sou Mere in St. Vincent takes
place, the dust, carried by the upper current, falls in
Barbados, though it lies 100 miles to windward of St.
Vincent (see PILOT BALLOON).
Ridge. An extension of a " high " area shown on a
weather chart, corresponding to the ridge running out-
wards from a mountain system. It is the opposite of a
trough of low pressure.
Rime. Ice crystals, like small needles, which form on
trees and buildings in foggy, frosty weather. The needles
point to the direction of the wind, and, in favourable
situations, the summit of Ben Nevis for example, may
accumulate and form large and heavy masses of ice.
River. Geographically a river is simply the flow of
water from the higher levels of the land to lower levels
and is thus, in meteorology, only part of the great circu-
lation of water through evaporation and condensation ;
but, from the point of view of climate, the variations of
river-level are interesting and important as they represent
the result of meteorological causes operating over a large
region. The seasonal variation is often different from
what might be expected, for example, the River Thames
is at its highest in February, four months after the normal
period of greatest rainfall. The great historic example
of seasonal variation of river-flow is that of the Nile, upon
which the fertility of lower Egypt depends; its annual
rise begins at Assuan in June and reaches its maximum in
the beginning of October. The Tigris and Euphrates, like-
most Continental rivers, show their rise in the spring with
the melting of the snow in the regions of the head waters.
13204 H
See CLIMATIC SUMMARIES appended to The Weather
Map.
Roaring Forties. The belt between latitudes 40
and 50 South latitude,' characterised by prevailing
boisterous Westerly winds.
St. Elmo's Fire. Brush-like discharges of electricity
sometimes seen on the masts and yards of ships at sea
during stormy weather ; it is also seen on mountains on
projecting objects. It may be imitated by bringing a
sharp pointed object, such as a needle, near a charged
Leyden jar.
Saturation. When applied to the air this term
indicates that all the moisture possible as water vapour at
that temperature is present. A reduction of temperature
would lead to the condensation of some of it to liquid
drops, while a rise of temperature would make the air
" dry " and enable it to take up more.
Screen. In order to allow thermometers to indicate
as nearly as possible the temperature of the air, and
therefore to avoid the disturbing effect of the sun's rays
and of neighbouring objects, louvred screens are used
which allow of a free circulation of the air. The pattern
used at the Meteorological Office observatories is a modi-
fication of that designed by Thomas Stevenson, C.E., one
of the founders of the Scottish Meteorological Society.
It is known in this country as the Stevenson screen, on
the continent as the English screen.
Scud. A word used by sailors to describe small
fragments of cloud that drift along underneath nimbus
clouds. The meteorological term is fracto-nimbus
(Fr. Nb.). See CLOUDS.
Scua. 227
In mountainous districts, after the passage of a depres-
sion, nimbus often breaks up into scud, which may
persist with sunny weather for many hours.
Sea-breeze. A breeze that blows from the sea during
the day in fine weather and drops at night (see BREEZE).
In bright weather the warmed air over the land-surface
rises, and there is an inrush of the cooler air from the sea
to take its place ; at night, when the temperatures are
more or less equalised, the wind dies away.
Sea-tevel. The level surface which the sea would
have if the waves were smoothed out. Mean sea level
(M.S.L.) is the mean position occupied by this surface
during the whole year. In England M.S.L. is an arbitrary
level at Liverpool. See LEVEL.
Seasons. In meteorology the seasons are taken to be
as follows :
1. Spring ... March, April, May.
2. Summer ... June, July, August.
3. Autumn ... September, October, November.
4. Winter ... December, January, February.
If an element is described as having simply a seasonal
variation, it implies that it goes through its changes in a
period of one year.
The selection of months to represent the seasons accord-
ing to the farmer's year is guided by the consideration
that each season shall comprise three months. The
uniformity in length opens the way for some paradoxical
cases. The warmest week of summer may be in the
spring, late May, or in autumn, early September, and
the coldest week of winter may be in the autumn, late
November, or spring, early March. From the point of
view of weather, we have in this country about five
months of moderate winter weather between October and
13204 H 2
228
Glossary.
April, and four months of summer weather from the
middle of May to the middle of September, a short
spring and a short autumn ; but the seasonal variations
are not nearly so large here as they are in continental
countries, and the change from winter to summer and
vice verm is much less abrupt.
NORMAL TEMPERATURES AT SEA-LEVEL OP EACH WEEK
OF THE SEVERAL SEASONS FOR ENGLAND SOUTH-EAST.
Week No.
Winter.
Spring.
Summer.
Autumn.
a
a.
a.
a.
i
278
278
287
288
2
78
79
87
87
3
78
79
88
87
4
77
80
89
86
5
. 77
80
89
85
6
77
81
89
84
7
77
81
89
83
8
77
82
89
82
9
78
83
89
82
10
78
83
89
81
ii
77
84
89
80
12
77
'85
8q
- 79
13
77
86
88
79
Mean
277
282
289
283
RAINFALL IN MILLIMETRES.
mm.
mm.
mm.
mm.
England, S.E.
172
131
1 60
216
Scotland, N.
413
251
268
396
Seasons. 229
The seasonal changes for England South-East, are set
out in the accompanying table of normal temperatures
for each week of the year, to which the seasonal rainfall
of Scotland North, as well as of England South-East, has
been added. The temperatures are given only to the
nearest whole degree of the absolute scale, so that minute
differences are not apparent.
If we allow for winter the temperatures 277, 278, 279,
i.e., from 39 F. to 43 F., and for summer the temperatures
287, 288; 289, i.e., from 57 F. to 61 F., we see from the
table that winter temperatures last from the 12th week of
auturdii (middle of November) to the 3rd week of spring
(middle of March) ; then come ten weeks of slow tran-
sition, a degree each fortnight, to summer temperature in
the first week of summer (beginning of June) ; the
summer temperatures last until the third week of autumn,
about the 21st September, then there are eight weeks of
rapid transition of a degree each week until winter temper-
ature is reached in the middle of November.
Some interesting particulars of the temperature of the
several seasons in the British Isles are given in Tempera-
ture Tables of the British Isles, M.O. Publication 154
(1902). Diagrams are given for the daily temperature at
four observatories : Aberdeen, Valencia, Falmouth and
Richmond (Kew Observatory), and they show a lag of
temperature behind the Sun very similar to that noticed
in the diurnal changes of temperature which are figured
in the same volume.
For rainfall in the South-East of England autumn is the
rainy season with 216 mm., compared with 131 mm. in
the spring, while in the North of Scotland winter is the
rainiest season though spring is again the driest.
In considering the seasonal variations of rainfall it is
230
Glossary.
important to distinguish between the day rainfall and the
night rainfall. We may contrast the two from the
summary of forty years' observations at Kew Observatory.
Average Daily Rainfall.
A.
M.
P.M.
Midt. to
6 a.m. to
Noon to
6 p.m. to
6 a.m.
Noon.
6 p.m.
Midt.
mm.
mm.
mm.
mm.
January...
0-37
0-37
0-36
0-36
February
0-38
*35
'34
0-29
March ...
0-31
0*30
0-32
'34
April
*33
0-34 -
0-39
0-30
May 0-34
'35
0*42
0-26-
June ... ... ... i 0*43
Q'45
'54
0-51
July 0-39
0*42
0-67
0-48
August ... ... ... ; 0*36
September 0-49
0*42
0-37
0*59
o*43
0-42
o 46
October ...
November
December
o-59
0-49
0*46
0*40
0-38
0-60
0-48
0*41
0-50
0-49
0-40
It will be seen that there is a maximum in October for
each of the four quarters of the day, but the October
maximum for the afternoon and for the evening is a sub-
sidiary one. The maximum for the whole year belongs to
the afternoon in July, when the figure 0*67 is reached.
The reader is recommended to draw " isopleths " on
this table, that is, lines of equal rainfall, *60 mm., *55 mm.,
50 mm., *45 mm., *40 mm., *35 mm., and '30 mm. He
will find the grouping of the rainy and of the dry parts
of the day in different months of the year very suggestive.
Seasons. 231
The idea of four seasons in agriculture appropriate to
these islands, the winter for tilling, the spring for sowing
and early growth, the summer for maturing and harvest-
ing and the autumn for clearing and preparing, depends
upon the peculiarity of our climate. Where the land is'
ice-bound in winter or rainless in summer another dis-
tribution has to be made.
Between the tropics there is nothing that can properly
be called summer and winter ; the seasons depend upon
the weather and rainfall, and not upon the position of the
sun, and the periods of growth are adjusted accordingly.
In India, or the north-western part of it, the divisions of
the year are the cold weather, the hot weather, and the'-
rains.
It is curious, for example, that the period for growing
wheat in Western Australia is locally the winter period,
and coincides in actual time with our own, which is a
summer period.
Secant. In a right angled triangle the ratio of the
hypotenuse to one side is the secant of the angle between
the two. See SINE.
Secondary. A small area of low pressure accom-
panying a larger " primary " depression. The secondary
may develop into a large and deep cyclone, while the
primary disappears. See ISOBARS and Plate XII.
Seismograph.. An earthquake recorder, or instru-
ment for automatically recording the tremors of the earth.
Serein. Fine rain falling from an apparently clear
sky. It happens very rarely.
Sliamal. From an Arabic word originally meaning
u left-hand " and thence 4k North " used to denote the
North-Westerly winds of summer over the Mesopotamian
plain. See The Weather Map, p. 60.
232 Glossary.
Shepherd Of Banbury. The nominal author of
" rules to judge the changes of the weather." The
following is taken from u The Complete Weather Guide,"
by Joseph Taylor, 1814 :
" Who the shepherd of Banbury was, we know not ; nor indeed
have we any proof that the rules called his were penned by a real
shepherd : both these points are, however, immaterial : their truth is
their best voucher. Mr. Claridge (who published them in the year
1744) states, that they are grounded on forty years' experience, and
thus, very rightly, accounts for the presumption in their favour.
' The shepherd,' he remarks, ' whose sole business it is to observe what
has a reference to the flock under his care, who spends all his days,
and many of his nights in the open air, under the wide-spread canopy
' of Heaven, is obliged to take particular notice of the alterations of
the weather ; and when he comes to take a pleasure in making such
observations, it is amazing how great a progress he makes in them,
and to how great a certainty he arrives at last, by mere dint of
comparing signs and events, and correcting one remark by another.
Every thing, in time, becomes to him a sort of weather-gage. The
sun, the moon, the stars, the clouds, the winds, the mists, the trees,
the flowers, the herbs, and almost every animal with which he is
acquainted, all these become, to such a person, instruments of real
knowledge.' "
The rules enumerated are typical of all rules based on
experience of the weather ; what of truth or error there is
in them the reader may judge ; they are as follows :
I, SiTN. If the sun rise red and fiery Wind and rain.
II. CLOUDS. If cloudy, and the clouds soon decrease Certain
fair weather.
III. Clouds small and round, like a dapple-grey, with a north-wind
Fair weather for two or three days.
IV. If small Clouds increase Much rain.
V. If large Clouds decrease Fair weather.
VI. In Summer or Harvest, when the ivind has been South two or
three days, and it grows very hot, and you see Clouds rise
with great white Tops like Towers, as if one were upon the
Shepherd of Baribury. 233
Top of another, and joined together with black on the nether
side There will be thunder and rain suddenly.*
VII. If two such Clouds arise, one on either hand It is time to
make haste to shelter.
VIII. If you see a Cloud rise aga'ulst the Wind or side Wind, when
that Cloud comes up to you The Wind will blow the same
way that the Cloud came. And the same Rule holds of a
clear Place, when all the Shy is equally thich, except one
Edge.
IX. MIST. If Mists rise in low Grounds, and soon vanish Fair
Weather.
X. If Mists rise to the Hill-tops Rain in a Day or two.
XI. A general Mist before the Sun rises, near the full Moon Fair
Weather.
XII. If Mists in the New Moon Rain in the Old.
XIII. If Mists in the Old Rain in the New Moon.
XIV. RAIN. Sudden Rains never last long : but when the Air
grows thick by degrees and the Sun, Moon and Stars shine
dimmer and dimmer^ then it is like to rain six Hours
usually.
XV. If it begin to rain from the South, with a high Wind for two
or three Hours, and the Wind falls, but the Rain continues,
it is like to rain twelve HoiJrs or more, and does usually
rain till a strong North Wind clears the Air. These long
Rains seldom hold above twelve Hours, or happen above once
a year.
XVI. If it begins to rain an Hour or two before Sunrising, it is
likely to be fair before Noon, and to continue so that day ;
but if the Rain begins an Hour or two after Sunrising, it
is likely to rain all that day, except the Rainbow be seen
before it rains.
XVII. WINDS. Observe that in eight Yeari Time there is as much
South- Wext Wind as North- East, and consequently as
many wet Years as dry.
XVIII. When the Wind turns to North-East, and it continues two
* See photograph of Cumulo-Nimbus under CLOUDS.
234 Glossary.
Days without Pain, and does not turn South the third Day,
nor Pain the third Day, it is likely to continue North- East
for eight or nine Days, all fair ; and then to come to the
South again.
XIX. After a Northerly Wind for the most part of two Months or
more, and then coming South, there are usually three or
four fair Days at; first, and then on the fourth or fifth
Day conies Rain, or else the Wind turns North again, and
continues dry.
XX. If it turns again out of the Smith to the North- East with Rain,
and continues in the North-East two Days without Rain, and
neither turns South nor rains the third Day. it is likely to
coutinue North-East two or three months.
XXI. If it returns to the South within a Day or two without Rain,
and turns Northward with Rain, and returns to the South
in one or two Days as before, two or three times together
after this sort, then it is like to be in the South or bouth-
West two or three Months together, as it was in the North
before.
The winds will finish these turns in a fortnight.
XXII. Fair Weather for a Week with a Southern Wind is like to
produce a great Drought, if there has been much Rain out
of the South before. The H ind usually turns from the
North to South with a quiet Wind without Rain; but
returns to the North with a strong Wind and Rain. r lhe
strongest Winds are when it turns from South to North by
West. When the North Wind first clears the Air, which is
usually once a Week, be sure of a fair Day or two.
XXI II. SPRING AND SUMMER. If the last eighteen Days of Feb-
ruary and ten Days of March be for the most part rainy,
then the Spring and Summer Quarters are like to be so
too ; and I never knew a great Drought but it entered in
that Season.
XXIV. WINTER. If the latter End of October and Beginning of
November be for the most part warm and rainy, then
January and February are like to be frosty and cold,
except after a very dry Summer.
XXV. If October and November be Snow and Frost, January and
February are likely to be open and mild.
Shepherd of Banbury. 235
The CORRELATION COEFFICIENTS (q.v.) for one or two-
of the above rules have been worked out, but they are dis-
appointing :
XXIII. For 38 years, S.E. England, between rainfall of last 18 days
of February and first 10 days of March, and spring 1 rain-
fall, the correlation coefficient is + 0'14 ; between the
rainfall for the same period and summer rainfall, + (V07.
XXIV. For 64 years at Greenwich, between October-November
temperature and that of the following January-February r
-1- 05 ; between October-November temperature and that
of the following December-January-February-March,
+ 0-25.
None of these values indicates a connection of any
significance; in the case of XXIV. the Shepherd's pro-
position is negatived.
Silver Thaw. An expression of American origin.
After a spell of severe frost the sudden setting in of
a warm damp wind may lead to the formation of ice
on exposed objects, which being still at a low temperature
cause the moisture to freeze upon them and give rise to a
" silver thaw."
Simoon. A strong, hot wind, accompanied by clouds
of dust, experienced in the Sahara and the Arabian desert.
It is probably due to convection movements similar to
those in thunderstorms, but there are no clouds, rain or
thunder and lightning ; this is probably owing to the
extreme dry ness of the air over the desert.
Sine. The ratio of the vertical height of a distant
object to the distance of its top from the observer is the
same for all objects which have their tops in the same
line of sight. It is one method of specifying the angles
which the objects " subtend " at the eye of the observer,
which could also be specified by the ratio of the vertical
236 , Glossary.
height to the horizontal distance of its foot (tangent), or
by the ratio of the horizontal distance of the foot to the
distance of the top from the observer (cosine). The
values of these ratios are of great importance in survey-
ing, and are called the trigonometrical ratios of the angles.
They are formally defined as follows :
Let A be a line drawn from the observer to the top
of the distant object, BC the vertical, AC the horizontal.
Then ABC is a right angled triangle with C as the right
angle.
The sine of the angle A (sin A) is the ratio BC to AB.
The cosine of the angle A (cos A) is the ratio AC to AB.
The tangent (tan A) is the' ratio BC to AC.
The secant (sec A) is the ratio of AB to AC.
The cosecant (cosec A) is the ratio AB to BC.
The cotangent (cot A) is the ratio AC to BC.
If the angle is greater than a right angle some of these
ratios are negative, and the following convention is
adopted. A positive angle is measured from AC in the
direction opposite to the motion of a watch hand. The
line AB is always counted positive, AC is positive if C
falls on the right of A, negative if C falls on the left, and
BC is positive if B falls above AC, and negative if B is
below AC. Thus if the angle is between 90 and 270
AC is negative, and if between 180 and 360 BC is
negative.
Sine Curve. This curve is obtained by plotting,
against horizontal distances representing angles, vertical
ordinates representing their SINES. Its simplest equation
is y = a sin a?, the more general equation y = a sin (x a)
represents the same curve shifted forwards through a dis-
tance corresponding to the angle a. The importance of the
Sine Curve. 237
curve in Meteorology is due to the fact that it represents
the simplest form of PERIODIC variation ; its shape, in
fact, is that of the conventional " wave." The diurnal
and annual march of temperature, for example, would, in
so far as they depend only on solar altitude, each be
represented by a sine curve, and are so represented in
theoretical work. Any periodic variation, however
complex, can be represented by a number of sine curves
superposed. The process of finding a set of sine curves to
represent a given variation is called HARMONIC ANALYSIS,
p. 145 and p. 311 (#.^.).
SirOGCO. A name used on the Northern shores of the
Mediterranean indiscriminately for any warm Southerly
wind, whether dry or moist. Such winds blow in front
of depressions advancing Eastward. The typical Sirocco
however is hot and very dry, and is probably in many
places a F6HN wind.
Sleet. Precipitation of rain and snow together. See
p. 341.
Snow. Precipitation in the form of feathery ice
crystals. See p. 342.
Snow Crystals. Thin flat ice-crystals of a hexagonal
form. There are many patterns, and when snowflakes
of various size are observed on any one occasion they
differ only by containing a larger or smaller number of
these crystals.
Solar " Constant." The amount of radiant energy
which would be received in one second from every square
centimetre of cross-section of a beam of solar radiation if
it had undergone no absorption in the atmosphere. Recent
investigations indicate that the solar "constant" is not
invariable. The mean value is about 135 milliwatts per
square centimetre.
238 Glossary.
Solar Day. See EQUATION OF TIME.
Solarisation. Exposure to direct sunlight; the same
as INSOLATION (q.v.). See also RADIATION, p, 330.
Solar Radiation Thermometer. A thermometer
whose bulb is blackened with lamp black, placed in a
vacuum, and exposed to the direct rays of the sun. It is
used for obtaining some indication of the intensity of the
Sun's RADIATION.
Solstice. The time of maximum or minimum decli-
nation of the sun, when the altitude of the sun at noon
shows no appreciable change from day to day. The
summer solstice for the northern hemisphere, when the
sun is farthest north of the equator, is about June 21st, and
the winter solstice, when it is farthest south, is about
December 2Ist. After the summer solstice the days get
shorter until the winter solstice and vice versa.
Sounding;. Generally means a trial of the depth of
the sea, but in meteorology it is used for a trial of heights
in the atmosphere with measures of pressure, temperature,
humidity or wind -Telocity. u Soundings of the ocean of
air " can be carried out by means of kites, PILOT BALLOONS
or BALLONS-SONDES.
Spells of Weather. Long spells of the same type
of weather are often experienced. Anticyclonic weather
may maintain itself for weeks. Depressions often follow
one another on nearly the same track for weeks or even
months ; if they pass North of us we get a warm
Southerly, alternating with a Westerly, type, and accord-
ingly rains, gales and fine intervals succeeding one
another with some regularity, as in the Autumn and
Winter of 1915 to 1916. If the depressions pass to the
Spells of Weather.
South we get Easterly and North-Easterly winds, with
cold weather and rain, or snow in Winter, as in the cold
spell in February and March, 1916.
Spring. Meteorologically in the northern hemisphere,
the three months March, April and May. Astronomically,
spring is defined as the period from the vernal EQUINOX,
March 21st, to the summer solstice, June 21st. See
SEASONS.
Squall. A strong wind that rises suddenly, lasts for
some minutes, and dies suddenly away. It is frequently
associated with a temporary shift of the wind, and heavy
showers of rain or snow. The thundersquall is a cool
outrushing wind, probably katabatic, that often precedes
a thunderstorm.
Stability. A state of steadiness not readily upset by
small events. For stability of the atmosphere, see
ENTROPY.
Standard Time. Time referred to the mean time of
a specified meridian. The meridian of Greenwich is the
standard for Western Europe. The standard meridian of
other countries is chosen by international agreement, so
that it differs from Greenwich by an exact number of
hours or half hours.
State of tlie Sky. The fraction of the sky obscured
by cloud. It is usually measured on a scale, of (quite
clear) to 10 (overcast). A rougher classification suitable
for synoptic charts divides the cloudiness into four classes
represented by the symbols : b, be, c, and o, which corre-
spond to cloudiness of 0-3, 4-6, 7-8, and 9-10 respectively.
Statics. A branch of mechanics, dealing with the
forces which keep a body at rest.
240 Glossary.
Station. A place where regular meteorological
observations are made. The classification of British
stations is :
(1.) First order stations of the International Classifi-
cation. Normal Meteorological Observatories : at which
continuous records, or hourly readings, of pressure,
temperature, wind, sunshine, and rain, with eye obser-
vations at fixed hours of the amount, form, and motion of
clouds and notes on the weather, are taken.
(2.) Second order stations of the International Classi-
fication. Normal Climatological Station?: at which
are recorded daily, at two fixed hours at least, obser-
vations of pressure, temperature (dry and wet bulb),
wind, cloud and weather, with the daily maxima and
minima of temperature, the daily rainfall and remarks on
the weather. At some stations the duration of bright
sunshine is also registered.
(3) Third order stations of the International Classifi-
cation. Auxiliary Climatological Stations : at which the
observations are of the same kind as at the Normal
Climatological Stations, but (a) less full, or (&) taken once
a day only, or (c) taken at other than the recognised
hours.
StatOSCOpe. A very sensitive form of aneroid
barometer, used to show whether a balloon is rising or
sinking. The range of its index is very small and it has
to be set from time to time by opening a tap leading to
the interior of the box.
Storm. Is commonly used for any violent atmos-
pheric commotion, a violent GALE or a THUNDERSTORM, a
rainstorm, duststorm or snowstorm. A gale of wind is
classed as a storm when the wind reaches force 10.
Storm Cone. See GALE WARNING.
Strato-cumulus. 241
StratO- cumulus. The most common form of cloud
of moderate altitude, sometimes covering the whole sky.
It consists of flattish masses, often arranged in waves or
rolls. See CLOUDS.
Stratosphere. The external layer of the atmosphere
in which there is no convection. The temperature of the
air generally diminishes with increasing height until a
point is reached where the fall ceases abruptly. Above
this poirit lies the stratosphere, which is a region where
the temperature changes slowly in a horizontal direction,
and is practically uniform in the vertical (see BALLON-
SONDE). The height at which the stratosphere commences
is often about ten kilometres, but varies. It is higher in
regions nearer the equator.
Stratus. A sheet of low cloud without definite form ;
virtually fog above the level of the ground. See CLOUDS.
Summer. Meteorologically in the northern hemi-
sphere the months of June, July, and August. Astro-
nomically, the period from the summer solstice,
June 21st, to the autumnal equinox, September 22nd
See SEASONS.
Sun. The central body of the solar system, round
which the various planets revolve. Almost all meteoro-
logical processes depend directly or indirectly upon the
radiation received from the sun.
Sun-dOgS. Another word for MOCK SUNS or PAR-
HELIA ; i.e., images of the sun occurring most often on
the halo of 22 radius. Sometimes also used' for portions
of a rainbow.
Sun Pillar. A column of light extending for about
twenty degrees above the sun, most often observed at
242 Glossary.
sunrise or sunset. The colour is usually white, but some-
times red. It is due to the reflection of light from snow
crystals.
Sunset Colours. See BLUE OF THE SKY and
TWILIGHT, p. 344.
Sunshine. An important climatological factor that is
determined by a sunshine recorder, an instrument in
which the rays of the sun are focussed by means of a
glass sphere upon a card graduated into hours. To obtain
comparable results the instrument must satisfy a precise
spec iiicat ion. The sun will also record its appearance on
photographic paper and in many other ways, but in
dealing with climatological records it is of the first
importance that they should be made on a comparable
basis.
For the British Isles a set of Monthly Maps of normals
for the duration of sunshine, together with those for
temperature and rainfall, is given in an Appendix to the
WeMy Weather Report for 1913. The only point to
which attention will be called here is that when one
takes the values for the whole year, so that the possible
amount is the same for all stations, there is a gradual
falling off in the percentage of possible duration shown
on the. records, as one goes northward, from nearly 45 per
cent, in the Channel Isles to 25 per .cent, in the Shetland
Isles. This difference raises the question whether there
are any parts of the globe where the average percentage
of the possible duration of sunshine is zero, where in
fact the screen of cloud ia perpetual.
If there is a region of that character, judging from the
meteorological conditions that are known to us, we should
expect to find it somewhere near the Arctic Circle in the
North Atlantic and the North Pacific, and anywhere
Sunshine. M3
along the Antarctic Circle in the Southern Ocean. And,
on the other hand, we know that, except in those regions
where the belts are interrupted by the trade winds and
monsoons, there is hardly any interference with the sun's
rays by cloud in the high pressure belts along the Tropics
of Cancer and Capricorn. We are not able at present to
give the figures which represent these conclusions, but
they lead on to speculation as to the causes which account
for the distribution of cloud and sunshine and as to why
cloud and rain are not confined to special localities or
regions.
PERCENTAGES OP POSSIBLE DURATION OF SUNSHINE
FOR THE WHOLE YEAR FOR DISTRICTS IN THE
BRITISH ISLES (AVERAGES FROM RECORDS EX-
TENDING OVER THE 30 YEARS, 18811910).
Western Side.
Per
cent.
Middle Districts.
Per
cent.
Eastern Side.
Per
cent.
Scotland, W.
30
Scotland, N.
26
Scotland, E. ...
30
Ireland, N. ...
2Q
England, N.W. ...
32
England, N.E.
3i
Ireland, S. ...
32
Midland Counties
3i
England, E. ...
36
England, S.W. ...
38
England, S.E.
38
English Channel
43
Sunshine Recorder : See Observer's Handbook.
Sunspot-Numbers : the numbers which are used
to represent the variation in the sun's surface from year
to year as regards spots. The occurrences of dark spots,
sometimes large, sometimes small, which are to be seen
from time to lime on the sun's face between its equator
and forty degrees of latitude north or south, have long
244 Glossary.
been the subject of observation. An irregular periodicity
in their number was discovered by Schwabe of Dessau in
1851, using 25 years of observation. Professor R. Wolf of
Zurich, by means of records in a variety of places, made
out a continuous history of the sun's surface from 1610 to
hin own time, which is now continued by his successor,
Professor Wolfer. The sunspot-number N is obtained by
the formula N = k (lOg 4 /), in which g is the number
of groups of spots and single spots, /is the total number
of spots which can be counted in these groups and single
spots combined, k is a multiplier, representing " personal
equation " which depends on the conditions of obser-
vation and the telescope employed. For himself when
observing with a three-inch telescope and a power of 64
Wolf took k as unity.
The method of obtaining the number seems very
arbitrary, but from the examination of photographic
records by Balfour Stewart and others it is proved that
the numbers correspond approximately with the " spotted
area " of the sun. One hundred as a sunspot-number
corresponds with about 1/500 of the sun's visible disc
covered by spots including both umbras and penumbras.
(See " The Sun," by C. G. Abbot, 1912.)
Spots are now regarded as vortical disturbances of the
sun's atmosphere. They have a definite relation to the
amplitude of the regular diurnal changes in ter-
restrial magnetism : their exact relation to magnetic
storms is still unknown. Very many attempts have also
been made to connect the phenomena of weather with
the sunspot-numbers, Indian famines dependent on
Indian rainfall, cyclones in the South Indian Ocean,
Scottish rainfall, commercial catastrophes have all been
Sunspot- Numbers.
245
the subject of investigation. The mean period of fre-
quency of spots is ll'l years and anything with a period
approximating to 11 years or a multiple or sub-multiple
thereof, may suggest a connexion with sunspots. The
most recent and the most effective relation that has come
to the knowledge of the Meteorological Office is the direct
relation between the sunspot-number and the variation
of level of the water in Lake Victoria at Port Florence.
The CORRELATION in this case is + '8.
The following is the list of sunspot-numbers since
1750 :
TABLE OP SUNSPOT-NUMBERS, 1750-1916.
i
i
2
3
4
5
6
7
8
9
1750
83
48
4 8
3i
12
10
i
10
32
48
54
1760
63
86
61
45
36
21
ii
38
70
106
1770
101
82
66
35
31
7
20
92 ,
154
126
1780
85
68
38
23
10
24
83
132
I3i
118
1790
90
67
60
47
41
21
16
6
4
y
1800
H
34
45
43
4 8
42
28
10
8
2
1810
i
5
12
14
35
46
4i
30
24
1820
16
7
4
2
8
17
39
50
62
6 7
1830
7i
48
28
8
13
57
122
138
103
86
1840
63
37
24
ii
15
40
62
98
124
96
1850
66
65
54
39
21
7
4
23
55
94
1860
96
77
59
44
47
30
16
7
37
74
1870
139
in
102
66
45
17
ii
12
3
6
1880
32
54
6c
64
64
52
25
13
7
6
1890
7
36
73
85
78
64
42
26
27
12
1900
10
3
5
24
42
64
54
62
49
44
1910
19
6
4
i
10
46
55
246 Glossary.
Surge. First used by Abercromby to denote the
general alteration of pressure that seems superposed
upon the changes related to a low pressure centre.
Synoptic. Giving a general or " bird's eye" view.
Synoptic charts show the weather at one point of time, or
its mean values for the same interval, over a large area
upon a single map.
Tangent. A straight line that touches a curve and
does not cut it even when produced. Trigonometrically,
the ratio perpendicular : base. See SINE.
Temperature. The condition which determines the
flow of heat from one substance to another. Difference
of temperature plays the same part in the transfer of heat
as does difference of pressure in the transfer of water.
Temperature must be clearly distinguished from HEAT,
heat being a form of ENERGY, temperature a factor which
affects the availability of the energy. Temperature is
measured by a THERMOMETER.
Temperature-Gradient. A change of temperature
with distance (see GRADIENT) ; but the usual meaning of
the term is the lapse rate or rate of decrease of tempera-
ture that is found as greater altitudes are reached.
In most parts of the earth near the surface a fall of
temperature of 1F. for every 300 feet occurs, so that a
tableland on the summit of a mountain 3,000 feet high
will have a mean temperature 10F. lower than stations
near sea level in the same neighbourhood.
In the free atmosphere the temperature gradient is
usually measured in degrees centigrade per kilometre of
height. The decrease is a little slower than that found by
mountain observations. Ic amounts to about 6C. per
kilometre in England in the lower strata, but increases to
Tempera ture- Graaien t . 247
from 7 to 8 in the strata that lie between 5 and 9 kilometre
height. Above 11 or 12 k. the fall ceases altogether. In
the tropics the temperature gradient of 7 or 8 per kilo-
metre is continued up to 15 k. height or more. See Tables
under BALLON-SONDE and DENSITY.
Tension of Vapour. See VAPOUR.
Terrestrial. Having reference to the earth. The
term Terrestrial Radiation refers to the heat radiated from
the earth. *
Thaw. The term used to denote the cessation or
break-up of a frost ; usually the result of the substitution
of a South-Westerly type for a North-Easterly type in
this country, or of the sudden incursion of a CYCLONE
from the west. In more northern latitudes the u spring
thaw " is a periodic event, denoting the seasonal progres-
sion, the unlocking of ice-bound seas and the melting of
the snow. In these latitudes, in Western Europe, though
not in Canada or Asia, the sun is generally at sufficient
ALTITUDE about noon, except near mid-winter, to effect a
partial thaw by day, even in the midst of a protracted
frost, if the sky is clear.
Thermodynamics. That part of the science of heat
which deals with the transformation of heat into other
forms of energy. See ENERGY and ENTROPY.
Thermogram. The continuous record of temperature
yielded by a thermograph.
Thermograph. -- A self-recording thermometer
generally consisting of a " Bourdon " tube or a bimetallic
spiral with a suitable index. A large spirit thermometer
with a float is also used. A mercury thermometer can
also be arranged to give a photographic record, as at the
observatories of the Kew type.
Thermometer. An instrument for recording tern-
248 Glossary.
perature, usually by means of the changes in volume of
mercury or spirit contained in a glass tube with a bulb
at one end, but not infrequently by the change of an
electrical resistance. Generally the temperature of the air
is required, and this is not easily obtained, particularly in
sunny weather. Readings of thermometers exposed in a
Stevenson screen, however, are sufficiently accurate for
practical purposes.
Thunder. The noise that follows a flash of lightning,
attributed to the vibrations set up by the sudden heating
and expansion of the air along the path of the lightning.
The distance of a lightning flash may be roughly estimated
by the interval that elapses between seeing the flash and
hearing the thunder, counting a mile for every five
seconds.
It is somewhat astonishing in common experience at
what little distance thunder ceases to be audible : the
interval between flash and sound seldom reaches a full
minute,* which would set a limit of twelve miles to the
distance of audibility. Considering the violence of the
commotion in the immediate neighbourhood of a flash it
might be expected that the sound would be perceptible
at far greater distances. Two considerations affect the
question first, it is a common experience with balloonists
that sounds from the balloon are less easily audible on the
ground than sounds from the ground to the balloon and
this observation is confirmed by the experience on
mountains that sounds from below are more easily audible
upwards than sounds from above downwards ; the second
consideration is that in thunder-weather there are great
discontinuities in the structure of the atmosphere, so that
the distortion of the rays of sound, which partly accounts
* Capt. Cave cites an occasion on which not less than two minutes
elapsed.
Thunder. 249
for the smaller audibility of sounds from below, is much
exaggerated on the occasion of a thunderstorm.
Thunderstorm.* In the British Isles, except at the
stations on the Atlantic Coasts, well developed thunder-
storms occur most frequently in the summer, and especially
during the afternoon. The barometric disturbances with
which they are associated are generally too limited in area
to be called cyclones, but like cyclones they frequently
move towards the East. They are nearly always accom-
panied by heavy rain, which is sometimes preceded by a
squall that blows outward from the advancing storm,
while the barometer rises suddenly and then remains
comparatively steady. The squall brings with it cool air.
See LINE SQUALL.
Lightning is seen, and thunder heard, before the arrival
of the storm itself, but the flashes are generally most
brilliant during the heavy rain. The thunder then
follows the flashes after a very short interval, showing
that the discharge has taken place at no great distance.
The thunder-cloud seems to be an extreme develop-
ment of the cumulus cloud, in which the ascending
currents have reached to a considerable height and spread
outwards at the top. In consequence they often have the
shape of an anvil. The thunder type of cumulus has a
rounded summit, with a clearly defined border. Obser-
vations of the time and place of occurrence of thunder-
storms show that they are generally long and comparatively
narrow, and move broadside across the country. The
precise conditions that lead to their formation are not
understood. In some parts of the tropics thunderstorms
are frequent and very violent.
* See photographs under CLOUDS and MAMMATO-CUMULUS.
250
Glossary.
IMMUNITY PROM THUNDERSTORMS IN VARIOUS PARTS
EXTENDING MAINLY OVER THE 25 YEARS
Table of " odds against one", expressing the random chance
STATION.
Spring.
Summer.
Autumn.
Winter.
SCOTLAND.
( Sumburgh Hd. (Island)
A I Deerness (Island)
"S -{ St or no way (Island) ...
Wick (E. Coast)
I Fore Augustus (Inland)
767
135
192
IIO
108
164
30
77
30
56
569
57
134
103
606
45o
68
102
225
225
( Nairn (N.E. Coast)
^ | Aberdeen (E. Coast) ...
\ Braemar (Inland)
K | Dundee (E. Coast)
ILeith (E. Coast)
192
135
74
35
32
20
25
15
23
284
142
162
84
228
1,120
1,120
281
750
375
fLaudale (W. Coa*t) ...
43 1 Rothesay (Island)
\ Glasgow (W. Coast) ...
^ | Pinmore \
L Douglas (Isle of Man)
34
44
68
92
53
24
33
26
45
24
162
97 '
49
33
80
204
118
250
IRELAND.
f Malin Hd. (N. Coast)...
| Blacksod Pt. (W. Coast)
{ Markree Castle (Inland)
| Armagh (Inland)
LDonaghadee (E. Coast)
92
109
72
97
36
58
32
25
46
120
152
175
505
175
134
73
78
391
1,120
f Dublin (E. Coast)
c/2 J Valencia (S.W. Coast)...
t Roche's Pt. (S. Coast)...
61
92
82
18
60
43
99
78
108
250
750
150
g g /Scilly (St. Mary's) ...
jfj 5 I Jersey (St. Aubin's)
82
36
42
16
65
28
68
58
Thunderstorm.
251
OF THE UNITED KINGDOM AS SHOWN BY OBSERVATIONS
1881-1905 (COMPILED BY F. J. BRODIE).
of a thunderstorm on any day in the several seasons of the year.
STATION.
Spring.
Summer.
Autumn.
Winter.
r N. Shields (E. Coast) ...
pq J Durham (Inland)
a | York (Inland)
* I Spurn Hd. (E. Coast) ...
74
27
27
32
20
10
13
II
95
11
62
375
562
2,250
375
rHillingdon (Inland) ...
W . I Yarmouth (E. Coast) ...
^ 1 Norwich (Inland)
I. Cambridge (Inland) ...
21
45
19
27
8
13
9
10
37
87
40
47
205
562
209
750
. fWorksop
g S 1 Cheadle
3 P J Churchstoke
q ,g "J Loughborough
5 o Cheltenham
1 Oxford
27
17
36
3i
29
40
IO
Q
15
II
14
H
61
25
76
65
87
63
225
125
220
1,000 f
161 "
750
f London (Brixton)
. j Margate (E. Coast) ...
K . { Dungeness (S. Coast) ...
00 i Southampton (Inland)
I Hurst Castle (S. Coast)
25
55
53
45
74
ii
20
17
17
29
53
103
5i
58
76
225
1,120
562
225
322
fc f Ay sgarth (Inland)
J | Stonyhurst (Inland) ...
g ^ ^i Liverpool (W. Coast) ..,
W ^ | Llandudno(W. Coast)...
L Holy head (W. Coast) ...
27
23
74
68
72
13
IO
27
29
30
39
27
114
60
69
237
94
321
141
374
( Pembroke (W. Coast)...
1 Falmouth (S.W. Coast)
00 / Cullompton (Inland) ...
209
177
72
9 2
56
30
95
73
108
562
150
150
252 Glossary.
Time.* For all common purposes Greenwich mean
civil tinie is now used in all places in Great Britain,
except at Canterbury, and clocks are set by telegraphic
signal from Greenwich. Previously, each town or village
clock kept its own local mean time and had to be set by
the local time keeper, usually the parson, with the aid of
a sundial or some other means of ascertaining the time
from the sun. In Ireland clocks are set according to
Dublin time, which is 25 minutes after Greenwich time.
In meteorology the hours of the civil day are numbered
from 1 to 24, the counting beginning from midnight.
Thus the hours of observation for telegraphic reporting
are, lh., 7h., 13h., 18h., with a subsidiary observation at
21h. Meteorologists are closely interested in good time-
keeping, because punctuality is of importance, both with
climatological observations and with those that are made
for the maps used in forecasting. For the former local
time, and for the latter Greenwich time is taken as the
standard. The records of self-recording instruments,
when the sheet is changed once a . week, are for many
purposes useless unless marks are made on the trace at
definite times, so as to allow for irregularities in the
running from day to day. See STANDARD TIME.
Tornado. A short lived, but very violent wind. In
West Africa the tornado is the squall which accompanies
a thunderstorm ; it blows outward from the front of the
storm at about the time the rain commences, and in all
parts of the world similar squalls occur, associated with-
thunder. It is also the name applied to small but very
* In 1916 " Summer Time," one hour in advance of Greenwich Time,
was used in the United Kingdom from May 21st to September 30fch ;
in 1917 from April 8th to September 17th ; between the limiting dates
(September 30th, 1916, and April 8th, 1917), GLM.T. was used in Ireland.
Tornado. 253
violent whirlwinds of one or two hundred yards diameter.
These whirlwinds often do immense damage in the
United States, where they are. known as cyclones,
completely destroying every tree and building in their
track. They are not unknown in England, but are less
frequent and less violent than in North America.
Torricelli, Evangelista.- The inventor of the
barometer, born at Piancaldoli in 1608. At the age of 20
he went to Rome to study mathematics. In 1641 he met
Galileo, and remained with him as his amanuensis till the
death of Galileo three months later. Torricelli then
became professor to the Florentine Academy ; he lived in
Florence till his death in 1647. Torricelli explained the
fact, already known, that water will only rise about 32 feet
in the pipe of a suction pump ; he argued that if this was
due to the pressure of the atmosphere the column of
mercury that would be supported would be a little under
2| feet, since mercury is 13^ times as heavy as water. He
performed the experiment that confirmed his theory. He
also enunciated various fundamental principles in hydro-
dynamics.
Trade Winds. The word " trade " in this expression
is said to mean " track " and trade winds are winds which
keep to a fixed track. We naturally turn to tropical or
subtropical regions for track winds. The easterly wind
on the margin of the ice in the Antarctic is very persistent
but not very steady. The best known examples of track
winds are the North East Trade and the South East
Trade of the Atlantic Ocean. In a publication of the
Meteorological Office, M.O. 203, on the Trade winds of the
Atlantic Ocean the areas selected for the observations are
for the North East Trade from 10 N. to 30 N. between
30 W. long, and the West Coast of Africa, and for the
254 Glossary. '
South East Trade the two pairs of ten-degree squares
to 20 S., to 10 W. and 10 to 30 S., to 10 E.
The Canary Islands and Cape Verde Islands come in the
region selected for the North East Trade, and St. Helena
in that selected for the South East Trade. At St. Helena
there is a self-recording anemometer at a point 1,960 feet
above sea level, which is maintained for the Meteorological
Otfice.
The coast of Africa disturbs the regularity of the North
East Trade in the Eastern part of the area selected, but
the monthly results for the whole area give a wind with
a mean direction for the whole year of N. 30 E., varying
between N. 18 E. in May and N. 48 E. in January, and
a mean velocity for the whole year of 1O6 miles per hour
(4* 7 ro/s), varying from 7*4 miles per hour (3*3 m/s) in
October to 13*5 miles per hour (6 rn/s) in April. The
South East Trade shows a mean direction for the year of
S. 38 E., varying from S. 35 E^in February and October
to S. 41 E. in August and November, and a mean velocitv
of 14'2 miles per hour (6*4 m/s) varying from 13*1 miles
per hour (5'9 m/s) in January to 15 miles per hour
(6*7 m/s) in April, June and August. Thus, the North
East Trade shows more variation in direction than the
South East, and its velocity exhibits a marked seasonal
variation, with a maximum in April and a minimum in
October, which has no counterpart in the South East.
These are taken from observations made by ships at sea,
the velocities being determined by a scale of equivalents
of ** Beaufort estimates." See BEAUFORT SCALE. When
the measures of the direction and velocity at St. Helena
are taken they show the monthly values oscillating about
S. 40 E., from S. 35 E. in October to S. 42 E. in April,
and a very marked seasonal change of velocity from
Trade Winds. 255
13 miles per hour (5*8 m/s) in May to 20 miles per hour
(8*9 m/s) in September. This is very nearly the counter-
part of the seasonal variation of the North East Trade.
The mean of the velocities of the two " trades" works out
at about 11*6 miles per hour (5'2 ni/s) throughout the
year. 'The flow which is represented by these winds
comprises two streams about 1,000 miles wide, the courses
of which are kept steadily from N.E. or from S.E. for
about 2,000 miles. These steady currents carry an enor-
mous amount of air. Taking the run at 300 miles per
day over a thousand mile front the flow for a thickness
of 1 mile would be 300,000 cubic miles a day ; it would
take rather less than 10 years for the whole of the
atmosphere to pass through ; if it be two miles thick the
circulation would be complete in five years. And on
the same assumption, the two trades acting together yet
they use only one-twentieth part of the belt of the earth's
surface available for approaching the equator from North
and South would deliver the equivalent of the whole of
the atmosphere in the course of about two and a half
years.
So far, we have considered only the trade winds of the
Atlantic Ocean between the African and American coasts.
Similar winds under similar conditions exist to the West-
ward of the American coast where, it may be remarked,
the coast line bends away, for a 1,000 miles or more,
to the Westward after crossing the equator from the
Southward, very much in the same way as the African
coast line does, so that the North- East Trade wind of the
Eastern Pacific lies to the West of, and not opposite to, its
South-East partner, just as in the Atlantic. These are
the only regions where the recognised characteristics of
the Trade winds are well marked. In the Indian Ocean
256 Glossary.
they are replaced by the Monsoon winds, which are con-
tinued across the equator from the North East in the
winter and from the South East in the summer, when
the air current from the south is carried forward as
a South-West monsoon over India. A suggestion of
" Trade " conditions appears in the Western Pacific to the
North East of Australia, but it is less well marked than
in the Atlantic and Eastern Pacific.
It should be noted that in the West Indian region in
June the wind varies from North. East to South East,
and the same is true off the coast of tropical South
America in December. Locally it is still known as the
Trade wind, although it may be blowing from the South
East, away from the equator.
The explanation of the origin of the Trade winds which
is given in all books on Physical Geography is due
originally to Edmund Halley, a personal friend of
Newton's, secretary of the Royal Society, and subse-
quently Astronomer Royal at the beginning of the
eighteenth century. It attributes the flow of air south-
ward and northward on either side of the line to the
replacement of air which has been heated by the warmth
of the equatorial belt, and has, in consequence, ascended
to the upper air and passed away. * Jete Hadley, also a
personal friend of Newton's, associated with him in the
invention of the sextant, explained the easterly com-
ponent by bringing the rotation of the earth into account.
Whatever real ground there may be for a flow of air
towards a belt of high temperature along the equator on
the ground of local heating, it appears clear from the
maps that the great arterial currents which we have
described, and which are commonly understood as Trade
winds, are really parts of the general circulation of the
Trade Winds. 2f>7
Atmosphere, governed by the distribution of pressure. A
map of the distribution of pressure and winds over the
globe, such as that for January in the Barometer Manual
for the Use of Seamen (M.O. publication No. 61) shows
that there are two belts of high pressure on either side of
the equator, about latitude 30 N. and 30 S. respectively.
These belts are not continuous but form a succes-ion of
anticyclonic areas, each with its appropriate circulation.
The southern hemisphere pr-sents the simpler arrange-
ment, because the land areas which cause disturbance of
the order are less extensive. Along the parallel of 30 S.
latitude we have anticyclonic areas with centres, (1) at
100 W., 30 west of South AmerL-a, (2) at 10 W., nearly
midway between South Africa and South America,
(3) a system with double centre between the Cape and
Australia, which extends along the Southern shore of
Australia and develops a secondary centre there. The
regularity of the distribution is thus much distorted by
the Australian continent. The channels of low pressure
between the anticycionic areas are breaches in the belt of
high pressure through which the great arterial currents
of the trade winds flow over the Eastern Atlantic and
Eastern Pacific. And these great currents are in reality
features of a circulation round the isolated regions of
high pressure. There seems little possibility of any
alternative for the distribution thus described. Halley's
explanation of the trade winds supposes a low pressure
area over the equatorial belt, continually maintained by
the convection of the rising air, and continually fed by a
flow of air from a belt of higher pressure north or south.
So that the flow of air is thought of as from high pressure
to low pressure. Whatever may be the actual state of
13204 I
258 Glossary.
things close to the equatorial belt, the arterial currents
of the trade winds are clearly shown by the map to be
great rtreams of air with 2,000 miles of run, and with
high pressure on the one side and low pressure on the
otht r, such as we may find in all cases of well established
air currents over the earth's surface, whether they last
only lor a few hours, a few days, as in the intermediate
and polar regions, or the whole year, as in the regions of
the trade winds.
If we cross the great currents from west to east we
are travelling towards lower pressure ; from east to west
towards higher pressure. Obviously, we cannot continue
this process all round the globe, and going westward we
see that the pressure soon gets to a maximum and then
falls off again, and the falling off is associated with a
change in the direction of the current from south-east
to east or north-east, or from north-east to east and
south-east. The WIND -ROSES show that the western
boundary of the high pressure is a fluctuating boundary,
not a fixed one. Going Eastward we are brought up by
the great land areas of Africa, where our knowledge of
the distribution of pressure is little more than guessing
from a few isolated stations which are affected by the
uncertainties of REDUCTION TO SEA-LEVEL. But further
investigation must lead to a distribution which corre-
sponds with a low pressure at sea-level. That these great
currents are really part of a great circulation which is
governed by the distribution of pressure may be illustrated
by comparing with the steadiness of the winds at St.
Helena* (lat. 16 S., long. 5*42 W.), the following table
of the winds at Suva, Fiji, which is in latitude
18 S., long. 17826 h. :
* See Trade Winds of the Atlantic Ocean. M.O. publication No. 20H.
Trade Winds.
259
TABLE OF WIND DIRECTION AT I) A.M. AT SUVA,
FIJI, IN 1911.
i N. N.B.
E
S.E.
S.
s.w.
w.
X.W. Calm.
January .. 6 3
: 5
4
12
February .. 4 , 5
5
2
2
9
March .. 4 G
2
2
3
13
April .. 7 12
I
I
4
4
May .. 2 15
2
f
3
i
June .. 3 6
I
I
I
5
10
July .. i 5
4
I
I
I
4
ii
A.ugust .-4 6
4
5
I
4
7
September .. 2 10 8 3
3
2
2
October .. L 10 7
4 3
2
I
3
Xovember .. 16
6
3 .1
4
December .. 9 10 <)
2
I
Year ... 43 104 ! 50
37
8 19 ;
4 i
26 74
From this it appears that the wind conditions at the
two places are quite different, though their relations to
the equatorial belt of high temperature are altogether
similar.
We must, therefore, regard the trade winds as the main
streams of .air in the general circulation by which the
intertropical region is supplied. The greater part of the
supply turns eastward and gets away from the equatorial
region again by passing round the western boundaries of
the anti-cyclonic regions. Some part of it may go to feed
the rain storms of the doldrums, but what fraction of the
whole supply is so used is not known.
The extension of Halley's theory of the trade winds
13204 I 2
2t>6 Glossary.
provides that the air after ascending in the equatorial region
should flow back again away from th e equator,and on account
of the rotation of the earth the northward flow should be
diverted towards the east, and thus become a south-west
wind. Accordingly, south-west and north-west winds
are to be expected above the north-east and south-east
trades. Two thousand miles is a long way for the air to
travel with no more diversion than 45 from the path of
its desire when the rotation is taking place at the rate of
lr> sin \ degrees per hour unless the distribution of pres-
sure interferes, but the theory seems to be confirmed by
an observation made in 1856, by Piazzi-Smyth (then
Astronomer Royal for Scotland) of a south-west wind
at the top of the peak of Tenerife, 12,500 feet high, over
the north-east trade flowing below. The transition is at
about 10,000 feet, and the existence of a south-westerly
current over the north-east trade over the ocean was
verified by balloon observations by Teisserenc de Bort,
although the question was the subject of some discussion
at the time.
If, however, we regard the surface winds of the trades
as part of the general circulation of the atmosphere con-
trolled by pressure, we cannot ^do otherwise in the case of
the upper currents, consequently we ought to find our
explanation of the south-westerly current over the north-
east trade as evidence of low pressure over the central
region of the Atlantic north and south of the equator,
and high pressure over the African land adjoining, giving
rise to a gradient for equatorial winds up above, or for
polar winds below. We should then have to conclude
that the high pressure belts of the tropics are reversed in
the upper regions, a corclusioii that carries with it some
consequences which at first sight are not easily disposed of.
Trade Winds. 261.
The trade winds have an interest for meteorologists
quite independent of their geographical interest and of
their place in the general circulation. They are a sort of
laboratory in which one can study the properties of a
great current of air of known temperature and humidity
flowing steadily over the surface of the sea and affected
by the turbulent motion caused by the surface.
Professor Piazzi-Smyth, spent part of July, August and
part of September, 1856, in the main stream of the north-
east trade at Tenerife at a height of 8,900 feet or more.
He found the air remarkably dry, while below him at
a height of about 5,000 feet he could see the long strings
of cumulus clouds that are characteristic of the trade
wind forming a level horizontal layer upon which it
seemed that one might walk to the neighbouring Canary
Islands if it were not for a gap between the cloud sheet
and the cloud actually in contact with the mountain,
which was some 1,000 feet below the trade wind cloud.
The wind at 8,900 feet was generally light. Cirrocumulus
clouds, moving from south-west, were sometimes visible
in the sky above at a height estimated at 15,000 feet.
Occasionally the north-easterly wind got nearly to gale
force at the high level, and on other occasions the wind
blew strongly from the south-west there. The interme-
diate region between the north-east trade and the south-
west counter trade was found to be generally a region
of light winds, while the trade wind clouds were formed
at the middle height of the trade.
It would appear, therefore,- that we have in the body of
the trade wind, an analogy, but on a smaller scale, of the
separation of the troposphere from the stratosphere which
is universal. The boundary between the two is the limit
262 GUssary.
of convection 'from the surface. In the trade winds the
boundary of convection is marked by the layer of clouds,
above that level the moist air does not penetrate. The
uniformity of level suggests that the limit is depen-
dent upon the turbulent motion due to the eddies caused
by the friction at the surface, the effect of which extends
upwards to a height which depends upon the length of
the "fetch." The convection is therefore in this case
partly dynamical, and it is curious that the clouds>formed
are often spoken of as rollers.
Piazzi-Smyth speaks of summer and winter conditions
as though he were dealing with a climate of temperate
latitudes instead of a region to the south of the line of
tropical high pressure.
The observations of St. Helena, which are made at about
2,000 feet above sea level in the south-east trade, show
persistent south-easterly winds, with a mean humidity
for the yearj|f of 89 per cent., ranging from 88 in January
to 91 in March, and the normal amount of cloud works
out at 8'5 (on the scale 0-10, or 85 per cent) for the
year. The mean temperature reaches a maximum of
291'9a, 66-lF., in March, and a minimum of 287-la,
57'3F., in September, and the rainfall has a normal
maximum of 131 mm. in March and a minimum
of 40 mm. in November. Hence, there is a definite
seasonal variation, but it lags behind the corresponding
changes in higher latitudes of the same hemisphere.
Trajectory. The path traced out by a definite particle
of air in a travelling storm, or the horizontal projection
of the course followed by a pilot balloon : the trajectory,
as worked out from theodolite readings, may be plotted
on squared paper, and the direction and velocity of the
wind at any given height deduced therefrom.
Tramontanes. 263
Tramontana. An Italian word for the northerly
winds of Italy which blow from the mountains.
Transparency. The capacity for allowing rays of
light or some other form of radiation to pass. Thus glass
is transparent for the visible radiation of light. Rock-
salt is specially transparent for the rays of radiant heat.
See VISIBILITY.
Tropic. One or other of the circles of 23^ of latitude
north and * south of the equator, which represent the
furthest position reached by the sun in summer and
winter in consequence of the tilting of the earth's axis
with reference to the plane of the ecliptic. The term
applies also to the zone of the earth lying between them.
The northern circle is called the* tropic of Cancer, the
southern the tropic of Capricorn.
Tropical. Belonging to the regions of the tropics, or
Hinilar to what is experienced there. The word tropical
is often used for the region between the tropics, which is
more strictly called intertropical.
Tropopause. The lower limit of the STRATOSPHERE.
Troposphere. A term suggested by Teisserenc de Bort
for the lower layers of the atmosphere. The temperature
falls with increasing altitude up to a certain height (see
TEMPERATURE GRADIENT), and the part of the atmosphere
in which this fall occurs is called the troposphere. In
these latitudes (50) it extends from the surface for a
thickness of some 7 miles, or 11 kilometres ; in the tropics
the thickness may reach 10 miles, or 16 kilometres.
Trough. The period of lowest barometer during the
passage of a depression. Taking the fluctuations of the
barometer to be analogous to the fluctuations of level
264 Glossary.
caused by waves, which is, however, not a very good
analogy, the trough of the wave suggests as its counter-
part the lowest reading of the barometer. With the
barometer the passage of the trough is generally marked
by phenomena of the type of a LINE SQUALL, a sudden
rise of pressure, veer of wind, drop of temperature, and one
or more squalls ; nothing of that kind takes place in the
trough of a wave.
Twilight. See p. 344.
Twilight Arch. On a clear evening after sunset a
dark arch with a pink edge may be seen to rise from the
eastern horizon ; the distinction between the darkness
below the arch and the brighter sky above it becomes
rapidly less as the arch rises in the sky. The dark space is
really the shadow of the earth. In mountainous countries
shadows cast by mountains between the sun and the
observer may be seen to rise from the twilight arch.
The pink edge of the arch is due to reflection from
particles in the atmosphere which are illuminated by rays
of the sun that have lost nearly all their blue light from
lateral scattering (see BLUE OP THE SKY).
Type. Different distributions of atmospheric pressure
are characterised by more or less definite kinds of weather,
and accordingly when a certain form of distribution is
seen on a chart the weather is described as belonging to
such and such a type. The types are defined as cyclonic,
anti- cyclonic and indefinite, and by terms denoting the
direction of the isobars. Thus, a " southerly type" denotes
a weather chart on which the isobars are shown as more or
less parallel lines running north and south. A northerly
type will also have isobars running north and south ; the
distinction will be that in the southerly type barometric
pressure will be high in the east, whereas in the northerly
Type. 265
type the higher pressure will be in the west. In each
season each type has more or less definite kinds of weather.
Thus, the anti-cyclonic type will have dry weather, the
cyclonic wet ; the southerly type will in general be warm,
the northerly cold.
Typhoon. A word of Chinese origin applied to the
tropical cyclones occurring in the western Pacific near
Japan and the Philippine Islands. They are extremely
violent circular storms of 50 to 100 miles diameter, and
travel slowly. Exactly similar storms are known as
hurricanes in the West Indies, and as cyclones in the Bay
of Bengal. The hurricanes of Mauritius are also similar
to typhoons, See HURRICANE.
Upbank thaw. A state of affairs in which the usual
fall of temperature with height is reversed, a thaw, or an
increase of temperature occurring on mountains some-
times many hours before a similar change is manifested
in the valleys.
It is due to the superimposition of a warm wind
blowing from a direction differing from that of the
surface wind, and occurs most usually at the break-up
of a frost, on the approach of a cyclonic system, but
sometimes during the prevalence of anti-cyclonic con-
ditions, when a down-current of air is dynamically
heated in its descent from a great height. Under these
conditions, at 9 a.m. on February 19th, 1895, at the end
of a great frost, the temperature at the summit of 3- Ben
Nevis was 9'8a., 17'G F. higher than at Fort William,
4,400 feet below.
It is probable that this INVERSION of the normal
temperature gradient is the cause of the phenomenon
known ;is GLAZED FROST (q.v.}.
266 Glossary.
V-shaped depression. Used to describe isobars
having the shape of the letter V, which enclose an area
of low pressure. The point of the V is always towards
the south or east.
Vapour-pressure. The pressure exerted by a
vapour when it is in a confined space. In meteorology
vapour-pressure refers exclusively to the pressure of
water- vapour. When several gases or vapours are mixed
together in the same space each one exerts the same
pressure as it would if the others were not present, and
the vapour-pressure is that part of the whole atmospheric
pressure which is due to water-vapour. See AQUEOUS
VAPOUR, RELATIVE HUMIDITY.
Vapour-tension. A now obsolete term for vapour-
pressure. There seems now to be no reason why this should
be called tension.
Vector. A straight line drawn from a definite point
iii a definite direction. Thus a radius vector of the earth
in its orbit is the line drawn from the sun to the earth. A
vector quantity is a quantity which has a direction, as well
as magnitude, and of which the full details are not known
unless the direction is known. In meteorology the wind
and the motion of the clouds are examples of vector
quantities ; the directions, as well as the magnitudes, are
required, whereas in the case of the barometer or the
temperature the figures expressing magnitude tell us
everything. They are called scalar quantities. All vectors
obey the parallelogram law. That is to say, that any
vector A may be exactly replaced by any two vectors
B and C, provided that B and C are adjacent sides of any
parallelogram, arid A the diagonal through the point
where B and C meet. Also the converse holds.
The position of an airship a given time after starting is
Vector. 267
an example. The two vector quantities that bring about
the final result are the velocity and the direction of the
airship through the air, and the velocity and direction of
the air, i.e., the wind. Suppose an airship pointing S.W.
and with speed of 40 miles an hour. After two hours
its position on a calm day is 80 miles S.W. of the starting
point. Now suppose the airship has to move in a S. wind
of 30 miles an hour ; after two hours this wind alone
would place the airship 60 miles N. of the starting
point. The real position will be given by drawing two
lines representing these velocities and finding the opposite
corner of the parallelogram of which they form adjacent
sides. See COMPONENT.
Veering". The changing of the wind in the direction
of the motion of the hands of a watch. The opposite to
BACKING.
Velocity. Velocity is the ratio of the space passed
over by the moving body to the time that is taken. It is
expressed by the number of units of length passed over
in unit time, but in no other sense is it equal to this space.
It can be expressed in a variety of units. For winds
metres per second, kilometres per hour and miles per
hour are most common. When a velocity is variable a
very short time is chosen in which to measure it. Thus
the statement " at 11 a.m. the wind was blowing at the
rate of 60 miles per hour " means that for one second or
so at just 11 a.m. the wind had such a rate, that had that
rate continued for an hour the air would have travelled
()() miles in that hour.
From the time of its establishment in 1854 until the
final evaluation of the Beaufort Scale in 1905 it was the
custom of the Meteorological Office to measure wind
velocity by the cup anemograph which gave the " run "
268 Glossary.
of an hour in miles. Miles per hour were accordingly
the accepted unit of velocity, but when it became certain
that for anemometers of the standard size (9-in. cups and
2-ft. arms) the " factor " was not 3 but 2-2, so that the
miles were not really miles after all, some change of
nomenclature was necessary to mark so great a change of
habit. The pressure-tube anemograph gave the oppor-
tunity of measuring the velocity at any instant instead of
the run during an hour : a gust that lasts only part of a
minute is more appropriately measured by the distance
which the wind travels in a second than by the distance
which it might travel in an hour if it remained unchanged
throughout the hour. Moreover in all questions of
dynamical calculation the second is the unit of time.
Gunners use the foot per second as their unit. The
metre per second is more suitable for units on the C.G.S.
system, and is now used in all the publications of the
Meteorological Office.
Vernier. A contrivance for estimating fractions of a
scale division when the reading to the nearest whole
division is not sufficiently accurate. The vernier is
a uniformly divided scale which is arranged to slide
alongside the main scale of an instrument. Informa-
tion as to the graduation of a vernier and the method of
reading is given in The Observer's Handbook.
Vertical. The direction of the force of gravity or of
the plumb line, so called because it refers to the vertex,
/.#., to the zenith, A vertical line is perpendicular to the
surface of still water, which is horizontal. When produced
it passes through the zenith and close to the centre
of the earth. Two vertical lines can therefore never be
parallel, although if they are near together they are very
nearly .po. A vertical plane at any point likewise passes
Vertical. 269
through the zenith and very close to the centre of the earth.
A vertical circle is a GREAT CIRCLE passing through the
zenith and the nadir ; and the vertical circle also passing
through the east and west points is called the prime
vertical.
Viscosity. The property of a liquid or gas whereby it
resists the tangential motion of its parts. See DIFFUSION.
Visibility. A term used in describing the effect of
the atmosphere, and the amount of light in the sky, on
the maximum distance at which an object can be seen,
and the clearness with which its details can be made out.
The visibility of the atmosphere depends chiefly on the
amount of solid or liquid particles held in suspension by
the air. On a cloudy day it is usually equally good in all
directions, but on a sunny day the visibility is usually
better, i.e. 9 objects can be seen more clearly, when looking
away from the sun than when looking towards it.
In England the visibility of the atmosphere is usually
bad towards the end of a spell of fine, calm weather ; but
in these cases the occurrence of a shower of rain
frequently clears the air and gives rise to good visibility.
On the other hand during rainy weather the visibility is
frequently bad, even when it is not actually raining.
The visibility of objects on the ground, when looked at
from an aeroplane, is sometimes bad even when the
visibility between two points on the ground is good and
the sky is cloudless. This condition usually arises in
calm, anti-cyclonic weather and is due to a layer of haze
at a definite height above the ground.
The occurrence of haze during fine dry weather can
frequently be connected with the proximity of a large
town. A light north-easterly wind sometimes carries
haze to a distance of 70 miles south-west of London, QA
270 Glossary.
one such occasion, it was found that at Farnborougli, 3~>
miles from London, the haze extended to a height of 7,000
feet. Above that height the atmosphere was perfectly clear.
It frequently occurs that an aeroplane can be seen from
the ground at a time when the ground cannot be seen
from the aeroplane. This condition arises when there is
a low haze or mist which pre vents a large part of the sun's
rays from reaching the ground. The aeroplane itself is
brightly lighted by the direct rays of. the sun, while the
light reflected upwards from the top of the haze towards
the aeroplane overpowers the feeble rays from the lees
brightly lighted ground The effect is similar to that of
a lace curtain over a window, which enables the occupants
of a room to see out, while the interior cannot be seen
from the outside.
Occasionally it happens that an aeroplane can see the
ground while remaining invisible itself. The condition
arises only on sunny days, but its cause is not understood.
The limit of visibility depends chiefly on the number
of dust particles per cubic centimetre of the atmosphere.
An apparatus for counting this number has been designed
(see DUST-COUNTER), and used by Mr. John Aitken, F.R.8.,
who has found as few as 10 arid as many as 7,000 dust-
particles per cubic centimetre in the open country. The
distance of the furthest visible object was found to depend
on the number of particles in the atmosphere, and on its
humidity. For a given depression of the wet-bulb
thermometer, the limit of visibility multiplied by the
number of particles per c.c. of air was found to be
roughly constant. This constant, however, increases as
the air becomes drier.
For a given depression of the wet bulb, therefore, the
number of particles in a column 1 sq. cm. in cross section
Water. 271
and stretching from the observer to the limit of visibility
is constant. Mr. Aitken's estimates of its values for
different degrees of humidity are shown in the accom-
panying table.
Depression of wet-bulb. C Number of particles of dust
to produce complete haze.
2 to 4 12 x 10 9
4 to 7 17 x 1C 9
1 7 to 10 22 x 10 9
Vortex. See p. 347.
Water. The name used for a large variety of sub-
stances such as sea-water, rain-water, spring-water, fresh
water of which water, in the chemical sense, is the chief
ingredient. Chemically pure water is a combination of
Hydrogen and Oxygen in the proportion by weight of one
part to eight, or by volume, at the same pressure and tem-
perature, of two to one, but the capacity of water for dis-
solving or absorbing varying quantities of other substances,
solid, liquid or gaseous, is so potent that the properties of
chemically pure water are known more by inference than
by practical experience. They are in many important
respects different from those of the water of practical Jife.
The most characteristic property of ordinary water is
that we find it in all three of the molecular states ; we know
it in the solid state as ice, as a liquid, (over a considerable
range of temperature so well recognised as to be used for
graduating thermometers), and as a gas. Thus, freezing and
boiling are the common experience of many specimens of
the water of ordinary life, and yet it is difficult to say in
what circumstances, if any, perfectly pure water can be
made to freeze or to boil.
272 Glossary.
Ordinary water is a palatable beverage, and is a medium
in which a variety of forms of vegetable and animal life
can thrive, but pure water freed from dissolved gases is
perfectly sterile and quite unpalatable.
Ordinary water has a mass of 1 gramme per cubic centi-
metre (62*3 Ibs. per cubic foot) at 277a. Sea- water contains
dissolved salts to the extent of as much as 35 parts per 1000
parts of water, and its density varies from 1*01 to 1*05 g/cc
Rain-water is the purest form of ordinary water, it
contains only slight amounts of impurity in the form
of ammonium salts derived from the atmosphere. Spring-
water contains varying amounts of salts dissolved from
the strata of soil or rock through which it has percolated,
The most common of these salts is carbonate of calcium,
which is specially soluble in water that is already aerated
with carbonic acid gas; sulphates of calcium and other
earthy metals are also found, and sometimes a considerable
quantity of magnesia. These dissolved salts give the
waters of certain springs a medicinal character. In some
districts underground water is impregnated with common
salt and its allied compounds to such an extent that it is
no longer called water, but brine.
When impure water evaporates, the gas that passes
away consists of water alone, the salts, which are not
volatile, are left behind ; similarly when w r ater freezes in
ordinary circumstances the ice is formed of pure water,
the salts remain behind in the solution ; so that, except for
the slight amount of impurity due to mechanical processes,
pure water can be got from sea- water or any impure water,
either by distilling it, or by freezing it.
Besides the solicj constituents which give it a certain
degree of what is called " hardness," ordinary water con-
tains also small quantities of gases in solution, presumably
Water. 273
oxygen and carbonic acid. When the water freezes the
ice consists of pure water, and the dissolved gases collect
in crowds of small bubbles.
The thermal properties of water, in the state of purity
represented by rain-water, are very remarkable. Starting
from ordinary temperatures, such as 290a. (62*6F.)
and going upwards in the scale, the water increases in
bulk, and part evaporates from the surface, until the boiling
point is reached, a temperature which depends upon the
pressure, as indicated on p. 300. Then the water gradually
boils away without any increase of temperature, but with
the absorption of a great amount of heat. Going down-
wards, the bulk of the water contracts slightly until the
temperature of 277a. is reached (4C., 39'1F.) : that is
known as the temperature of maximum density of water.
From that point to the freezing point of water, there is a
slight expansion of one eight-thousandth part, and in the
act of freezing there is a large expansion amounting
to one-eleventh of the volume of water. It is in
consequence of this change of density in freezing that
ice floats in water with a one-eleventh of its volume pro-
jecting, if the ice is clean, solid ice, and the water of the
density of fresh water. Salt water would cause a still
larger fraction to project, but floating ice carries with it a
considerable amount of air cavities and sometimes a load
of earth so that the relation of the whole volume of an
iceberg to the projecting fraction is not at all definite.
Water-atmosphere. A general term used to indi-
cate distribution of water- vapour above the earth's surface.
The limitation which is imposed upon the quantity of
water-vapour in the atmosphere by the dependence of
the pressure of saturation upon temperature, places the
distribution of water- vapour on a different footing from
274 'Glossary.
that of the other gases. The atmosphere is enriched with
water- vapour by EVAPORATION at the surface and it is
distributed by the process of CONVECTION, but that pro-
cess does not extend beyond the TROPOSPHERE, and the
water- vapour beyond that limit must be attributed to the
action 6f diffusion. Above the surface saturation is pro-
duced only by the reduction of temperature on rarefaction
caused by convection, so that we cannot expect CLOUDS
to be formed beyond the range of convection. Hence
for all the ordinary purposes of meteorology which are
concerned with the formation of clouds and other forms
of precipitation, the water-atmosphere is limited by the
boundary of the troposphere.
Waterspout. The term used for the funnel-shaped
tornado cloud when it occurs at sea.
Waterspouts are seen more frequently in the tropics
than in higher latitudes. Their formation appears to
follow a certain course. From the lower side of heavy
Nimbus clouds a point like an inverted cone appears
to descend slowly. Beneath this point the surface of the
sea appears agitated, and a cloud of vapour or spray forms.
The point of cloud descends until it dips into the centre
of the cloud of spray ; at the same time the spout
assumes the appearance of a column of water. It may
attain a thickness (judged by eye) of 20 or 30 feet, and
may be 200 to 350 feet in height. It lasts from 10 minutes
to half an hour, and its upper part is often observed to be
travelling at a different velocity from its base until it
assumes an oblique or bent form. Its dissolution begins
with attenuation, and it finally parts at about a third of
its height from the base and quickly disappears. The
wind in its neighbourhood follows a circular path round
the vortex and, although very local, is often of consider-
Wares. 275
able violence, causing a rough and cimfused, but not
high, sea.
Water- Vapour .See AQUEOUS VAPOUR.
Waves. Any regular periodic oscillations, the most
noticeable case being that of waves on the sea. The three
magnitudes that should be known about a wave are the
amplitude, the wave length, and the period. The ampli-
tude is half the distance between the extremes of the
oscillations, in a sea wave it is half the vertical distance
between the trough and crest, the wave length is the
distance between two successive crests, and the period is
the time-interval between two crests passing the same
point. In meteorological matters the wave is generally
an oscillation with regard to time, like the seasonal
variation of temperature, and in such cases the wave
length and the period become identical.
If a quantity varies so as to form a regular series of
waves it is usual to express it by a simple mathematical
formula of the form y = a sin (/?. + ). Full explanation
cannot be given here, it must suffice to say that the
method of expressing periodic oscillations by one or
more terms of the form a sm(nt + a) is known as
44 putting into a sine curve," u putting into a Fourier
series," or as "HARMONIC ANALYSIS." See p. 311.
Any periodic oscillations either of the air, water,
temperature, or any other variable, recurring more or less
regularly, may be referred to as waves. During the
passage of sound waves the pressure of the air at any
point alternately rises above and falls below its mean value
at the time. A pure note is the result of waves of this
sort that are all similar, that is to say, that have the same
amplitude and wave length. The amplitude is defined in
this simple case as the extent of the variation from the
276 Glossary.
mean, while the wave length is the distance between
successive maximum values. The period is defined as
the time taken for the pressure to pass through the whole
cycle of its variations and return to its initial value.
Another good example of wave form is provided by the
variations in the temperature of the air experienced in
these latitudes on passing from winter to summer. This
is not a simple wave form because of the irregular
fluctuations of temperature from day to day, and the
amplitude of the annual wave cannot be determined until
these have been smoothed out by a mathematical process.
Fourier has shown that any irregular wave of this sort is
equivalent to the sum of a number of regular waves of
the same and shorter wave length. In America u heat
waves " and " cold waves " are spoken of. These are
spells of hot and cold weather without any definite
duration, and do not recur regularly.
Waves of Explosions are among the causes which
may produce a rapid variation of pressure which begins
with an increase, and is followed by a considerable de-
crease. The transmission is in the same mode as that of
a wave of sound. The damage done by a wave of ex-
plosion is often attributed to the low r pressure which
follows the initial rise. In the same way the destructive
effect of wind is sometimes due to the reduction of
pressure behind a structure resulting in the bulging
outwards of the structure itself in its weaker parts.
Weather. The technical classification of different
kinds of weather as given by the letters of the Beaufort
notation, set out in detail in the Introduction, p. 10.
Weather Maxim. A popular saying or proverb in
connexion with the weather, sometimes expressed in
rhymes. The best are the sailors' maxims which, at the
Wmtlior Ma.rim. 277
Meteorological Office, whether rightly or wrongly, are
associated with Admiral FitzRoy. The relation with
modern meteorology is often easily apparent.
First rise after low
Foretells a stronger blow
is quite characteristic of the passage of the TROUGH of a
depression.
If the wind backs against the sun.
Trust it not for back ic will run
is appropriate for the anticipation of a cyclonic depression
in the Northern Hemisphere.
Long foretold, long last,
Short notice, soon past
is also good meteorology in relation to travelling depres-
sions.
A useful essay might be written on the sailor's maxim
(( noted by Sir G. Nares
When the rain's before the wind
Your topsail halyards you must mind,
But when the wind's before the rain
You may hoist your topsails up again.
Some of the land maxims also represent fair conclusions
from experience.
If hoar frost come on mornings t.vain
The third day surely will have Jain
provides a fair indication of the gradual transition from
Easterly to Southerly weather.
A yellow sunset is regarded as a sign of stormy weather.
Admiral FitzRoy's version of the maxim is " A bright
yellow sunset presages wind ; a pale yellow wet.''
A voluminous collection of maxims and legends has
been compiled by Mr. Richard Inwards, a former President
of the Royal Meteorological Society, under the title of
Weather Lore," Admiral FitzRoy was perhaps the J;tsr to
278 Glossary.
attempt to draw up a scheme of weather prognostics
according to the precepts of experience, as he was the first
to introduce forecasts based on weather maps. Professor
W. J. Humphreys of the United States Weather Bureau
has given a physical explanation of many of the best
known weather signs in the atmosphere.
An examination of Mr. Inwards' collection makes it
apparent that the weather wisdom of ancient saws does
not lend itself to systematic presentation. Variants of
the same maxim sometimes contradict one another. A
large number have to do with the saints' days of the
calendar, and so with seasonal variations. The St.
Swithin's legend has obvdous reference to the transition
from spring drought to autumn rainfall (see SEASON), and
the fact that the hour of heaviest rainfalLin the year [in
London] is the third hour of the afternoon in July.
Many maxims are based upon the prevalent notion that
every unusual occurrence is a sign of something to come.
In modern days we prefer to regard the state of the crops
and the behaviour of birds as the natural consequence of
the past and present* not as the controlling cause of the
future. No doubt, if the course of events in the physical
universe is unique, that is to say, if the present is the
only possible sequel to the past, then the relation of the
future to the present is of the same order as the relation
of the present to the past, and while we are looking for
the one we may find the other. But what are offered as
signs are obviously insufficient as causes. When we read
Hark ! I hear the asses bray,
We shall have some rain to-day,
we are supposed to regard the braying, not as a cause of
rain, but as an evidence of superior intelligence in the
quadruped, stimulated by sensations which are too delicate
Weather Maxim. 279
for our senses ; but as a matter of practice it is doubtful
if any serious action was ever based on that intelligent
expression of the emotions.
Wedge. Short for wedge of high pressure : an exten-
sion of a high pressure region, more or less in the shape
of a wedge, that separates two neighbouring areas of lower
pressure. See Plate XVI.
Weight. The force with which the earth attracts
bodies near its surface. In dynamics we distinguish
between the amount of material substance which a body
contains, and the force with which gravity attracts it, but
experiments made by Galileo long ago led ultimately to
the conclusion that apart from the resistance of the air,
all bodies, large or small, are similarly affected by GRAVITY,
so that every part of a composite mass is now recognised
as separately affected by gravity, the result for the whole
being simply proportional to the amount of material
substance, irrespective of its particular nature Bodies
immersed in a fluid, as a balloon in air, may rise, that is,
apparently have less than no weight, because they dis-
place fluid, air in this case, which weighs more than the
bodies themselves. It should be noted that it is the
weight of the heavier surrounding air that furnishes
thrt driving force for the ascent of the balloon.
Wet Bulb. An ordinary thermometer having the
bulb coated with muslin that is kept moist. The evapora-
tion cools the bulb, and makes the reading lower than
that of a similar plain thermometer. See PSYCHROMETER.
Whirlwind. A quite small revolving storm of wind
in which the air whirls round a core of low pressure.
Whirlwinds sometimes extend upwards to a height of
many hundred feet, and cause dust storms when formal
over ii dcs<Tt.
280 Glossary.
Wind. The motion of the air. It appears certain that
the general winds of the earth are maintained by the
unequal warming of different parts of the earth by the
sun, but the exact manner in which they arise, and the
reason of their distribution, is not clearly understood.
Some local winds may be explained, as, for instance,
the wind accompanying the descent of an avalanche,
or the land and sea breezes, but the problem of the
general circulation is very much more difficult. In the
open sea, away from the disturbing influence of the
great continents, the general trend of the winds is as
follows : Round the equator are light variable winds,
and on either side to 20-30 north or south are to be found
the TRADE WINDS, moderate in force from the N.E. in the
northern and from the S.E. in the southern hemisphere.
Further towards the poles in about latitude 40 and 50
there are winds blowing chiefly from westerly points, but
by no means steady. They often reach the force of a gale.
Concerning the polar regions comparatively little is known.
The calm equatorial belt and the trade winds on either-
side follow the movements of the sun, being furthest
north in our summer, and furthest south in winter.
There is at present no exhaustive analysis of the facts
which have been collected concerning the force and
direction of the winds of the British Isles. The diagrams
on pp. 281-285 represent the monthly frequency of winds
of different forces at Deerness (for the northern Area), at
Holyhead (for the Irish Sea), at Scilly (for the mouth of the
English Channel), and at Yarmouth (for the East Coast).
A diagram is also given for the frequency of the wind in
January at ten stations. The monthly average duration of
winds of gale force at these stations is given under GALE.
The real fact which these diagrams illustrate is that no
Wind.
281
BEAUFORT NUMBERS
01T[2|3|4|5|6|7|8|9||0
METRES PER SECOND
Monthly Average Wind Frequency at Deernes&,
282
Glossary.
' 1 2 ! 5 ' 4 ! 5 ! 6 ! 7 ! 8 ! 9 ! 10
METRES PER SECOND ~
Monthly Average Wind Frequency at Holy head,
Wind.
Monthly Average Wind Frequency at Scilly.
284
Glossary.
Q_
<
30-
20 :
30-
20- :
10-
BEAUFORT NUMBERS
01234 56783 10
METRES PER SECOND
YARMOUTH
BEAUFORT NUMBERS
0|l|2|3|4|5|6h|8|9|K)
METRES PER SECOND
I
i
7 YEARS -
Monthly Average Wind Frequency at Yarmouth.
Wind.
285
v o
c^
aJ i
H-S
30,-H
20
10fi
30-
2 Of
30-H
20
10
30-H
20
iof
BEAUFORT NUMBERS
0|l|2|3|4|
METRES PER SECOND
Average Wind Frequency in January for Ten Stations.
Note : From an investigation lately completed it appears that on the
average the observed wind of force (i is related to the theoretical or
sjfeostrophic wind at various stations as follows :
Valencia, 62 per cent. Aberdeen, 58 per cent. Scilly, 63 per cent.
Yarmouth, 63 per cent. Spurn Head, 71 per cent.
286 Glossary.
exact scientific meaning can be attached to the comparison
of measurements of wind when the observations are made
close to the surface of the earth. It is certain that near
the ground or near buildings the velocity of the air is
changing rapidly with height. It has for example
recently been determined by special observations of wind
quite near the surface that the velocity at 4 ft. is from
83 to 90 per cent, of the velocity at 6 ft. above ground
according to the nature of the ground. The velocity
doubles itself, more or less, within 500 metres ; the actual
ligure depends upon the time of day among other things.
So a measure of wind is as much dependent upon the
exposure of the particular point at which the observation
was taken as of the unrestricted flow of air in an unob-
structed position. In recent years at the Meteorological
Office we have found the wind computed from the isobars,
the geostrophic wind, a much more, satisfactory standard
of reference than the anemometer readings.* Further
information about wind is given under the following
headings : BEAUFORT SCALE, EDDY, FRICTION, GALE,
GRADIENT WIND, GUST, ISOBARS, LINE SQUALL, SQUALL.
See also references in the index to Tables on p. 357.
Wind at the earth's surface is subject to considerable
diurnal variation, being greatest in the early afternoon
and least before dawn (see DIURNAL) : at the tops of
mountains and presumably at higher levels generally
the reverse is the case, as the following tabular summary
of the observations at the top of the Eiffel Tower (300
metres high) clearly shows :
* Strictly speaking*, in dealing 1 with winds at a considerable height,
we should employ the system of isobars appropriate to that particular
level.
Wind.
28?
<"O N O ** "*t~ ** vrt t*.vC coco t-* O LT O^' -^ ) -i T^ w O^ ^ t^O ' co
r^O xr> rh co w xr>woo t^uo i.nvo O C^ "-i rfoo w O CFi O\ OOO 1 oo
O O^oo O O t^ O O oo coo roo vrjao >-. co ooo >rh ONOO oo t>.
fO-^OOO > >O | -'OOOOrO^ t> t>OO cOOO Cl rJ-VO ifl vT> rf- rf- O^>
CT> w> O -^- ~t- -d-vjD vnt^c-i C^l^i-< ir>i-iOOO coi-i i-ioo r^oo >-*
i^r^QO r^.vOUD*O xr>wO C^i^Oxrjmi-^H-i O^Tj-^c^criO ^ OO
ON
M x^oo mvo oo i-tvoci O O^ooo cor>.r^oo TJ-MCO covo Cfr 1-1
>-< O O"*OO t^. t^ t>. COOO O O LO<O t> O> CO C7>00 CO vn t^. iri co co
O O G^ ^ O^> O\ O^> O^oo OO t> t~^ t~>. r> t>oo OOG^OOOOOO O^
-+O O covO Tt"vQ t^.M^i-iO-<O>O-i-i coo O >-" vri HH vrj
COHH OOO t>.vO cO-n\rit-i ^-vO t>.t^O M rt-O CTi-^-OO C?> ^O .
OoOOOl^-OOO rJ-CTiO^MVO covnw W rj-n-vo c^t^OJ O
^- O O"^ x> O O oo i~^ *- w wi x>. o\ -H ci -^- vn r->. ci ^ -i- ^ co co O
oo oo co oo oo r>o '-o >-o vr>yp O O CT^O !-> r^- <>> r^oo oo oo oo oo
_--,-._ w 1-1
noo oo O r
>. t^ 1>.OO OO OO OO i
O vnx^.oooo ->J~>^O t^vr>cor>.O vnO voi^.-ioo >-<oo O corj-w
r-^o rj-coM i-iOO O O MO r^t^CNOt-i >-oo Thoo O\ O oo O
m co cooo O O C^oo oo w>O i-< r>. r^ 1-1 C>o d O co ^ co O >-
M -t O CTvC^COO CO >-0 . ON"-" W r}-l^r>.oO i-ir>.O HH TOrt--l-
O O O O^<^O ^oooo i>.oooooooooooo ^<^O O O O O
o o ooo opoo<^ocoooocoooooc^c^oo ooo o
c^r^rj-inr-ric<j t^.-h-^-ThooO ^ooo VT>OSTJ-T>I-VO *-* t^O
\C \n i/-j -f- voo O 10 co O^ TT ^\ r^ t^ O^> "'J-oo coo oo oo oo r- r
OOOOOOOOOC^C^oooOOOQOO^O^OOOOOOO O
ji o
I
288 Glossary.
Winter.- In meteorology the months of December,
January, and February. Astronomically, winter com-
mences on December 21st, and ends on March 21st. See
SEASONS.
Wireless Telegraphy has two types of application
in meteorology, according as we deal with electromagnetic
waves produced artificially or naturally. The former is
exemplified by the wireless weather-reports from vessels
at sea, enabling synoptic charts to be extended to ocean
regions without the long delays otherwise involved, and
by the distribution of meteorological information from
high power stations. The other application is in the
detection of distant thunderstorms. Very early in the
historyipf Wireless Telegraphy it was found that lightning
flashes* emitted electromagnetic waves capable of affecting
the detecting device then in use the coherer. A coherer
may be|made to actuate an electromagnet the armature of
which carries a pen for recording on a revolving drum, so
that. every lightning flash within 200 miles or more may
thu? automatically record itself. In the modern wireless
receiver the electromagnetic waves set up by lightning
cause clicks in the telephone. It seems probable that
a large proportion if not all of the irregular and trouble-
some noises called atmospherics, strays, or X's, which are
formidable obstacles in long distance wireless telegraphy
may be referred to distant lightning.
Zenith. The point of the sky immediately " over-
head," or in the vertical produced upwards ; the opposite
of nadir, which is the point in the sky below one's feet,
or in the vertical produced downward beyond the earth's
centre and out the other side.
Zodiac.
289
Zodiac. The series of constellations in which the sun
is apparently placed in succession, on account of the
revolution of the earth round the sun, are called the
Signs of the Zodiac, and in older writings give their
names and symbols to the months, thus :
March is associated
April
May
June
July
August
September
October
November
December
January
February
with Aries, the Ram.
Taurus, the Bull.
Gemini, the Heavenly Twins.
Cancer, the Crab.
Leo, the Lion.
Virgo, the Virgin.
Libra, the Scales.
Scorpio, the Scorpion.
Sagittarius, the Archer.
Capricorrius, the Goat.
Aquarius, the Watercarrier.
Pisces, the Fishes.
Owing to precession, the position of the sun relative to
the above constellations has altered a good deal since
classical times. The sun now enters Aries in April.
Zodiacal Light. A cone of faint light in the sky,
which is seen stretching along the Zodiac from the Western
horizon after the twilight of sunset has faded, and from
the 'Eastern horizon before the twilight of sunrise has
begun. In our latitudes it is best seen from January to
March after sunset, and in the Autumn before sunrise.
In the TROPICS it is seen at all seasons in the absence of
moonlight. It is supposed to be due to the reflection of
sunlight from countless minute particles of matter revolv-
ing round the sun inside the Earth's orbit. Its light is
usually fainter "than that of the Milky Way.
13204 K
290 Glossary.
SUPPLEMENTARY ARTICLES.
Absolute Humidity. Aqueous vapour has a very
large annual variation but a small diurnal variation,
whether one considers the amount of vapour present, or
the contribution which it makes to the atmospheric
pressure. This is readily seen in consulting the two
accompanying tables, which give respectively the quan-
tity and the pressure of aqueous vapour at Richmond
(Kew Observatory).
According to the first table the quantity of aqueous
vapour in the atmosphere in the hottest months, July
and August, is nearly double that in the coldest months,
January and February. The quantity is slightly greater
in the afternoon than in the morning hours, but the
excess of the afternoon maximum over the morning
minimum represents only 6 or 7 per cent, of the mean
value.
As the second table shows, the mean vapour-pressure
in July and August is fully double that in January and
February. The diurnal variation is also a little more
marked than it was for the quantity of vapour. The
maximum pressure in the afternoon occurs decidedly later
in the day at midsummer than at midwinter. The
morning minimum, on the other hand, occurs a little
earlier in summer than in winter.
Absolute
291
j -utWK
f; : f ; ^ & b
b & t 5 t
"^ ;f
S E S C & b
b b\ t~x <b <^>
K
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c; c t 5 c a i
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292
Glossary.
'A*CI
*
NO
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OC
* 2 JT JT
2
o
oo
**
ON
NO
Nb
K
g
bN "M g g
s
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K
NO
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5 1
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s
K
ON
3
00
NO
00
NO
r^
CO
00 C4 p p
Vj
H
cc
K
Os
5
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NO
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NO
K
oc
"ON Vi "<* "*
s
b
ro
do
K
oo
bs
3
ON
NO
00
NO
N
00
ON >^ O M
ON Vl j* Vt-
?
b
-h
CO
g,
g
2 s
ON
NO
ON
NO
K
b H Vi- V*-
vT
b
cc
K
ON
ON
oo
ON
NO
O
**
M W^ p P
b H "* V
00
b
oc
r
Os
ON
ON
NO
fc
cc
O Vl Vh rr>
i
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b
oc
K
ON
ON
NO
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K.
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K
io
M * OS 00
s
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CO
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e
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r
p
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ff
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g
IT
Tf
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ON
S
K
p
2
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b
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JJ
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s
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Os
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M
e,
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j
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o <^ *o t^
NO
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OS
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s
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rr>
ic
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O
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s
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s ^ ?
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oc
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Accumulated Temperature. 293
Accumulated Temperature. A term used to de-
scribe the excess or defect of temperature in relation to a
selected base-level, prevailing over a more or less ex-
tended period of time, e.g., a week, a month, or even a
year. Accumulated temperature is employed mainly in
connection with agricultural statistics, and the base-level
adopted in this and in most Continental countries is a
temperature of 279a, equivalent to 42F., or 10 above the
freezing point. This temperature was suggested many
years ago by Prof. A. Candolle, an eminent Swiss
physicist, as the level above which the growth of
vegetation commences and is maintained. Temperatures
above 42F. may therefore be regarded as effective, inas-
much as they tend to the promotion of active plant -
growth. Temperatures below 42F. may be regarded as
non-effective at the best, and at certain seasons of the year,
when the defect is large, as positively injurious. In the
Weekly Weather Report the amount of effective and non-
effective heat is expressed in what are described as u day-
degrees." A "day-degree " Fahrenheit signifies 1 above
or below 42F. continued over a period of 24 hours, or, in
inverse proportion, 2 continued over 12 hours, 3 over 8
hours, and so on. At the Meteorological Office the
amount of Accumulated Temperature above and below
42F, is computed from the daily readings of the maxi-
mum and minimum thermometers, in accordance with
formulae proposed more than 30 years ago by Sir Richard
Strachey, at that time Chairman of the Meteorological
Council. The actual method of computation is described
in the preface to the Weekly Weather Report for 1884
and subsequent years. As examples of the results
attained, the following statistics may be of interest. In
the table the amount of Accumulated Temperature above
294
Glossary.
and below 42F. is given respectively for the exceptionally
warm summer of 1911, in contrast with the cool summer
of 1907 ; and for the cold winter of 1916-17 in contrast
with the mild winter of 1912-13.
The period embraced is, in each case, the 13 weeks
comprised as nearly as may be within the three months
June to August and December to February. The winter
of 1916-17 was, it need scarcely be said, prolonged far
beyond the ordinary winter boundary. The averages
with which the actual results are compared are those for
the 35 years 1881-1915.
ACCUMULATED TEMPERATURE.
Above 42F.
Below 42F.
Day
degrees.
Difference
from
average.
Day
degrees.
Difference
from
average.
Summer of
1911 ...
1,940
+ 297
?? >
Winter of
?> ?>
1907 ...
1916-17
1912-13
M23
54
278
220
-126
+ 98
598
'85
+*37
-176
Atmospheric Electricity treats of the various elec-
trical phenomena observed at or near the earth's surface.
If we regard the earth as a sphere of radius R, carrying
an electrical charge of uniform surface density <r, the elec-
trical force on unit charge at any radial distance / not less
than R is the same as if the whole charge ^vR 2 were
Atmospheric Electricity. 295
collected at the centre, and is thus C x 4:7rrrR*fr 2 , where C
is a numerical constant depending on .the units adopted.
On the electrostatic system C is unity. Charges of like
sign repel one another, and it is usual when talking of
electrical force to regard it as the force experienced by a
positive unit. Thus the above force is upwards or down-
wards according as <r is positive or negative. In fine
weather v is normally negative and so the force is down-
wards. The force at any surface-point is really deter-
mined by the charge in its immediate vicinity, and thus
whether * be uniform or not the force is given correctly
by 47To-(7, where fa is the surface density at the point.
If we take a point at a height k small compared with R
and suppose p to be the mean electrical charge per unit
volume throughout the height /*, we find in a similar way
that the electrical force at the point is 4;r(7 (tr + pJi).
If F denote the attraction towards the earth on a unit
charge at height h, in order to raise it to a slightly greater
height h' an amount F (h' h) of work must be done,
which is transformed into a rise of potential (i.e., capacity
to do work). If the potential rises during the operation
from V to V, then F(h'-h)= V - V. Thus the force
downwards is F = (V - F)/(fc'-A).
It is usual to consider the change of potential per metre
of height, which is known as the potential gradient. If
we write P for ( V i - F)/(A' A), we finally get
where P,, and P h are the values of the potential gradient
at ground level and at height h respectively. A is here a
numerical constant, equal to 4?r in the electrostatic
system of units.
P is positive, i.e. the potential increases as we leave the
ground, if (the force on a positive charge is directed down-
296 Glossary.
wards, i.e. if <r is negative. This is almost always the case
in fine weather. As we go up, P will increase if p is nega-
tive like <r, but diminish if p is positive. In fine weather P
diminishes as we go up, or p is positive. At Kew Observatory
the potential gradient in fine weather averages about 300
volts per metre. At most other stations somewhat lower
values have been observed. The potential gradient has
a large annual variation, being lowest in summer. There
is also a large diurnal variation, with usually two maxima
and two minima in the 24 hours. At Kew the lowest
value occurs in the early afternoon in summer, but in the
early morning in winter.
As we go up from the earth, the potential gradient
normally diminishes, i.e. each successive metre adds less
to the potential, but all contributions being in the same
direction the potential goes on mounting, and at the levels
attained by aircraft may reach hundreds of thousands of
volts. Any body remaining at one level gradually assumes
the potential of the surrounding air, the process being
accelerated if the body is provided with sharp edges or
points, or with an engine emitting fumes, or if it is
discharging ballast. But if a body makes a large rapid
change of level, it may depart widely in potential from
the surrounding air, and in an extreme case this miy lead
to discharge by sparking and consequent danger to an
airship.
Besides its regular changes, potential gradient near the
ground shows numerous if not perpetual irregular changes.
Clouds sometimes carry large electrical charges, and heavy
passing clouds usually cause large fluctuations of potential.
During rain the potential gradient at ground-level is
often negative.
The charge in the air evidenced by the change in
Atmospheric Electricity. 297
potential gradient with height may be carried by
drops of water, but it is also carried by ions. There
are always at lea'st two kinds of these present in the
atmosphere, usually known as light (or mobile) ions and
heavy (or Langevin) ions. The latter seem the more
numerous, especially near smoky towns, but they move
very slowly, and as carriers of electricity are relatively
unimportant. Also the numbers of heavy ions carrying
positive and negative charges seem approximately equal,
thus they- neutralise one another so far as influence on the
potential gradient is concerned. On the other hand there
is usually a marked excess of light positive ions, as
suggested by the falling off of the gradient as height
increases. The number of light positive and negative
ions combined near ground level is usually of the order
of 1,000 per c.c.
Air is often regarded as a non-conductor of electricity,
and it is a very poor conductor compared with copper ;
it conducts, however, to a certain extent. The negative
charge on the earth and the positive ions in the atmosphere
attract one another, a process equivalent to the passing of
a current from the air to the earth. This current is
extremely small if reckoned in amperes per cm. 2 only in
fact about 2 x 10~ 16 on the average but for the earth as a
whole this means fully 1,000 amperes, if we can accept
the few stations where observations have been made as
fairly representative.
The process by which this current is maintained is at
present a mystery. The difficulty is analogous to that
which puzzled philosophers who saw rivers flowing into
the sea without the sea becoming fuller. Two explana-
tions which seemed promising, though neither suggested a
sufficiently regular source of supply, appear to have broken
298 Glossary.
down. It was suggested that rain might restore the
balance by bringing down negative electricity. But the
observations hitherto made too limited as yet perhaps to
be wholly conclusive indicate that while rain not
infrequently brings down negative electricity, it brings
down on the whole more positive. Another suggestion
was that the balance might be restored by lightning.
Mr. C. T. R. Wilson has, however, recently devised a
method of determining the sign and the quantity of
the electricity brought to the earth by a lightning
flash, and his results suggest that on the whole the
charge brought down is at least as often positive as
negative.
Observations made from balloons indicate that at heights
above 1 or 2k a rapid increase takes place in the ionisation
of the atmosphere. At heights of 90 to 150k AURORA is
a frequent phenomenon in high latitudes, and it is natural
to suppose that at such heights the electrical conductivity
is much greater than near the ground.
Aurora takes a great many forms. In addition to
arcs, bands, rays and isolated patches, there are some-
times displays resembling curtains or draperies, also so-
called " coronae," representing a concentration of rays
directed towards a limited space of the heavens.
Of the accompanying figures, one shows an arc, the
other a curtain. The arc is the most symmetrical and
stable form of aurora, sometimes persisting with little
visible variation for several minutes. Auroral curtains,
when seen in the zenith in high latitudes, seem very thin
in the direction perpendicular to their length, und are
usually in rapid motion. The lower edge, both of arcs
and curtains, is usually much the best defined.
Aurora is very rare hi the south of Europe, and is but
To face p. 298.
P
tf
.tf
5
^ 3204
To face p. 291).
PQ
fe
H
K
<D
tf
H
PQ
O
.0
>. .
H
d
PH
O
O
CM
fi
Aurora. 299
seldom seen, in the south of England. But the frequency
increases rapidly as we go north, and in Orkney and
Shetland Aurora is a comparatively common phenomenon.
A /one of maximum frequency passes from the North of
Norway to the South of Iceland and Greenland. Aurora
is also common in high southern latitudes, but its distri-
bution there is still imperfectly known. The visibility
of weak aurora is much affected by moonlight, and even
the strongest aurora seems to be invisible if th-3 sun is
above the. horizon. Thus there is some difficulty in
assigning definite laws for its frequency of occurrence.
In the British Isles and similar northern latitudes it is
most common in the late evenings and near the equinoxes ;
but in the north of Norway and Greenland it appears to
be most frequent near mid-winter. Of late years Pro-
fessor Stormer has devised a method of photographing
aurora, includins 1 reference stars in the photograph, and
by means of photographs taken simultaneously at the two
ends of a measured base he has obtained numerous results
for the altitude. Heights exceeding 200 k. are not unusual,
but a great majority of the heights lie between 90 k. and
130 k.
Aurora is undoubtedly an electrical discharge, and we
thus infer that at heights of 100 k. at least in high lati-
tudes the atmosphere must often, if not always, be a
vastly better conductor of electricity than it is near the
ground. Some distinguished travellers have claimed to
see aurora come down between them and distant moun-
tains. If this be the case, aurora must occasionally come
down to much lower levels than any measured by
Stormer, and there may be truth in the belief held by
some meteorologists that cirrus cloud is sometimes the
seat of aurora.
300 Glossary.
When visible in England, aurora is nearly always
accompanied by a magnetic storm, but this is not the case
when it is confined to high latitudes. The spectrum of
aurora consists of a number of lines, one of which in the
green is particularly characteristic, bat it has not yet been
identified with certainty with that of any known gas. It
is not at all improbable that eventually we may learn much
from the spectrum of aurora a~s to the constitution of the
atmosphere at heights far greater than those accessible to
balloons.
Boiling-points. It is customary to Bay that a
thermometer is graduated so that the freezing-point and
boiling-point of water com^ at specified figures, e.g., 32
and 212 on the Fahrenheit, or 273 and 373 on the abso-
lute scale, and the importance of defining the pressure
under which the water is boiling is frequently over-
looked. In the process of boiling, bubbles of vapour are
formed in the interior of the liquid. If the pressure of
the air above the liquid is low the bubbles are formed and
grow more readily, i.e., at a lower temperature than when
the pressure is high, so that the boiling-point is lowered
by decrease of pressure, raised by increase of pressure.
Except on rare occasions pressure at sea level in
England is between 1,040 mb. and 960 mb. Under the
pressure 1,040mb., water would boil at 373*73 a, under
960 mb. at 371-49 a. Under 1,000 mb. the boiling-point is
372*63 a. The standard pressure used in the definition of
the temperature 373 a (or 212F.) is that due to 760mm.
of mercury at the freezing point ol ! water at sea level in
latitude 45, i.e., 1,013*2 mb.
When we leave sea-level and ascend to places where
the pressure is much lower, the temperature at which
Boiling-points. 301
\vater boils is reduced. For example, at Pretoria, 5,200 ft.
above sea-level, the average pressure is 871 mb., corre-
sponding with a boiling-point 369a.
This change in the boiling-point in the course of an
ascent is mado use of in the measurement of heights by
the HYPSOMETER, a method which is convenient, as the
apparatus required is more portable than a mercury-
barometer^ and more reliable than an aneroid. At 2 ,600 f t. r
the greatest height reached by the Duke of the Abruzzi on
the Himalayas, the pressure would be about 420 mb. and
the boiling-point 350a. At great altitudes there ,is much
difficulty in cooking, owing to the comparatively low tem-
perature of the boiling water.
It should be mentioned that the figures which have
been given above refer to the boiling of pure water. The
addition of salts in solution raises the boiling-point. Sea
water of average density boils at about 0*5a. above pure
water under the same pressure. By adding 40 grammes
of salt to 100 grammes of water the boiling-point can be
raised by 9a.
Calorie or gramme-calorie i.e., the heat required to
raise the temperature of 1 g. of water by la. at 288a. is often
used in connection with measurements of solar radiation.
The amount of solar radiation is accordingly often given as
so many gramme-calories per square centimetre per minute.
At the Meteorological Office we use instead the milliwatt
(per square centimetre), because that is the accepted unit of
"rate of working" in Thermodynamics. Even in the
total absence of cloud the amount of solar radiation which
reaches a given area on the earth's surface at a given time
is very variable. It depends on the altitude of the sun
and the transparency of the earth's atmosphere. Thus it
302 Glossary.
is by no means a simple thing to estimate the ' Solar
Constant," i.e., the radiation which would be received in
one minute by a square centimetre outside the earth's
atmosphere, at the mean distance of the earth from the
sun, the incidence of the radiation being normal. From
a long series of investigations, Dr. Abbot, of the
Smithsonian Institution, Washington, U.S.A., has arrived at
1*93 calories per square centimetre per minute or 135 milli-
watts per square centimetre, as the mean value of the solar
constant. Dr. Abbot concludes, however, that solar radia-
tion, outside the earth's atmosphere, is not really constant,
the fluctuations being presumably due to changes in the
solar atmosphere.
Contingency. In expressing the relationship between
two variables, the correlation co-efficient " r " can only be
used when both variables are given quantitatively. The
correlation ratio rj can be used when one or both variables
are expressed quantitatively.
If, however, both variables are given qualitatively, it is
not possible to use either the correlation co- efficient or the
correlation ratio. In such cases it is usual to calculate the
co-efficient of mean square contingency, Ci. This co-
efficient Ci is of the same nature as the correlation
co-efficient r.
The method of calculating d can be obtained from any
text-book on the Theory of Statistics". An excellent
astronomical example is given on page 167, " The Com-
bination of Observations," Brunt (Camb. Univ. Press), in
which it is shown that there is a close relationship be-
tween spectral type and the colour of stars. See Correlation.
Correlation Ratio. A measure of the relationship
* An Introduction to the Theory of Statistics by G, Udny Yule,
Correlation Ratio. 303
between two variables. In calculating a correlation co-
efficient it is assumed that the regression is linear, i e.,
that the observations themselves, or the means of grouped
observations when plotted, lie approximately along a
straight line. In the calculation of the correlation ratio iy,
linear regression is nob assumed. The correlation ratio
may be denned as a generalised coefficient which measures
the approach towards a curved linear line of regression of
any form.
If /;, r be respectively the correlation ratio, and correla-
tion coefficient, found from a set of observations, // 2 -r 2 is
a measure of the deviation from linearity of the curve of
regression. See Computer's Handbook, pp. V 29-52.
Duration Of Rainfall. A climatic feature of some
importance. Many forms of self-recording raingauge
have been designed to determine the distribution of rain
in time, but it is only in recent years that adequate atten-
tion has been paid to the matter, and the pattern of gauge
has not yet been standardised in this country. During
the 34 years, 1881-1914, at the headquarters of the
British Rainfall Organization, in North-west London, the
average annual duration of rain (snow and other forms of
precipitation included) was 433 hours, about seventeen
days, or 5 per cent, of the year, the extreme annual values
ranging from 299 hours to 689 hours (12| days to 29 days,
3vr per cent, to 8 per cent.). In the wet regions ot
Cumberland, Wales and Western Scotland, the annual
duration exceeds 1,000 hours (11*5 per cent, of the year)
and in the wettest parts in the wettest years sometimes
approaches 2,000 hours, or between a quarter and a fifth
of the year.
The duration values for the year 1915 (a wet year) are
-iven for the Observatories of the Meteorological Office,
304
Glossary.
together with the total amount of rainfall and the mean
rate of fall per hour :
Rainfall.
Duration.
Rate per hour.
mm.
hrs.
mm.
Kew ...
805
461 1*75
Falmouth
1,322
744 i'7 8
Eskdale
1,224
790 i "55
Aberdeen . . . 805
589 1*37
Valencia ... 1,516
761 1-98
Armagh
741
530
1-40
The highest duration value for the year 1915, accord-
ing to tables set out in the annual publication British
Rainfall was 1,303 hours, or 15 per cent., near Kinloch-
leven, Argyll, a few miles S.E. of Ben Nevis, and the
lowest value 442 hours at Nottingham.
In the London district the mean rate of fall increases
with the mean temperature, reaching its maximum in
July and minimum in January. The actual duration,
on the average, is least in July and September, when
it is little more than 25 hours, and greatest in November,
when that figure is nearly doubled.
The CORRELATION COEFFICIENT between annual
amount and annual duration of rainfall for 35 years
in N.W. London is +'81.
Dust. The atmosphere is permeated by dust up to
great heights ; the dust may be derived from deserts, -or
from any dry surface, from evaporated sea-spray, from
Dust. 3()f>
plants in the shape of pollen grains, from the debris of
meteorites, from volcanoes. Dust is important mete-
orologically in that water vapour condenses on dust
particles. Dust -free air may be cooled considerably
below the dew-point without condensation occurring.
Dust Counter. A.itken has devised three types of
instrument for estimating the number of dust-particles in
a sample of air ; a large model with artificial illumina-
tion, suitable as a permanent part of the equipment of a
first-class observatory ; a portable model suitable for
testing locally polluted ah, e.g., in sanitary work ; and a
pocket instrument for use in country districts where the
variations in the dustiness are small. This last instrument
will be described briefly, as it embodies the same principle
as the others, but with fewer working parts.
Into a receiving chamber, lined with moist blotting
paper, a measured quantity of the air to be tested is intro-
duced. This chamber contains a horizontal glass stage,
having fine cross lines 1 mm. apart etched upon it so as to
divide the surface into a network of squares. A sudden
reduction of the pressure of the saturated air in this
receiver, which is effected by means of a pump, causes the
aqueous vapour to condense upon the dust-particles, and
the small raindrops so formed fall upon the glass stage.
The average number that fall upon one of the small
squares is then counted, with the aid of a lens let into
the roof of the receiver, and so an estimate of the number
of dust-particles in 1 cc. of the air can be made. The
method of measuring the volume of the sample of air ijs
ingeniously simple. When the piston is drawn down in
making a stroke of the pump, the air in the receiver
expands by a fraction which is read off upon a scale
306 Glossary*
engraved upon the barrel of the pump for this purpose.
If now the receiver is put into communication with the
outer air, a sample of air having this volume enters and
restores the pressure inside to that of .the outside air. The
complete instrument can be packed in a box 4| x 2^ x 1
inches and weighs barely half-a-pound.
The following table shows the number of dust-
particles found by Aitken in 1 cc. of air at various
localities :
Cannes (April) ... 1,500150,000
Simplon Pass (May) 50014,000
Summit of Rigi (May) 200 2,35( )
Eiffel Tower (May) ... ... 226104,000
Paris (Garden of M.O.) (May) ... 134,000210,000
London (Victoria Street) (June) ... 48,000150,000
Dumfries (Oct.-Nov.) 39511,500
Ben Nevis (August) 335 473
False Cirrus." False cirrus may be defined as a
type of cloud resembling cirrus but occurring at lower
altitudes. It consists of snow, may occur at any height,
and may be divided into two main classes :
(1) Consisting of isolated tufts of large masses of
considerable height.
(2) Spread out in extensive sheets.
Type (1) is commonly seen on the tops of showers.
The rounded top of a cumulus, consisting of minute
particles causing diffraction rings, often turns to false
cirrus; this may afterwards be carried for considerable
distances both vertically and horizontally. As the shower
dissolves away, the false cirrus may remain for some time
* Contributed by Lieut. C. K. M. Douglas, R.F.C.
False Cirrus. #07
afterwards, and may consist of white tufts or dull grey
masses, with edges resembling cirrus. It occasionally
forms direct, and not from other clouds. For instance,
on April 1st, 1917, near Edinburgh large tufts of false
cirrus formed at a height of about 6,000 feet, apparently
caused by convection currents. Snow showers resulted
but only a few flakes reached the ground.
Type (2) is distinguished by its more regular formation
and covers large areas. It sometimes has a dull uniform
grey appearance, being then usually classed as " alto-
stratus " ; it has then often very great thickness. Some-
times it assumes the well-known cirrus form, resembling
cirro-stratus sheets, but never causing a halo. No very
definite dividing line can be drawn between "false
cirrus " and cirro-stratus. False cirrus sheets may change
to thin wavy clouds resembling cirro-cumulus, causing
diffraction rings ; this most often happens in the evening,
and the clouds may afterwards dissolve away entirely.
Dense masses of false cirrus from the south with surface
winds from the north-east often precede heavy thunder-
storms.
Glacier. A river of ice flowing slowly but irresistibly
down the valleys of those regions which have a perpetual
supply of snow to feed the head of the glacier. The
explanation of the gradual flow of ice down valleys under
the action of gravity, forms a special section of physics
and is another illustration of the peculiarities of the
material of which water is composed. Glaciers are not
only of climatic importance but in dynamical meteorology,
with rivers, they deserve consideration as showing the
line along which air flows when the excess of density
cine to cooling is the primary reason for the movement.
#08 Glossary,
From the analogy we may conclude that a gully is no
protection against katabatic winds but rather the reverse.
Gravity relates to the attraction between material
bodies. The law of universal gravitation is that every
mass attracts every other mass with a force which varies
directly as the product of the attracting masses and in-
versely as the square of the distance between them. It is
convenient to regard the attracted "body as of unit mass.
The law then implies that the force exerted is independent
of the temperature or velocity of the attracting body.
Both these conclusions have been attacked of late years,
but it is not questioned that they are sufficiently exact for
meteorological purposes.
It is easily shown mathematically that a sphere whose
density varies only with the distance from the centre
attracts an external body exactly as if the whole ma^s
were collected at the centre, and that a similarly consti-
tuted spherical shell i.e., a mass bounded by two con-
centric spherical surfaces while attracting an external
body as if its mass were collected at the centre, exerts no
attraction at any internal point. Let us apply this to a
point in the atmosphere at height h above the ground,
regarding the earth as a perfect sphere of radius 72, and
assuming the density, whether of the earth or the atmo-
sphere, to vary only with the radial distance. The atmo-
sphere outside the spherical surface of radius R + h exerts JU i.
no attraction, while the earth's mass M' within the surface *
of radius R + h attract as if collected at the centre. Thus
the attractive force is G(M + M')l(R + h) 2 , where G is a
constant. This becomes g (1 + M'jM) (l + /i/#)~ 2 , where
c/ =GMjR- is the corresponding force at the earth's sur-
face, i.e., wholly within the atmosphere. Counted 171
Gravity. 309
kilogrammes M' is large, but even if we went to the
confines of the atmosphere M\M. would be less than
1/1 million. Thus the attraction of the atmosphere may
be neglected, at least for meteorological purposes. The
variation with height in the earth's own attraction is
much more important. At all heights attained by
balloons we may neglect ft 2 /jR 2 , and so replace (l + /*/jR)~-
by 1 -2A/R. But at a height of say 10 miles this repre-
sents a reduction of one part in 200 in gravity.
In reality the earth is not a sphere, but approaches to a
spheroid whose equatorial radius is 10*7 k. longer than its
polar radius. Also it rotates, and the " centrifugal force "
due to the rotation reduces gravity, especially near the
equator. The earth's surface is also irregular in outline,
and the density variable, at least near the surface. Thus
the formulae actually advanced to show the variation of
gravity at different parts of the earth's surface are com-
plicated.
The following formula, due to Helmert, is perhaps the
best known. In it g denotes the acceleration of gravity
in C.G.S. units, i.e., in cm/s 2 :
=978- 000(1+ 0-00531 sin 2 </>) x
2h 37* c: It' (6-0) \
" K 2/2 A 2./2A y )
Here <p is the latitude, R the earth's mean radius, h the
height above sea level, h' the thickness of surface strata
of low density,. A the earth's mean density (approxi-
mately 5'6 times that of water), c mean density
of surface strata (usually taken as 2*8), the
actual density of surface strata for the region, and y
3.10 Glossary.
a so-called orographical correction arising from neigh-
bouring mountains and so on. At sea level, supposing
3=0, and tj negligible, this becomes
=978-000 (1 + 0-00531 sin 2 f),
or more conveniently </= 980 '5966 2*5966 cos 2<l>.
Thus g has its mean value where cos 2< vanishes, i.e.,
where 2^=90, or </>=45. This explains why it is usual
to reduce gravity to latitude 45. This means reducing
some measure actually made e.g. of the height of the
barometer to what it would have been if gravity had
possessed its mean value. The formula does not, of
course, imply that gravity has the same value at every
spot in latitude 45, irrespective of its height above sea
level or other local peculiarities.
The determination of g absolutely at any spot with the
precision which the formulae suggest is extremely difficult,
but relative values of /, or, differences between its values
at different places, can be determined with very high pre-
cision by means of pendulum observations.
If t and t' be the times of oscillation of a certain pen-
dulum at two stations, the corresponding values g and g'
of gravity are connected by the relation g'\g(t\t'}*.
This enables gravity at any station to be determined in
terms of gravity at a base station. For accurate work
corrections have to be applied to the observed times of
swing to allow for departures of temperature and pressure
from their standard values, also for chronometer- rate and
flexure of the pendulum-stand. When all the known
corrections are carefully made a very high degree of
accuracy is obtainable. For instance, taking 981*200 as
the value of g at Kew Observatory, this being the value
accepted for the purposes of the Trigonometrical Survey
Gravity ,, ,3il
of India, the last two comparisons instituted between
Greenwich and Kew, the one made by the United States
Coast and Geodetic Survey, the other by the Trigono-
metrical Survey of India using two different sets of
half-second pendulums, gave for g at Greenwich the
respective values 981-188 and 981*186. -
Pendulum and other geodetic observations have led to
a theory of isostasy which has received strong support of
late years, especially in the United States. According to
this theory if we start at about 100 k. below sea level we
find between there and the free surface an approximately
uniform quantity of matter irrespective of whether the
free surface is mountainous or not. A lesser density
under lofty mountains and a higher density under deep
seas act as compensations.
While the mass of a body is independent of its position,
its weight, i.e., the gravitational attraction exerted on it
varies with g and so increases as we pass from the equator
towards either pole. Denoting by g<p the value of g in
latitude </> we obtain from the formula
^0=983-19, 00=978-00.
In other words, gravity at the poles exceeds gravity at the
equator by 1 part in 189.
Harmonic Analysis. There are many meteoro-
logical phenomena which recur with some approach to
regularity day by day. If the changes of such a variable
us temperature are represented by a curve, then the
portions corresponding to successive days bear a strong
likeness to one another. If for the actual record for each
day the record for the average day were substituted, the
variation for u long period would be represented by a
31;!
Glossary.
c^p-KEW OB5ERVATORY, RICHMOND, SURREY -*^>
-NORMAL DIURNAL VARIATION IN TEMPERATURE IM MARCH
a
ZB2
280-
figure i jf
"rom Hourli^ Mean
i a
282
NSET
278
22
282
280
278
\^
278
276
294
at
280
278
276
*""-* ^.^^^ SUNRISE .s
^^
, , i , ,
i i i i i
| , 1 , ,
, , 1 i ,
'IRST TERM OF HAR
FIRST TWO
Figure 2 .*
^lOINIC SERIES
^
/^^^
...^
"^x^
''Ti^""---^ ,*_-'
.-''/^
^^:
, i i , ,
, i 1 . i
i i 1 i i
NORMAL DIURNAL VARIATION IM TEMPERATURE IN JULY.
292
290
288
286
/
P^~~
\
200
288
286
~/_
\ SUNSET
TV,
/
~\^^ SUNRISE
7
t 1 1 I 1
1 1 1 1 !
.III!
1 1 1 1 1
GMT 2 4 6 8 10 12 14 16 18 20 22 24
Hariri unit' Ann.lt/s-is. 313
curve in which the part corresponding with each day was
like its fellows. The simplest curve possessing this
property of continuous repetition is a curve of sines. As an
example the variation of temperature at Kew Observatory,
Richmond, in July, may be cited. The sequence of change
throughout the average day is shown in the lower part of
the figure. The representative curve is not unlike a
curve of sines, but it is not quite symmetrical. The rise
which commences at sunrise and lasts until after 15 h.
is more. steady than the drop which is rapid in the evening
and slow after midnight. A good approximation to the
temperature on the average day is given, however, by the
expression 289 '9 + 3 '7 sin (15*+ 224^) where t is the time
in hours reckoned from midnight. It will be seen that
the lowest value is reached at the time given by
15-f2247j:=270 i.e. at 3 h. 6 m. and the maximum comes
12 hours later at 15 h. 6 m. The substitution of a sine-
curve for the curve based on the observations would make
the minimum too early by an hour, but would not affect
the maximum so much.
The curve showing the diurnal variation of temperature
in March (in the upper part of the figure) is not so near
to a sine-curve as that for July, The rise in temperature
from minimum to maximum takes little more than six
hours. The best sine-curve for representing the variation
is given by the formula
0=278 -74 + 2 -47 sin (15^ + 222)
and is shown by the broken line in the figure. It will be
seen that the agreement is by no means close. To
obtain a more accurate expression for the temperature, an
additional sine term with a period of 12 hours may be
314 (jlossary.
introduced. The best formula containing such a term is
0=278 '74+ 2 -47 sin (15* + 222) + 0-63 sin (30* + 39)
The new term 0*63 sin (30* + 39) is positive in the early
morning and in the early afternoon, so that it delays the
drop to the minimum and makes the maximum earlier.
In Figure 2, the continuous curve which corresponds
with the proposed formula crosses the simple sine-curve
at intervals of six hours. The resemblance to the curve
based on the observations is greatly improved. A closer
resemblance would be obtained if additional terms
0-08 sin (45* + 330) + 0-12 sin (60* + 190)
were included in the formula.
The harmonic representation of a diurnal inequality
may be expressed in either of the alternative forms
r/i cos (15 xf) +a 2 cos (30 x *) +a s cos (45 x *)
+a 4 cos(60x*) + . . .
+ &! sin (15 x *) + 6 2 sin (30 x *) + & 3 sin (45 x *)
+ 6 4 sin (60 x*) + . . . ,
Pi sin (15 x * + Ai) + P 3 sin (30 x * + A 2 ) +
P 3 sin(45x* + J 3 ) + P 4 sin.(60x*-M 4 ) + . . . ,
where t denotes the time in hours counting from some
fixed hour, usually midnight. The latter is the form
which has been adopted in the previous part of this note,
as it best exhibits the physical significance of the results,
but the first form is that employed for the actual
numerical calculation of the harmonic coefficients. We
first calculate the a, b coefficients and then derive the
P, A coefficients from the relations
where n may be 1, 2, 3, 4, &e.
Harmonic Analysis. 315
For brevity, let 0, 1, 2 .... 23 represent the algebraical
departures from the mean value for the clay of an element
such as temperature or pressure at the successive hours
midnight (or 0), 1, 2 .... Then using the following
closely approximate values 0*966 for cos 15 or sin 75,
0-866 for cos 30 or sin 60, 0-707 for cos 45 or sin 45,
0*259 for cos 75 or sin 15, and noticing that 0*5 is the
exact value of cos 60 or sin 30, we have the following
mathematical expressions for the a, b coefficients of the
first 4 orders :
12ai=(0-12) +0-966{(i + 23)-(ll + 13)} +0-866 {(2 + 22)
-(10 + 14)} + 0707{(3 + 21)-(9 + 15)}
)-(8 + 16)}+0-259{(5 + 19)-(
+ (11 + 13)} + 0-5{(2 + 22) -(4+20) -
+ (10 + 14)},
12&i=(6-18) + 0-966{(5-19) +(7-17)} +0-866{ (4-20)
+ (8-16)}+X)'707{(3-21) + (9-15)}
+ 0-5{(2-22) + (10-14)} +0-259{(l-23) + (ll -13)},
-(10-14)} +0-5 {(1-23) +(5-11)) '-(7-17)
-(11-13)},
316 Glossary.
The terms are arranged in pairs for facility of calcula-
tion, as will be better understood on consulting the
numerical example given presently. It will be noticed that
in the case of the b coefficients we have to do invariably
with differences, and that the sum of the two numerals
(representing the observational hours) which form the
pair is invariably 24. Similarly in the case of the a
coefficients, with two apparent exceptions, we have the
sum of observational values at hours which together
amount to 24. The exceptions (0 12) are only apparent ;
they really represent {(() + 24) (12 + 12). The hours
and 24 alike represent midnight.
Take for illustration the case already given of the
diurnal variation of temperature at Kew Observatory
during March. The 24 hourly differences from the mean
of the day on the average of the 45 years 1871 to 1915
were as follows :
0,12 1,23 2,22 3,21 4,20 5,19
-1-35 -1-51 -1-75 -1-91 -2-10 -2'19
+ 2-03 -1-04 -0-72 -0-29 +0-11 + 0-70
6,18 7,17 8,16 9,15 10,14 11,13
-2-31 -2-14 -1-62 -O49 + O43 4-1-41
+ 1-42 +2-26 +2-69 +2'93 +2-77 +2-58 -
The headings denote the hours to which the observa-
tional data immediately below refer. For instance, the
departures from the mean value of the day at h. and 12 h.
were respectively 1*35 and + 2*03.
Referring to the formulae it will be seen that we want
the algebraical sum and difference of the two entries
Harmonic Analysis. 317
which appear in the same
column. These are respect-
ively :
sums
+ 0-68
-2-55
-2-47
-2-20
1-99
-1-49
-0-89
+ 0-12
+ 1-07
+ 2-44
+ 3-20
+ 3-99
differences
-3-38
-0-47
-1-03
-1-62
-2-21
-2-89
-3-73
-4-40
-4-31
-3-42
-2-34
-1-17
For calculating the b coefficients we require only the
differences ; for the a coefficients in addition to the sums
we require only the first difference.
We thus have
12i= -3-38 + 0-966 (-2-55-3-99) + 0-866 (-2-47- 3-20)
+ 0-707 (-2-20-2-44) +0*5 (-1-99-1-07)
+ 0-259 (- 1-49-0-12).
Employing Crelle's Tables, or logarithms, or straight-
forward multiplication, we get 12a : = 3*38 6*32
-4-91 -3-28 -1-53 -0-42 = - 19-84,
and so a\ = 1*653.
Similarly we find 12a 2 = +4'83, and so a 2 = + 0'403 ;
12a 3 = -0-53 03= - 0-044 ;
12a 4 = -0-32 a 4 =- 0-027.
Coining next to the &'$, we have
UPi =-3-73 + 0-966 (-2-89-4-40) +0-866 (-2-21-4-31)
+ 0-707 (-1-62-3-42) + 0-5 (- 1-03 -2-34)
+ 0-259 (-0-47 -1-17),
= -3-73 - 7-04 - 5-65 - 3-56 - L-69-0-42 == - 22-09,
and so b\ = 1-841.
318 Glossary.
Similarly we liiul
12& 3 = + 5'86, and so // 2 =+0'488 ;
126,= + 0-79 6 3 = +0-066;
.12& 4 =-l-39 b, = -0-116.
The deduction of the P, -4, constants is simple. Take.
for example, the case of PI, A\, 'i.e. the 24-hour term. We
have tan A l = i = ~= + O8978.
bi ' 1'841
The angle whose tangent is +0*8978 may be 4155 / or
415,Y + 180, i.e. 2215f) / .
To determine which, we notice that a\ and bi are both
minus, so that sin A\ and cos .4i are both minus. Thus
AI lies in the third quadrant, between 180 and 270, and
consequently is 22155', or to the nearest degree 222.
The formula PI = aifemAi gives us PI=~ ^=2'47.
O'obo
We need not trouble about the sign, as the P's are all
positive. The values of A 2 , P 2 , A^ PS, A 4 and P 4 are
derivable exactly in the same way.
The process of finding the trigonometric series to give
the best representation of a periodic function is known as
harmonic analysis. The reverse process, determining the
value of the function at any time when the components
are known, is harmonic synthesis. Both processes can be
carried out by suitable machines, and also by arithmetical
computation from given data. The latter process is the
more .usual except in the case of the prediction of tides.
In any term P sin (nt + A) the coefficient P which
determines the range is called the amplitude, nt + A is
called the phase-angle, A being the phase-angle for mid-
night. It may be mentioned that the alternative form
Harmonic Analysis. 319
Pcos n(t t ) where t is the time of the maximum, has
certain advantages ; it was adopted by General Strachey
for the discussion of harmonic analysis of temperature in
the British Isles.
By comparison of the amplitudes and phase-angles for
different places and different seasons, climates may be
classified. For example, the amplitude of the whole-day
term for temperature in July at Falmouth is 2'la, and the
phase-angle for local apparent midnight is 250. In com-
parison ' with Kew, the amplitude is small and the
maximum occurs early. This difference in phase is
typical of the difference in conditions on the coast and
inland. It may be stated, however, that as regards tem-
perature, harmonic analysis has not yielded information
which can not be obtained more readily from the curves
showing the daily variation. With pressure more im-
portant results have been discovered.
For temperature the first or all-day term in the expan-
sion in trigonometric series is by far the most important.
With pressure the second term is comparable in size with
the first, and at most stations there are two maxima and two
minima in the course of 24 hours. The first term is found
to depend on the situation of the station, whether near the
coast or inland, in a valley or on a mountain-side, whereas
the second or twelve-hour term depends principally on
the latitude. The daily changes represented by the first
term are clearly understood, they are the effects of local
heating of the air. No adequate explanation of the surgo
of pressure which is represented by the second and higher
terms has been put forward.
320 Glossary.
The daily variation of pressure at Cairo in July is
represented graphically by the second Figure.
Daily variation of the Barometer at Cairo [Abbassia
Observatory] in July.
The departure of the pressure from the mean for the
day is given in millibars by the expression
92 sin (15* + 17) + : 66 sin (30*-+ 140)
+ '12 sin (45* + 348) +-05 sin (60* + 250).
The first term represents an oscillation with the
maximum and minimum at about 5h.and 17h. respectively.
It indicates that as the air is warmed in the daytime it
expands and overflows from the Nile valley over the sur-
rounding high ground and over the neighbouring seas.
The second term represents an oscillation with maxima
at lOh. 20m. and 22h. 20m. These hours are almost the
same all over the globe. The amplitude depends on the
latitude and to a certain extent on the time of year.
The mean value for the year at Cairo is 0*8 mb. It is
about 1*3 mb. at the equator, 0*5 in latitude 45, 0*35 in
Lond'on and 0*1 mb. in latitude 60.
The third term is interesting as it changes its phase by
180 at the equinoxes. The first maximum occurs at 2h. in
summer, the first minimum at the same hour in winter.
Harmonic Analysis. 321
It has been mentioned that the all- clay term depends
largely on local conditions. An interesting contrast is
offered by the British observatories. At Richmond,
Surrey, the amplitude of this term in July is about O3mb.,
and it has about the same value at Cahirciveen, but the
phases are opposite : at Richmond the maximum occurs at
f)h., whereas at Cahirciveen it is the minimum which occurs
in. the morning (at 7h).
Harmonic Analysis may be extended to the investigation
of changes which are caused by forces having different
periods. The classical instance is that of the tides. The ,
Tides being caused by the attraction of the sun and the
moon show as periods the solar and also the lunar day.
The process by which the heights and the times of tides
are foretold in practice defends on harmonic analysis
and synthesis.
Ice Owing to the large amount of heat absorbed in
melt ing (80 CALORIES for one gramme melted) a mass of ice
represents a powerful reservoir of cold. Masses of ice or
snow can attain to such dimensions in Nature that the
heat absorbed during melting is of climatological im-
portance. An excellent example is furnished by the ice-
bergs observed by Antarctic explorers. The largest of
these appear to be portions of the great Ross Ice-barrier
that have broken away during the summer months. They
are generally several hundred feet thick and may exceed
20 miles in length. The amount of heat required to meli
one 20 miles long, 5 miles broad and 600 feet thick would
be sufficient to raise the temperature of the air over the
whole British Isles from the ground up to a height of
1 kilometre (3,281 feet) by over 40C.
When ice forms upon a pond during frosty weather
the cooled water at the surface is continually replaced by
322 Glossary.
warmer water from below until the whole "mass has fallen
to 277a (39F), which is the temperature at which water
has its greatest density. The surface can then cool un-
disturbed until the freezing point is reached and ice
begins to form, but when ice first begins to form in a
flowing river is a complicated question about which
Professor H. T. Barnes, F.R.S., has written.
When the sea freezes the crystals formed contain no salt
but cannot easily be separated from the brine which is
mixed up with them, consequently the water obtained by
melting genuine sea-ice is salt. When, however, this ice
forms hummocks under the action of pressure the brine
drains out and leaves pure ice. Newly-formed sea-ice
has a surprising degree of flexibility due to the fact that
the crystals are separated by layers of brine or salt, and
even when it is several inches thick the surface can be
moved up and down unbroken by a swell. As the thick-
ness increases this can no longer happen, and the sheet is
broken up into pieces, which grind together and soon
form the beautiful *' pancake-ice " familiar io polar
explorers.
lonisation in the atmosphere arises from the presence
of free + and ions. Charged ions move along the
lines of force in an electric field, carrying their charges
with them. This is equivalent to an electric current.
The current increases with the number of free ions
present, and with their mobility (i.e., the velocity which
an ion possesses in a field of unit strength). An increase
of current for a given strength of field is equivalent to an
increase of conductivity in the medium.
There are at least two distinct kinds of ions in the
atmosphere, usually known as light or mobile ions, and as
heavy or Langevin ions, so-called after their discoverer.
Itmisation. 323
I'rof. Langevin. The heavy ions ;ire the more numerous,
at least, in the polluted air ordinarily. met with near large
towns, but their mobility is so small that they are of
minor importance so far as the conductivity of the
atmosphere is concerned. The light ion has a velocity
of the order of 1 cm. per second in a field of 1 volt per cm,
Ionic charges in the atmosphere are usually measured
with the Ebert apparatus. This consists of a hollow
cylindrical tube vertical in the more recent forms con-
taining a co-axial cylindrical rod, which is insulated and
connected to the fibres of a " string electrometer." The
rod is charged to a potential of from 100 to .20 ) volts, the
cylindrical tube being earthed. Air is pulled through the
tube by a turbine, the amount passing being recorded on
an anemometer. Supposing the rod charged negatively,
the positive ions in the admitted air are attracted to the
rod, and as the air passes give up their charge to it. The
length of the rod and the potential to which it is charged
are such as to ensure that no mobile ion will escape
capture. The readings of the electrometer taken before
and after the admission of the air, the capacity of the
electrical system being known, inform us how much the
charge on the rod has been diminished. A small part of
the loss determined by a separate experiment is due to
imperfect insulation ; the balance represents the free
electrical charge opposite in sign to that on the rod
existing in a measured volume of air. Two experiments
are made, the rod being charged negatively in the one
case, positively in the other. We thus get the free charges
present per c.c. in the atmosphere. These charges are
highly variable, but are usually of the order of 1000 x 10" 20
in electromagnetic units. The number of ions per c.c. is
often published and may be deduced by dividing the
1S204 L 2
324 Glossary.
charge, whether + or ,. by the charge on an ion, for
which the value accepted at present is 159 x 10~ L ' 2 in
electromagnetic units.
In all, or nearly all the earlier work done on the
subject, the ionic charge was given a value 29 per cent, less
than that now accepted, leading to an overestimate of the
number of ions. It is also the case that part of the charge
is derived from the heavy ions, though the percentage of
these caught by the Ebert apparatus is undoubtedly small.
The mobility of the ions may also be measured with the
Ebert apparatus, but the process is somewhat compli-
cated, and the accuracy of individual determinations is
not high.
Observations at the earth's surface have usually given a
decided excess in the number of -f ions. The + and -
ions naturally tend to combine, and their presence thus
points to the existence of some agency producing ions.
Several possible agencies are recognised, including radio-
active substances, and solar radiation, especially ultra-
violet light. Near the ground the radio-active substances
ordinarily present in the ground may be the chief source.
Balloon observations have shown that the ionisation
diminishes somewhat at first as we leave the ground, but
at heights of 1 k to 2 k it begins to increase again, and at
heights of 9 k or 10 k it seems to be very much larger
than near the ground. The ionising agent there is
presumably RADIATION of some kind from the sun.
Lightning", Protection from. The region between
the earth and a thundercloud is one of great electrical
stress. It may appear paradoxical that a lightning-con-
ductor, intended to protect a structure within that regicii
from damage, acts primarily by increasing this stress.
The lightning-conductor consists of a metallic point or set
L igh tning, Protection from. 325
of points connected by a metal rod to a conductor of con-
siderable area buried in moist earth. This conducting
point, projecting some short distance above the highest
point to be protected, increases the intensity of the stress
between itself and the cloud, till one of two things
happens. A silent " brush " discharge may take place, in
which the induced electrification of the earth streams
comparatively gently from the point (selected because of
its well-known property of facilitating brush-discharge),
and thus finally reduce the stress to a limit insufficient to
produce a violent lightning discharge. If, on the con-
trary, a lightning discharge does occur in the region, it
will pass to the point because of the increased stress, and
the rod will carry the current harmlessly to earth.
The old idea of an "area of protection" is no longer
tenable, and pointed conductors should therefore be pro-
vided on every vertical projection of the structure to be
protected, and at intervals along the ridge of a long roof.
These should be connected by metal rods or cables and
the rods connected at several places to earth. The con-
necting rods should run as straight as possible since
electrical inertia will make the discharge jump across an
air-gap in preference to rounding a sharp bend or loop in
the rods. Iron rods are preferable to copper both
electrically arid economically, and may be painted for
preservation. The rods should be held at a distance of
some inches from walls, but should be fixed, not by in-
sulators, but by metal holdfasts fastened in or on the
walls. Metal roofs, gutters, pipes and other masses of
metal should be electrically continuous and connected to
earth.
Inside the building, pipes, bell-systems and all large
metallic masses should also be earth-connected, since vio-
326 Glossary.
lent oscillations may be induced in them by a discharge
through the external conductor, and they may cause fire
by sparking to earth if they are not so connected.
The only complete protection is attained by enclosing
the structure in a "birdcage " arrangement of conductors,
well connected to earth at several points. A metal build-
ing, whose parts are electrically connected amongst them-
selves and to earth, is probably the most perfect protection
possible, but here also internal metal-work should be
earth-connected .
The personal danger from lightning in the open
country is at least twice as great as in towns, owing to the
number of buildings in the latter protected intentionally
(as by lightning conductors), or unintentionally (as by
overhead wiring for light, power, telephones, etc.). It
seems to be a definitely established fact that certain trees,
particularly the Oak, are more frequently struck than
others. The relative danger of lightning stroke, taking
the Beech as 1, is Oak 50-60, Scotch Pine 30-40, Spruce
10. but is of course much affected by environment.
Isolated and prominent trees are more frequently struck
than average forest trees.
The safest course in the open though perhaps not the
most comfortable is to lie in a ditch or furrow, failing
these to lie on the ground. To shelter under isolated
trees or on the edges of a wood is dangerous ; well inside
the wood the danger is probably not great. Immediately
under a line of over-head wires is also a relatively safe
area.
The protection of aircraft against lightning is a difficult
problem, complicated by such appendages as wire cables
in the case of balloons and radio-telegraphic aerials on
aeroplanes. The trail of hot gases from the engine
Magnetism. 327
exhaust acts as a comparatively good conductor, which
may work for good or harm according to the position of
the craft relative to the thundercloud, but on the whole,
by acting as a lightning-conductor and thus facilitating dis-
charge along a path passing through the machine, probably
adds a good deal more to the danger than it takes from it. *
Magnetism. The branch of knowledge relating to
magnetic phenomena. Terrestrial magnetism is con-
cerned with the earth's magnetism. The earth has been
happily .described as a great magnet : the distribu-
tion of magnetic force at its surface may as a first
approximation be regarded as that due to a uniformly
magnetised sphere, whose magnetic axis, however, is
inclined at some 10 or 12 to the earth's axis of rotation
(polar axis). The magnetic poles are the one to the north
of Canada, the other in the Antarctic. At these poles the
dip needle is vertical, and the horizontal component of
magnetic force vanishes ; the compass needle has no
guiding force and points anyhow. At what may be called
the magnetic equator, which is nowhere very far from the
geographical equator, the vertical force vanishes ; the dip
needle is horizontal, and the horizontal force has its
largest values. The horizontal force in London is only
about half that where the magnetic equator crosses India.
The compass needle points in the direction of the local
horizontal component of magnetic force, and this direction
is exactly north and south only along two lines or narrow
belts, one of which at present crosses A.sia Minor and
European Russia, while the other crosses the United States
and Canada. The position of these " Agonic " lines as
they are called, and the direction of the magnetic needle,
change gradually with time, having what is known as
" secular change." The inclination of the magnetic needle
328 Glossary.
to the geographical meridian is called the declination, or
sometimes, rather unfortunately, the magnetic variation.
As we travel westward from the Asiatic-European Agonic
line, the declination becomes increasingly westerly, until
we pass the western limits of Europe. Thus, at present,
approximate values are at Cairo If W., at Athens 3f W.,
at Pola 7i W., at Copenhagen 8^ W., at Brussels
12^ W., at London 15 W., and at Valencia Observatory
(Co. Kerry) 20 W. In Europe westerly declination is at
present diminishing at a rate of nearly 10' a year.
Besides the secular change, declination has a regular
diurnal variation, and also frequent irregular changes,
which when large are known as magnetic storms. In the
Northern hemisphere the prominent part of the regular
diurnal variation is a westerly movement from about 7h.
to 13 h. (1 p.m.), followed by a slower easterly movement.
Speaking generally the range of the regular diurnal
variation is least near the magnetic equator, where it may
average less than 3', and the greatest near the magnetic
poles, where it may exceed 1. It varies with the season
of the year. At Richmond, for instance, in the average year
it varies from about 3^' in December to 11' in August,
the mean from all months of the year being about
8'. It also varies considerably from year to year, show-
ing a remarkably similar progression to that of sun-
spot frequency. In a year of sun spot maximum the
range may be 50 per cent, or more larger than a few
years earlier or later at sunspot minimum. Few days are
wholly free from irregular changes of declination, and
these are sometimes much larger than the regular changes.
Thus the actually observed daily range at Kew sometimes
exceeds 1, and in high latitudes ranges of 5, or even 10
or more, are occasionally observed, In latitudes, however,
Magnetism. 329
below 60 in Europe departures of more than 30' from the
mean value for the year are rare. On the occasions of
very large magnetic changes AURORA is generally observed,
even in the South of England, and there is usually some
interference with ordinary telegraphy.
The direction of the compass-needle is practically
independent of the height above the ground, except in
localities where some large underground source of dis-
turbance exists. In such a case, moreover, the higher one
goes up the less is the effect of the local disturbance.
All objects of iron are liable to, become magnets,
especially when they are long and disposed with their
length nearly in the magnetic meridian, and are thus apt
to introduce errors in the readings of compasses in their
vicinity. This warning applies particularly to large
objects like ships, girders and large guns ; but even small
articles of iron, such as some buttons or spectacle frames,
when close to a compass, may cause quite a serious error.
Some forms 01 rock, especially dark-coloured basalt, are
strongly magnetic, and large disturbing effects are caused
by the strong electric currents used in connection with
electric tramways and railways when the system is a
direct-current one.
Precipitation. A term borrowed from chemistry to
denote any one of the results of the conversion of the
invisible water-vapour to visible water or ice," thus com-
prising not only rain, hail, snow, sleet, dew, hoar-frost
and rime, but cloud, mist and fog. In practice, however,
the use of the word is limited to appreciable deposit in
either the solid or the liquid form at the earth's surface,
the definition of "' Day of precipitation " being identical
with that of RAIN-DAY (q.v.y. At low levels it is rare
for appreciable deposit of water in the rain-gauge to
330 Glossary.
result from precipitation of cloud, mist or fog, but in
mountainous districts wet-fog and SCOTCH-MIST (q.v.}
are responsible for a considerable quantity of " rainfall."
See RAIN, HAIL, SNOW, etc.
Radiation, Solar and Terrestrial. Radiation is
the process by which heat is transferred from one body
to another without altering the temperature of the inter-
vening medium. All life upon the earth and all meteoro-
logical phenomena are dependent upon the radiant heat
and light received from the sun. See INSOLATION.
The earth itself is always radiating into space, and
according to Stefan's law r the rate of radiation is given
by o-T 74 , where T is the absolute temperature and <t a
constant. During the day normally the earth receives
directly from the sun and indirectly from the atmosphere,
including the clouds, more radiation than it gives out.
At night the reverse is the case. The rate of loss of heat
from a plate freely exposed at the earth's surface at night
tells us the balance of loss, i.e., the excess of the earth's
radiation outwards over what it receives from the atmos-
phere. With the aid of Stefan's law we can form an
estimation of radiation received from the atmosphere.
According to Professor Millikan, . who has recently
reviewed the literature of the subject, the most probable
value of <r in Stefan's formula in watts per sq. cm.
is 5*72xlO~ 12 , or in gramme - calories per sq. cm.
per minute 77 x 10~ 12 . For the radiation at 288a this
gives 77(288) 4 / klO~ 12 or 0*53 gramme-calories per sq. cm.
per minute, and similarly for any other temperature.
Combining this with observations of the balance of loss
experienced at different heights at different temperatures,
we have the following results, the heat-data being in
gramme-calories per sq. cm. per minute :
, Ho/fir and Terrestrial. 331
Height (metres)
44
950
3100
3100
Temperature absolute
288
267
261
271
Balance of loss
0-13
" J 5
0*20
0*20
Radiation by Stefan's law!..
o'39
0*36
0*42
Radiation received from at-
mosphere
0*40
0-24
o- 16
O*22
Recent experiments by Anders Angstrom show that
aqueous vapour exercises a potent influence in reducing
the balance of loss of heat by nocturnal radiation.
In a paper read before the Royal Meteorological Society
in February, 19 1 7, and published in the Quarterly Journal
for April, Mr. W. H. Dines, F.R.S., gives the following
values for the amount of solar radiant energy absorbed
and reflected by the earth and its atmosphere, and of
radiation of the earth's heat and its absorption and
transference.
The values are expressed in gramme-calories per sq.
centimetre per day.
Radiant energy received by the earth per day 720
(An amount capable of warming the
atmosphere 3a per day.)
Radiant energy absorbed by the earth per day ;
for the whole earth ... ... ... ... 300
(The Callendar instrument at the Meteoro-
logical Office gives about 200 ; Hann gives
166 for Kiev (50N.) ; ' and 46 tor Tauren-
berg Bay (79N.) ; 300 is the value for
London at the beginning of May or August.)
liadiant energy absorbed by the air ... ... 60
(The value seems low, but there is no
observational evidence against it.)
332 Glossary.
OutwHi'd radiation from the earth ... ... f)<K.)
(Obtained by using the most probable
value of <r in Stefan's law [vide supra'].)
Heat radiated by earth reflected back by
atmosphere ... ... ... ... ... 60
Heat radiated by earth absorbed by atmosphere 360
Heat radiated by earth transmitted beyond the
outer limit of atmosphere ... ... ... 80
Transference of heat from earth to air ... ... 200
Radiation from air downwards 340
Radiation from air upwards ... ... ... 280
Effect of the ivhole daily solar radiation when applied to
raise the temperature of the air in the first 1, 2 and 8
kms. of the atmosphere.
According to Angot (quoted from Hann) the following
are the proportionate values of the solar radiation per
cm 2 , in each latitude.
10 20 30 40 50 60 70 80 90
350 345 331 308 277 240 199 166 150 14;>
If the value of the solar constant be taken as 2 gramme-
calories the daily receipt of heat per cm. strip on the
equator is 2r x 2 x 60 x 24 where r is the earth's radius, and
the receipt per cm 2 , per day is %r x 2 x 60 x 24/27r/'=916.
The amounts for each latitude are therefore
10 20 30 40 50 60 70 80 90
916 904 869 807 725 629 521 435 393 380
Taking a mean pressure of 1014 mb. the water-equivalent
of the atmosphere is a layer of water 250 cm. deep, hence
dividing the above numbers by 250, the number of
tiJ.fn- and Tr-rrrxt./'ictl. 333
degrees that the whole radiation is capable of raising the
whole atmosphere is given below for each latitude.
10 20 30 40 50 60 70 80 90
3-7 3-6 3-5 3-2 2*9 2'5 2-1 1-7 1-6 1-5
With a mean surface temperature of 288a and a lapse-
rate of 6a per k. the percentage of the whole quan-
tity of air found under 1 k. height is 11*3, under 2 k.
21*7 and under 3 k. 30'8. The amounts for each latitude
are shown in the following table, the figures indicating
the rise af temperature in degrees that would occur if the
whole solar radiation were concentrated in the layer and
no loss of heat occurred.
Latitude 10 20 30 40 50 60 70 80 90
Under 1 k. ... 32'5 32'0 30'8 28'6 25'7 22-3 18-4 154 13'9 13'4
Under 2k. ... 16'9 16'6 16'0 14*9 13'4 11-6 9'6 8'0 7'2 7'0
Under 3k. ... 11-9 11-7 11-3 10-5 9*4 8-2 6-7 5-7 5'1 4'9
The above figures represent mean conditions as to
density. A fall of pressure will increase the values and
so also will a rise of temperature, because with a rise of
temperature a smaller proportion of the whole atmosphere
will be found in the given layer. The mean conditions
hold in about latitude 40 ; in the equatorial regions some
4 per cent, must be added to the values, and in latitude
'55 some 4 per cent, deducted.
The values given are interesting, but, it must be re-
membered that the whole solar heat is not absorbed by
the lower strata, probably only a small proportion of the
whole, also as the loss per 24. hours is about the same as
the gain in the 12 hours that the sun is on the average
above the horizon, the rise of temperature, quite apart
from convection, would be only half the values given in
the table.
334
Glossary.
Size and Rate of Fall of Raindrops.
Raindrops. The size of raindrops can be measured
If, for example, a shallow tray containing dry plaster of
Paris is exposed for a few seconds during rain, each drop
which falls into the tray will make a plaster cast of itself
which can easily be measured. A better method is to
collect the drops upon thick blotting paper. If, while
still wet, the paper is dusted over \Yith a dye powder a
permanent record will be obtained consisting of circular
spots whose diameter is a measure of the size of the drops.
By comparing the diameters of the discs produced by
raindrops with those produced by drops of water of
known size, the amount of water contained in the former
can be found. The following table contains some results ob-
tained by P. Lenard in this way at nine different times :
Drops.
Diameter.
Vol-
No. of drops per m.'- per second.
ume.
mm.
in.
mm. 3
CD
C2)
(3)
(4)
(5)
C6)
m
CS)
m
'5
0:9
o o6'6
;IOOO
1600
129
60
100
514
679
7
I'O
039
0-523
200
120
100
280
50
1300
423
524
233
i $
1-77
140 60
73
1 60
50
500
359
347
2'0
079
4-19
140
200
100
20
150
200
138
295
46
2*5
098
8-19
'O
29
20
156
205
7
3'o
118
14-2
o
o
57
200
138
81
o
3'5
138
22-5
o
o
O
O
O
o
28
32
4'
157
33'5
50
O
20
39
4'5
177
47-8
200
101
c
5*0 1
196
65*5
o
o
o
o
25
Total number ...
1480
1980
486
540
500
2300
1840
2190
500
Rate of rainfall
0-09
O'o6
O'll
0-05
0-32
0-72
0-57
0-34
0-26
(mm./min.)
Raindrops^ 335
(i) and (2) refer to a rain " looking very ordinary "
which was general over the north of Switzerland. The
wind had freshened between (1) and (2).
(3) Rain with sunshine-breaks.
(4) Beginning of a short fall like a thundershower.
Distant thunder.
(5) Sudden rain from small cloud. Calm ; sultry
before.
(6) Violent? rain like a cloudburst, with some hail.
(7), (8)' and (9) are for the heaviest period, a less heavy
period, and the period of stopping of a continuous fall
which at times took the form of a cloudburst.
We see then that in a general rain, such as the normal
type which accompanies the passage of a depression over
Northern Europe, by far the greater number of drops
have a diameter of 2 mm. or less. In short showers,
especially those occurring during thunderstorms, the
frequency of large drops is much greater. In such
showers the diameter of the largest drops appears to be
about 5 mm. We shall see later that there is a limit to
the size of drops determined by the fact that it is im-
possible for a drop, whose diameter exceeds 5*5 mm. or
rather less than a quarter of an inch, to fall intact.
The rate at which a raindrop, or any other object, can
fall through still air depends upon its size. When let
fall its speed will increase until the air-resistance is ex-
actly equal to the weight, when it will continue to move
at that steady speed (see EQUILIBRIUM). The manner in
which this u terminal velocity," as it is called, varies
with the size of the raindrops is shown in the following
table, due to Lenard.
336
Glossary.
TERMINAL VELOCITIES OF WATER-DROPS FALLING
IN AIR.
Diameter
Terminal
Diameter
Terminal
of drop.
Velocity.
of drop.
Velocity.
mm
in.
m/s. mi/hr.
mm. in.
m/s.
mi/hr.
0*01 0*0004
0-0032 0-007
3-0 0-118
6-9
15-4
O'l
0-0039
0-32 0-71
3*5 0-138
7'4
16-5
*5
0-020
3*5 7'9
4-0 .0-157
7'7
17-2
r-o
0-039
4*4 9*8
4-5 0-177
8-0
17-9
1-5
0-059
5-7 12-6
5'0 ' 200
8-0
17-9
2-0
0-079
5-9 13-2
5*5 0*216
8-0
17-9
2'5
0-098
6-4 14-3
We may look upon this table in another way. The
frictional resistance offered by the air to the passage of a
drop depends upon the relative motion of the two, and it
is of 110 consequence whether the drop is moving and the
air still, or the air moving and the drop still, or both air
and drop moving if they have different velocities. The
velocities given in the tables are those with which the air
in a vertical current must rise in order just to keep the
drops floating, without rising or falling. The above
results were, in fact actually determined by Lenard in
this way, by means of experiments with vertical air-
currents on drops of known size. We see that beyond a
certain point the terminal velocity does not increase with
the size of the drops. This is due to the fact that the
drops become deformed, spreading out horizontally, with
the result that the air-resistance is increased. For drops
greater than 5*5 mm. diameter, the deformation i^
Raindrops. 33?
sufficient to make the drops break up before the terminal
velocity is reached.
An important consequence of Lenard's results is that no
rain can fall through an ascending current of air whose
vertical velocity is greater than 8 m/s. In such a current
the drops will be carried upwards, either intact or after
breaking up into droplets. There is good reason for
believing that vertical currents exceeding this velocity
frequently occur in nature.
On account of their inability to fall in an air current
which -is rising faster than their limiting velocity, rain-
drops formed in these currents will have ample oppor-
tunity to increase in size, and the electrical conditions
will usually be favourable for the formation of large drops.
These large drops can reach earth in two ways ; either by
being carried along in the outflow of air above the region
of most active convection, or by the sudden cessation of or
a lull in the vertical current. The violence of the precipi-
tation under the latter conditions may be particularly
disastrous. (See also Cloudburst and Hail.}
Electrification of Waterdrops ly Splashing.
If drops of water are allowed to splash upon a metal
plate, the water acquires a minute positive charge of
electricity, and an equal negative charge is shared by
the plate and the air contiguous to the splashing drop. It
is possible to show this by means of delicate apparatus.
The largest charges are found when distilled water is
used, and even small amounts of dissolved substances in
the water make a considerable difference to the results
obtained. With sea-water, indeed, the effect is reversed,
the water becoming negatively charged after splashing.
338 Glossary.
The presence of a solid obstacle to cause splashing is not
really necessary to produce the separation of electricity.
The breaking up of a jet of water into spray and the
splitting of large drops of water in a current of air
produce similar effects. The last-named case is of par-
ticular importance in meteorology because it forms the
basis of the theory put forward by Dr. G. C. Simpson to
account for the production of the enormous electrical
stresses in the atmosphere which precede the discharge of
lightning in thunderstorms.
The first necessity for a thunderstorm is the formation
of a cumulus cloud, and this requires an ascending current
of air. In ascending the air expands and gets rapidly
cooler, with the result that before long the water- vapour
in the air begins to condense and form visible droplets.
The cloud is, in fact, the visible result of this condensa-
tion. Once formed, the drops rapidly increase in size and
would ordinarily fall as rain. But if the ascending
current is sufficiently violent the raindrops will not be
able to fall through it, but will be carried up with the air.
The vertical velocity required to hold up drops of all sizes
is 8 m/s., and there is no reason to doubt that such
currents can easily be produced. Now in such a current
it is impossible for a drop to grow beyond 5*5 mm. in
diameter (see Size and Rate of Fall of Raindrops). At
that point it becomes unstable and divides into droplets.
These in their turn go through the same process of
growing and dividing. Each time a division occurs the
droplets gain a positive charge, and the air which is
carried up with the current gain^ an equal negative
charge. In this way the waterdrops in the region of the
ascending current rapidly become very highly charged,
and as soon as the potential gradient anywhere amounts
Raindrops. 339
to 30,000 volts/crn. a lightning-flash will occur. Although
the charge produced by a single division is very small, we
have only to suppose that the same quantity of water may
take part in many hundreds of divisions and there is
nothing improbable in this to be able to account for the
production of sufficiently high potentials.
The negatively charged air will be carried right to the
top of the column and there dispersed. Its presence
should be shown by a negative charge on the rain which
falls some distance from the storm-centre, while that
falling near the centre should be positively charged. Such
observations as exist tend to support this conclusion of
the theory.
Regression Equation. A regression equation shows
the most probable form of the relationship between two
varying quantities insofar as such relationship can be
definitely determined from the set of statistical data on
which it is based. It is formed from the correlation
co-efficient and the matter is best explained by an
example. See also under CORRELATION.
The strength of the wind and the steepness of the
barometric gradient at the same time and place are closely
related, and a regression equation may be formed between
them. Let W denote the strength of the wind, W m its
mean value, and S W the departure from the mean, and
let 6r, 6r m and f G be the corresponding values for the
steepness of the gradient. Then if the gradient is known
the strength of the wind is given by a regression equation
in the form
W = W m + c G + .
In this equation t will in general have some value, posi-
tive or negative, differing on each occasion, and the a will
340 , Glossary.
be so chosen that the sum of the squares of the e's will be
as small as possible. The IF, c Gr and < are variable
quantities, the a a constant. It is usual, for the sake of
brevity, to write the equation c W = a c Gr, but it must
be remembered that the e has been omitted ; e is called the
residual error, and since e is often fairly large, it is not
permissible to wrUe J-^~=^4^frf. g G*
There are two errors involved in a regression equation.
The value of a can in general only be found at all
correctly when the number of observations is large. The
residual error e may be as large as the term a c G unless
the correlation co-efficient is nearly 1 or 1. When the
correlation is + 1 the term t is nothing ; also, when the
correlation is known to be large (it cannot be proved to
be large from a few observations) the value of a can be
determined with greater certainty than when the correla-
tion co-efficient is small.
The following are three examples all dependent on
fairly high correlation co-efficients.
Thickness of TROPOSPHERE = 10,600 + 112 3P 9 metres,
where P 9 denotes the pressure of the air at a height
of nine kilometres expressed in millibars. (Europe.)
Hay crop per acre = 28 + 4 c R cwts.,
where R denotes the spring rainfall in inches.
(East of England.)
Number of deaths in England during July, August and
September = 150,000 + 7,200 c T,
where T denotes the mean temperature in degrees
F of June, July and August. (On the assumption
that the present population is forty millions.)
Scotch-Mist. In mountainous or hilly regions, rain-
clouds (nimbus) are often adjacent to the ground, and
Scotch-Mist. 341
precipitation takes place in the form of minute water-
drops, the apparent effect being a combination of thick
mist and heavy drizzle.
The upland character of the greater part of Scotland
and the consequent frequency of occurrence of the phe-
nomenon in that country have secured for it the appella-
tion by which it is generally known in the British Isles.
The base of a true nimbus or rain-cloud rarely exceeds
about 7,000 ft. (2'1 k.) in elevation, and sometimes
descends to within a few hundred feet of sea-level, so that
Scotch>mist may be experienced in comparatively low-
lying regions.
In the uplands of the Devon-Cornwall peninsula the
same phenomenon, which is there of very frequent inci-
dence, is known as " mizzle."
Sleet. Precipitation of rain and snow together or of
partially melted snow. In America the name " sleet "
is used for small dry pellets of snow which might be
classed as soft-hail. Sleet is, perhaps, snow that passes
through a stratum of comparatively warm air (see
INVERSION) and, undergoing partial liquefaction therein
to an extent varying with the temperature and thickness
of the layer, reaches the ground in a semi-liquid con-
dition. If the stratum of warm air is not adjacent to the
ground, and if the surface-temperature is below the
freezing-point, a phenomenon similar to GLAZED FROST
(q.v.) may result, the re-freezing of the half-melted snow
occasioning the formation of a layer of ice on all objects
exposed to the precipitation. Marked instances of this
occurred in London during the winter of 1916-1917, and
road traffic, and in some cases even rail-locomotion, was
rendered difficult or impossible.
)U2 Glossary.
Snow. Precipitation in the form of feathery ice-
crystals ; other forms of ice-precipitation are the powdery
ice-crystals or needles which are commonly experienced
in the snow-storms of intensely cold weather on mountain
tops and in the arctic or antarctic regions in the snow-
storms, in fact, of which the " BLIZZARD " has become
the descriptive name; soft hail or graupel. /.., snow in
which the needles are agglomerated to form minute snow-
balls, sometimes striated in texture, which break with a
splash on reaching hard ground ; and true hail, which
began as rain frozen and sustained in rapidly-ascending
currents of dynamically cooling air. Snow may perhaps
be the result of the direct congelation of water- vapour,
the omission of the intermediate liquid-state being the
essential difference between the hail and snow processes.
Snowflakes are formed of one or more ice-crystals arranged
in symmetrical hexagonal patterns of which there is an
almost infinite variety. Many are figured as photo-
micrographs in the Monthly Weather Review of the
United States Weather Bureau, Washington, for 1902 (W.
A. Bentley). When snow falls with comparatively high
temperature, large, wet flakes often result ; with lower
temperature the flakes are smaller, and with the thermo-
meter reading far below the zero of the Fahrenheit scale,
we have the " snow-dust" or " ice-needles " ; fine ice-
crystals or needles also characterise the deposits which are
formed in foggy, frosty weather, particularly on mountain-
tops, where wreaths of such crystals sometimes grow out
to windward. The hexagonal formation of a snowflake
may be well observed under a low-power microscope ; it
will be noticed that each one of the constituent "spiculee"
is set at an angle of 60 degrees to its fellows. The ratio
which an inch of rain bears to an inch of snow depends
Snow.
upon the density of the snow ; as a rough approximation
for the most common kind of snow a ratio of 12 to 1 is
usually taken in this country. In exceptional cases the
divergences from this value are very wide indeed ;
according to Colonel Ward, the range may be from about
5 to 1 to about 50 to 1 that is to say, a foot of snow on
the ground may yield, when melted in the RAIN-GAUGE,
the water-equivalent to 2 '4 in. of rain at the one extreme,
or to 0-24 in. at the other. One foot of snow to one inch
of rain is, however, a convenient generalisation.
Soft-Hail. The English term for the form of ice-
precipitation known in German as Graupel. It consists in
reality of pellets of closely agglomerated ice-needles,
sometimes striated in texture, and thus falls under the
category of snow, rather than under that of hail. On
colliding with any hard substance, soft-hail breaks up
with a splash, and may thus be distinguished from true
hail, the form of which is not affected by the impact. The
French equivalent for " soft-hail " or " graupel " is
" gresil."
Sun-dial. Little use is made of the sun-dial at the
present time, except as an ornament for the garden. There
are various forms, the commonest being a horizontal stone
slab upon which a rod or style, called the gnomon, is set
up in the astronomical meridian, inclined to the horizontal
at an angle equal to the latitude of the place, or, in other
words, parallel with the earth's axis. The line traced by
the shadow of the style at each hour of the day is engraved
upon the slab. When the vertical plane through the style
lies correctly in the meridian, after applying a correction
for the EQUATION OF TIME (q.v.\ such a dial will give
344 Glossary.
mean solar time whenever the sun is visible. To obtain
Greenwich Mean Time a constant correction must be
applied depending upon the longitude of the place.
By taking account of the length of the shadow as well
as the line on the dial, the time of year can be indicated,
and some dials are elaborately graduated as a perpetual
calendar as well as time-keeper.
Twilight. Twilight is caused by the intervention of
the atmosphere between the sun and the earthV surface.
With no atmosphere, darkness would set in sharply at the
moment of sunset, and would give place suddenly to light
at sunrise, as on the moon. But when the sun is some
distance below the horizon the upper layers of air are already
illuminated, and are reflecting light to us. The amount
of reflected light diminishes as the sun's distance below
the horizon increases, because higher, and so less strongly
reflecting, layers alone are in direct sunlight.
So early as the llth century the period of Astronomical
Twilight, between sunset and the onset of " complete "
darkness, was determined as ending when the sun is 18
below the horizon, and this value has not been modified
by later observations. If we assume direct reflection as
the sole cause of twilight, this value, 18, would indicate
that the atmosphere above a height of some 80 kilometres
is incapable of reflecting an appreciable amount of light.
Long before the end of Astronomical Twilight, how-
ever, the light has become insufficient for ordinary
employments, hence another period, Civil Twilight, is
recognised, ending when the sun is about 6 below the
horizon, and conditioned by the insufficiency of light for
outdoor labour after that time.
The duration of twilight depends on the season arid the
Twilight. 345
latitude. At midsummer the sun is 23^ North of the
equator. Hence within the Arctic circle, latitude
90 23i=66|, the sun never 'nets, so thai; there is no
twilight! Between the Arctic circle and latitude
90-*3| -18 =48i there is a belt with no true night,
twilight extending from sunset to sunrise. At midwinter,
in the Arctic circle, the sun does not rise, but up to lati-
tude 90 23^ + 18=84i, there is an alternation between
twilight and night. North of 84^ there is continuous
night.
At London (latitude 51 ) Astronomical Twilight has a
minimum duration of about 1 hour 50 minutes on March
1st and October 1st, with a secondary maximum in mid-
winter of just over 2 hours, and lasts all night at mid-
summer.
At, the Equator the minimum duration is 1 hour 9
minutes, the solstitial maxima 1 hour 15 minutes.
Civil twilight at the Equator varies between 21 and 22
minutes, at London it has minima of 33 minutes in March
and October, maxima of 40 minutes in December, and 45
minutes in June.
The duration of either twilight at any latitude and
season may be found by using the equation
sin a sin sin S
cos h = -
cos cos o
where h = sun'e hour angle from the meridian,
a = sun's altitude,
= latitude,
c = sun's declination,
a = 50' at beginning of twilight (allowing for sun's
semi-diameter and refraction),
6 at end of civil, and 18 at end of astro-
nomical twilight ;
346 Glossary*
thus to find duration of civil twilight, find cos h for the
two cases, a = 50' and a = 6, convert the two values
of h to time, and the difference is the required duration.
The intensity of twilight depends to some extent on
cloud, dust, haze, or other obscurity in the atmosphere.
Dust in the higher layers, as in the case of the sunsets of
1883-5 (after the eruption of Krakatoa), may much
increase the intensity of illumination during twilight, by
increasing the reflected light. In a cloudless sky the
intensity falls off rapidly at first, then more slowly from
about 35 foot-candles at sunset to 5 foot-candles at the
end of civil twilight, and to '0001 foot candle at the end of
astronomical twilight; These intensities are to the light
of the fuJl moon in the zenith as 1750, 25, and "005
respectively to 1.
The optical phenomena of twilight occur in the follow-
ing sequence ; for explanations, reference should be made
to the corresponding articles in the Glossary. As the sun
sinks towards the horizon it is shining through an
increasing thickness of haze and dust-laden air, and
scattering (see BLUE OP THE SKY) causes less and less of
the blue light to reach us, so that the sun appears in-
creasingly red. A yellow band now appears on the
Western horizon, extending for about 60 to either side
of the sun. Gradually the yellow deepens to orange or
red as the proportion of blue decreases. As the sun passes
below the horizon the pink TWILIGHT ARCH (better
called the Anti-Twilight Arch) rises from the Eastern
horizon, the space under it being strikingly darker than
the rest of the sky. While this arch is rising in the East
the PURPLE LIGHT has appeared at an altitude of about
25 in the West, above the point of sunset. This light
attains its maximum intensity when the sun is about 4 Q
Twilight. 347
below the horizon, and disappears on the Western horizon
when the sun is about 6 below, at the end of Civil
Twilight. Just before its disappearance the purple light
has become a narrow arch over the yellow glow near the
horizon and thus forms the " Western Twilight Arch."
The purple light is often seen to be intersected by dark
blue stripes radiating from the position of the sun. These
are the shadows of clouds on or below the horizon, and
are frequently called Crepuscular Rays. On very clear
nights a second dark segment in the East and a second
Purple -Light in the West may be observed.
Evening conditions have been assumed above, but
obvious inversions make the discussions applicable to the
mornings.
Vortex. A special form of rotatory motion in fluids.
Two forms of vortex have figured much in mathematical
literature, the vortex-ring and the long straight vortex,
and both are believed to be represented in nature. The
mathematical vortex-ring in its simplest form is shaped
like a perfect anchor-ring or hoop, whose circular aper-
ture is very large compared with the diameter of the
circular wire of which it is composed. The cross-section
of the material of the ring a liquid or gas is every-
where a circle of radius 0, and the centres of all these
circles lie on a larger circle, the aperture, of radius n.
There is complete symmetry round the axis, i.e., the per-
pendicular to the plane of the aperture through its centre.
Any plane through the axis cuts the ring at right angles
in two circles of radius e, situated at opposite ends of a,
diameter of the aperture. If we take any one of these
circular sections the liquid within it is everywhere circu-
lating round and round within the circle. In the simplest
348 Glossary.
case its rotational velocity increases as its distance from
the centre, where it vanishes. But in addition to this the
ring moves bodily. If it is alone in an infinite liquid, its
centre travels in the direction of the axis, with a uniform
velocity which is greater the greater ale.
'The straight vortex in its simplest form is a right
circular cylinder, or pencil-shaped body, and if the
vorticity is uniform over the cross section the simplest
case the liquid spins round with a velocity proportional
to the distance from the centre of the section, i.e., the
liquid forming the vortex turns round exactly as if it were
a rigid body. A solitary straight vortex in an infinite
liquid has no inherent tendency to translatory movement.
The liquid forming the vortex simply goes on spinning
round the axis of the cylinder ; the liquid round it also
rotates round this axis, but with a velocity which dimin-
ishes as the distance from the vortex increases.
The assumption ordinarily made that the liquid is
infinite means that every part of the vortex is remote
from a boundary. But some forms of vortex motion are
possible in presence of a plane boundary, and a sphere
whose radius is large compared with the largest dimension
of a vortex may be treated as a plane. A vortex ring
with its aperture parallel to a plane boundary behaves as
if face to face in an infinite liquid with an equal " image "
vortex, whose distance is double that of the real vortex
from the plane. The two vortices repel one another.
Again a theoretically possible case is that presented by the
half of a complete vortex-ring cut in two, as it were, by
the boundary the plane of the aperture being perpen-
dicular to the boundary. The motion would be the same
as if the ring were really complete and no boundary
Vortex. a4l)
existed. Similarly there are two possible cases of a
straight vortex in presence of a plane boundary. The
vortex may be parallel to the boundary. The conditions
are the same as if it were in an infinite liquid facing
another equal vortex in which the spin is in the opposite
direction the distance between the two being double the
distance of the real vortex from the boundary. The
vortex tends to move parallel to the boundary, in the
direction perpendicular to its own length. In the second
case the vortex is perpendicular to and abuts on the
boundary ; it then behaves as if it extended to infinity on
both sides of the boundary, and so has no inherent
tendency to translatory movement. Whether the vortex
be straight or ring-formed, an essential feature of the
mathematical theory is that the liquid, once incorporated
in the vortex, remains in it. The beginning and ending
of the existence of the vortex are events outside the
compass of the mathematical theory.
Vortex rings are easily created by human agency. A
drop of one liquid falling into another suitable liquid
forms a vortex-ring, and smoke-rings are familiar to most
people. Whether they occur in nature is a more difficult
question. Delicate optical measurements suggest that
sunspots are whirls of electrified gases/ It is noticed that
sometimes sunspots move in pairs, and that the whirls in
them deduced from the optical measurements are in
opposite directions. It has been suggested that we have
here really to do with the horse-shoe or semi-ring vortex.
The two sunspots represent the portions of this which
are nearly perpendicular to the sun's surface, and the
connecting or crown portion extends into the more
rarefied solar atmosphere and is not recognisable from the
earth. This is merely a speculation, but the possibility
350 Glossary.
of. a similar phenomenon in the earth's atmosphere may
be worth considering.
What seems to be at least an approach to the long
straight vortex is exemplified by water spouts and by the
dust-whirls sometimes seen on warm days. But the
common belief that it is also exemplified by the ordinary
cyclonic storm does not seem well founded. Tho belief
is mainly based on the fact that the isobars during a
cyclonic storm are often roughly circular. The direction
of the wind, it is true, at some height above the ground,
approaches that of the isobars. But the centre of the
storm, i.e., the centre of the system of isobars, is not
stationary but moves with a velocity comparable with
that of the wind itself. The actual path of the air is
complicated. It is carried from without into the cyclone,
but does not remain in it. The mathematical vortex,
on the other hand, is composed of the same fluid from
start to finish. The mathematical vortex, moreover,
is a long thin body like a pencil. An ordinary
cyclone, even supposing it extends some distance into
the stratosphere, is a disk-like body, the height of which
is small compared with its diameter. Again, in the
straight mathematical vortex the velocity round the axis
is the same at the same axial distance in all cross sections.
Ordinarily the wind increases in velocity with the height
above the ground. Supposing the core of a cyclonic vortex
originally vertical, unless the motion of translation were
the same at all heights, the core would depart more and
more from the vertical, and, judging by what happens
with water-spouts, dissolution would soon ensue.
The conditions compatible with real vortex motion in
a cyclone are that the velocity should be independent of
the height, that the horizontal diameter of the body of
Vortex. 351
air possessing the motion should not be large, and that an
unchanged body of air should have the translational
velocity shown by the isobars. The necessary conditions
are certainly not fulfilled near the centre of the ordinary
large cyclonic storm. They seem much more likely to be
encountered in the small " secondaries " that are some-
times met with on the outskirts of large depressions, or
in the whirlwinds that occasionally leave a long narrow
track of devastation. It is difficult to ascertain the exact
meteorological conditions attending these special disturb-
ances. A weather map, to show them satisfactorily,
would have to be of exceptionally open scale and based
on an unusually minute knowledge of local conditions.
One or two cases of real vortical storms do seem, however,
to have been observed in the British Isles, notably a
storrn on March 24, 1895, which caused much damage to
trees in the Eastern Counties.
Wind Rose. A diagram showing, for a definite
locality or district, and usually for a more or less extended
period, the proportion of winds blowing from each of the
leading points of the compass. As a rule the " rose " in-
dicates also the Strength of the wind from each quarter,
and the number or proportion of cases in which the air
was quite calm.
The simplest form of wind rose is represented by the
accompanying figure, in which the number or proportion
of winds blowing from each of the principal 8 points of
the compass is represented by lines converging towards
a small circle, the proportion of winds from each direction
being indicated by the varying length of the lines. The
figures in the circles give the number, or percentage, of
cases in which the air was calm.
352 Glossary.
A "rose" may be, and occasionally is, devised
such a manner as to indicate the relation of other meteoro-
logical phenomena, such as cloud, rain, fog, &c., to the
direction of the wind. As a result of an investigation
recently undertaken in the Meteorological Office a series
of roses has been constructed showing that on the western
and southern coasts of the British Islands the bulk of the
fogs experienced are sea fogs, i.e., they occur with winds
blowing (sometimes with considerable strength) from the
surface of the ocean. On the north and east coasts summer
fogs also come from the sea, but winter fogs more often
from the land. The " roses " show further that over the
inland parts of England the fogs are radiation fogs, and
are accompanied by calm or very light winds blowing
from various quarters.
The publications of the Meteorological Office have in-
cluded from time to time wind roses of various designs.
Specimens of these are reproduced on the two succeeding
pages.
s j
he
Wind-Pose.
353
Showing- Average Di-
rection of Wind by shaded
areas converging- towards
centre of diagram and
Strength of Wind in
numbers of Beaufort Scale
by dots.
Prevalence of Calms
indicated by diameter of
central circle.
(Reproduced from Wind Charts of North Atlantic," published in 1859.)
Relative preva-
lence of Wind for
each point of the
compass shown by
length of arrows
converging to-
wards centre.
Force of Wind
by curve inter-
s e c t i n g wind
arro\* s.
Calms by pro
portion of shadec
to unshaded por
tions of large cen
tral area.
Additional in
formation relate*
to Ocean Currents
and other matters
of interest t<
Navigators.
(Reproduced from. " Charts of Meteorological Data for Lai. 20 N. to 10 S Long
10 40 W. t " published in 1876.)
13201
M
354
Glossary.
Irregularly shaped
areas around outside
circle indicate relative
prevalence of Wind
from various directions.
Radial lines converg-
ing towards central
area indicate Wind
Force.
Shaded portions of
outlying areas indicate
prevalence of Gales.
Small central circle
indicates by its diameter
the proportion of
Calms.
Star points around
this circle indicate by
their length the num-
ber of observations.
(Re produced from " Charts ofttie Ocean District adjacent to the Cape of Good Hope "
published in 1882J)
Arrows fly with the
Wind towards centre of
diagram.
Frequency . of Wind
from each direction is
indicated by length of
arrow.
Force of Wind is indi-
cated thus :
Light, Moderate, Strong.
Figures in centre of
diagram indicate percent-
age of Calms.
{Similar to Wind Roses published in current issues of Monthly Weather Report and
in " Monthly Meteorological Charts of North Atlantic and Mediterranean"^)
Conversion Tables.
355
II
co vo CM ON vo
CM CO Vh V$- vO
CO CO CO co co
o
to CO co CO CO
ON O w ^t r*
CO CO ^t" >-n vO
vb K. t^oo ON
p
O CM -*J-vO CO
i-O
b
CO
SCM -^-vO CO
b
to
O M rj-vO CO
b
CO
O c ThvO oo
CO
8,
O CM -^-VO CO
b
CO
II
vovb ** x>oo
o M
co O vO to O
o>b o ^ c<
r^ co O t-. > ^-
N CO Vt- ri- vo
M t^ Tf HH 00
vb vb t-^oo oo
W d OJ W W
rj- t-l OO VO C*
ON b b M w
CM CO CO CO co
8 CM Tj-vO <?O
(
O "<*-vO CO
b
CO
O M rj-vo CO
b
CO
O <N ^VO CO
CO
b
CO
O CM T}-VO co
b
CO
3 M
O vp to O i
;* O f* T- M
CM CO Cp T$- vO
r^ ^t- t-t co '^~
M oo m w co
ON ONO ^ M
vo C<| ON vO CM
n to to ^ vo
SJ
ON ON O O O
t-i
a;
II
O N "tf-VO OO
VO
O C< Tj-vO CO
O N -^-vO co
O CM T^-VO CO
CO
O CM -^-vO co
^
CM CM co Vj- rh
OO OO CO CO OO
^(- M CO VO w
vOSO VO t^-OO
OO vo M CO vO
CO ON b b '**
co CO ON ON ON
CM CM to Vh rh
vovb o -f^oo
co
g
p w
O c -^-vo co
O W -^-vO oo
O W ^"VO oo
CO
ON
O CM ^VO CO
;3 OD
t-i oo yo HI co
vo d CO vO tX
co ON ON b HH
HH f>J CO CO Tj-
SSvb K<
vo to O t>- "4-
oo ON b b >-"
t^ i^OO OO CO
a
H-t
O M Tj-vO 00
VO
oo
O W -^-vo CO
vp
CO
O W ^-VO 00
CO
O CM T^-VO 00
00
oo
a CM Tj-VO OO
m
00
,
CM 00 vo ex, ON
VO W ONVO to
ONVO co O t>
CO O t->. CO O
t^Th t-Th
||
oo oo ON O O
1-1 M W CO 4-
vO vO vO vO vO
VO vO VO vo vO
vO vO VOVO VO
i-i CM to to Th
vO vO VO vO vO
1
CM -^-O CO
CO
O w -^-vO oo
w
O M ^VO 00
CO
O CM T*-VO OO
co
CO
O CM -^-vO co
7*-
00
w !
356
Glossary.
O VO M j>. CM
OO co ON * O
vO M J>- CNJ 00
CO ON rhO
M t^ CM OO CO
c8
oo oo ON o^ o
O -! t-H d CO
CO Tf Th U^ VO
vo vo t^oo oo
ON ON O O M
CO CO ro CO cO
CO CO CO CO CO
CO CO CO CO CO
CO CO CO CO CO
CO CO CO CO CO
O vo M t*- CM
OO CO ON rj- O
VQ M t^ CM OO
co ON -i- O vo
M t>. M OO CO
O
o
vr unvo O t^
CO CO CO cO CO
r^oo oo ON O
CO CO CO co rj-
O hH HH CM CO
co co rf- vo xr>
^- "* Th Tj- ri-
VO O t^ J-^00
TJ- -t- Th Th T^-
EH
o
vnvo r^oo ON
M CM CO Tj-
O O O
ff^^??
O M W CO Tt-
vnvO t^OO ON
M ^ co oo co
ON rh O O M
t^ CM oo co ON
rh o vo M r^
CM OO co ON T|-
cS
TJ- rrf- xr> vr><o
ON ON ON ON ON
CO CO CO CM CO
vo t^oo oo ON
O"i ON ON ON o^
CM CM CM CM W
ON O O O
CM co CO CO co
CM co co rj- rt-
O O O O
CO CO CO CO CO
w> LOVO VO J^
O O O O
CO CO CO CO CO
l-l .t^ CO OO CO
ON rt-O w
J>. C-l OO CO ON
T- O vo 1-1 i^
CM OO co ON Tf-
9
W hH CO CM CO
CM CO CO CO CM
CO r^- U-J iTiVD
CM CM CM CN C^
VO t^ t^O OO
CM M <N CM C<J
ONO O W l-l
CM CO CO CO CO
CM CM CO CO -<^-
CO CO CO CO CO
&
O M CO CO Tj-
vnvo t^oo ON
O M CM CO Th
vovo r-^oo ON
O I-H co co T}-
o
CO 00 CO ON rh
O vo HH t>. CM
OO CO ON ^- O
VO t-t t-^ C^ OO
co ON -rj- O vO
06
O O >H i-" CM
OO OO OO OO OO
CM CM CO CO CO
CO CO ^h Tj- W>
00 00 OO 00 00
CM W W CM CM
ir>>O O t^oo
OO OO OO OO CO
CM CM W CM CM
OO ON ON O O
OO OO CO ON ON
W CM W CM CM
M M CM CO CO
ON ON ON ON ON
CM CM CO CO M
<M OO CO C^ Tj-
O VO ** t*. CM
oo co ON ^- O
VO t-t t^ CM 00
co ON rf- O vo
O
o
t>. t^oo oo ON
O M M 01
CM CO CO rt-xn
vnvo vo t^ r~*
oo oo ON O O
f=H
o
w>v5 t^-OO ON
O H CM CO TJ-
xr> vr \f) \n xr>
vnvo t^oo ON
xr> \jt vn UTJ \r>
O - CM co -ri-
w>vo t-^oo ON
vc vO vo VO VO
CO ON rh O ^D
-< t>* CM OO CO
ON "* O vo M
t>. CO OC CO ON
"tf- O vo M t^
03
O VO t^OO 00
ON ON O O "-<
M CJ CO CO rj-
^t- tn to vo vo
t>.oo 00 ON ON
w 01 o* a o*
CM CM CM CM CM
CM CM CM W <N
CM CM C<J CM CM
CO CO CO CO CM
t^ M vo O -=t-
ON cooo CM JT>.
H-4 VO -O SO -
^ CM OO CO ON
Th O VO M t-*
?
VO ^D >JT> A^> ^J-
1 1 1 1 1
CO CO CM CM HH
Mill
HH O O M
1 1 +
M CM CM CO CO
^t- u-> vr>vo vo
ft
O M M CO rj-
W CM CM CM C<
vnvo t^oo ON
CM W CM W N
O 1-1 CM co rh
CO CO CO CO CO
xr>vO J-^oc ON
CO CO CO C*5 TO
O -i CO CO Tj-
Index to Tables.
357
INDEX TO TABLES INCLUDED IN THE GLOSSARY.
Abbreviations. List of
Adiabatic expansion. Change in temperature
of air on.
Aqueous Vapour Mass of in saturated air ...
Pressure of do. do. ...
Amount and Pressure of, at
Kew.
Clouds. Types of
Correlation Coefficients. Selected examples ...
Density
Evaporation of Water
Fog and Mist. Average No. of observations
of Fog in British Isles.
Frequency in English Chan-
nel of
Gales at some British anemometer stations ...
Seasonal variation of
Gradients. Steep pressure
Gusts. Range of fluctuation of
Strongest recorded
Hurricanes, cyclones and typhoons recorded in
various parts of the World.
Insolation. Calculated Insolation reaching
Earth.
Pressure Units. Conversion table
Rain. Consecutive hours of rain in 1912
Rainfall during the four seasons in S.E.
England and X. Scotland.
Day rainfall and night rainfall
Relative Humidity. Frequency of occurrence
of various values of
Sunshine. Percentages of possible duration of
Temperature. Boiling points of water at
various pressures in the atmo-
sphere up to 8,000 feet.
Conversion table
Normal weekly temperatures for
S.E. England.
Some common temperatures ...
13204
See under :
p. 2.
Adiabatic,
Aqueous vapour.
Do.
Absolute Humidity.
Clouds.
Correlation.
Buoyancy.
Evaporation.
Fog.
Frequency.
Gale.
Gale.
Gradient.
Gusts.
Gusts.
Hurricane>
Insolation.
p. 355.
Persistent rain.
Seasons.
Seasons.
Relative humidity
Sunshine.
Hypsometer.
p. 356.
Seasons.
Absolute temperature.
N
358
Glossary*
Thunderstorms, Immunity from
Upper atmosphere. Normal pressure at var-
ious heights.
Average temperature at
different levels.
Average values of pres-
sure, density and tem-
perature of air over
regions of high and
low pressure.
Normal factors for the
density of air at various
heights.
Limit of height for the
expenditure of ballast.
Depression produced on
airships by rain or
snow.
Wind. Monthly normals of wind velocity at
some French and British stations.
Normal hourly wind velocities at Kew
Spells of N.E.-S.E. winds of specified
duration over S.E. England and N.
France.
Frequency of winds from different
quarters over S.E. England and N.
France.
Distance between isobars for various
geostrophic winds.
Equivalents of wind force
Wind direction at Suva, Fiji
Hourly velocity at the top of the Eiffel
Tower.
See under :
Thunderstorms.
Ballon-sonde.
Ballon-sonde.
Density.
Buoyancy.
Buoyancy.
Buoyancy.
Normal.
Normal.
Frequency.
Frequency.
Isobars.
Beaufort scale.
Trade winds.
Wind.
Printed .under the authority of His Majesty's Stationery Office
BY DARLING AND SON, LIMITED, BACON STREET, E.2.
* METEOROLOGICAL GLOSSARY," M.O. 225 ii. (Fourth Issue.)
CORRIGENDA.
In the Fourth Issue the plates representing various forms
of pressure distribution which in the previous issue were
placed with the separate articles : ANTICYCLONE, COL, DE-
PRESSION, SECONDARY DEPRESSION, V-SHAPED DEPRESSION,
WEDGE in alphabetical order are now put together in the
article ISOBARS, and should have been re-numbered in order
to correspond with the text. But the re-numbering, and in
consequence the order, has failed. The following corrections
should therefore be made in the numbering and order of the
plates :
DEPRESSION should be Plate XI. instead of XIII., and face
page 174.
SECONDARY DEPRESSION should be Plate XII. instead of XIV.,
and face page 175.
ANTICYCLONE should be Plate XIII. instead of XL, and face
page 176.
COL should be Plate XIV. instead of XII., and face page 177.
The foot-note on page 177 should be omitted.
Page 75, line 1, "Sir Gilbert Walker" should read -Dr.
Gilbert Walker."
Page 132, last line, " 65 millibars " should read " 5 millibars."
P.ige 256, line 24, " John Hadley," etc., should read " George
Had ley, who was a brother of John Hadley," etc.
Page 262, line 18, " years" should read " year."
Page 271. In the column headed u Depression of Wet Bulb,"
the temperature scale F. should be indicated.
Page 295, line 1, R 1 should read R 2 .
line 12, /o<r should read <r.
line 26, V 1 should read V'.
Page 308, line 27, "Earth's mass M' " should read "earth's
mass plus that of atmosphere (M + M')."
Page 330, line 28, should read " 77 (288) 4 x lO" 12 ."
SW
Page 340, line 7, the equation should read 3G = .
a
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