Supplement to “ Nature,” April 14, 1923 ix 

produce such an obscurity that midday is as dark as 
midnight. The total abolition of smoke from London 
would not reduce the occasions on which mist and fog 
occur, but many fogs would remain mists, and we 
should never have a “ London particular.” 
For the formation of mist and fog it is necessary that 
the temperature of the air should continue to fall after 
the dew point of the air has been reached. The extent 
of the fall below the original dew point determines the 
density of the obscurity, neglecting for the moment 
the effect of impurities. This cooling can be brought 
about in several ways, of which only two are of real 
importance. Fogs may be caused by warm air blowing 
over a cold sea or a cold land surface, and the method 
by which the temperature of the air is then reduced on 
account of turbulence was first explained in a brilliant 
paper by G. I. Taylor, which has become a meteoro- 
logical classic. The fogs of London are, however, 
almost entirely due to the loss of heat from the lower 
layers of the atmosphere into a clear sky above. The 
air radiates its heat, its temperature falls, and condensa- 
tion takes place as already described. Other methods 
of fog formation, such as the mixing of warm and cold 
air, are of secondary importance and never give rise to 
more than patchy local mists or light fogs. 
Clouds.—Adiabatic cooling plays no part in the 
formation of mists and fogs, because the pressure 
changes in any given layer of the atmosphere are 
relatively small and slow. Appreciable adiabatic cool- 
ing can take place only when air is raised in the atmo- 
sphere, and then the cooling may be large and rapid. 
When air not saturated rises in the atmosphere its 
temperature is reduced by about 1° C. for every 100 
metres of ascent. When the ascent is carried far 
enough the dew point is reached, after which any 
present. As the ascent is carried beyond the point of 
condensation more and more water is deposited, with 
a consequent increase in the size of the drops. This is 
the manner in which clouds are formed, and there are 
very good reasons for saying that it is the only way. 
Thus there is a fundamental difference between the 
method of formation of clouds and fogs: fogs form 
without any ascent of the air, while clouds are never 
formed without it. Thus it is not correct to describe 
current, owing to the increase in relative humidity. 
But when saturation is reached they are still very 
small, say about 1-1 x 10~5 cm., that is, they are smaller 
than the wave lengths of light (5xxzo0~5 cm.), and 
therefore cannot be directly observed, and produce little 
obscurity in the air, which still appears relatively clear. 
Drops of this size need supersaturation to grow, but we 
see from Table I. that only x per cent. supersaturation is 
necessary. They are, however, unstable, for as they 
grow they need less supersaturation. Thus as soon as 
the air is sufficiently supersaturated to be in equilibrium 
with the nuclei the slightest further rise causes the drops 
to grow very rapidly to the size in which they are in 
equilibrium with saturated air. The height at which 
this change occurs is the height of the base of the cloud. 
Rain.—Before we are able to form a clear idea of the 
processes which give rise to rainfall it will be necessary 
to consider the laws of the fall of water drops through 
the air. 
It is well known that in a vacuum all bodies fall at 
the same rate with a constant acceleration, so that their 
velocity constantly increases. When, however, bodies 
fall through a resisting medium, such as air, they more 
or less quickly reach such a velocity that the resistance 
of the air equals the pull of gravity, after which they 
fall with a constant velocity, which is different accord- 
ing to the density and shape of the falling bodies. 
Experiments have been made to determine the rate of 
fall of water drops through air at atmospheric pressure, 
and the following “ end velocities ” have been found. 
TABLE IV. 

Velocity calcu- 
Radius, | Sioa by Stokes, | Velocity observed by | Velocity observed by 
cm./sec. 
00005 0-3 RY 
0-001 13 ae 
0:005 32 sh 
0-010 126 BS 
0-020 r 180 
0-030 hs 
0040 4 “< 
0°050 393 440 
0-100 577 59° 
o-150 692 690 
O-175 749 737 
0°225 ee 805 
0:273 os 798 
0318 780 
clouds as fogs of the upper atmosphere. 
The very sharp line of demarcation between the air 
under a cloud and the cloud itself needs explanation, 
There is no slow transition between the clear air and 
the cloudy air, as one would expect if clouds were due 
to the gradual increase in the size of drops from small 
nuclei to relatively large cloud particles. We must 
picture the hygroscopic nuclei collecting more and more 
water around them as they rise with the ascending 

Three important points are to be noticed about these 
results. 
(a) The extremely small velocities with which small 
drops fall. The average radius of the drops in clouds 
from which rain is not falling is approximately o-oor cm, 
Such particles, according to our table, would fall only at 
the rate of a little over a centimetre a second. 
