THE USE OF CLOUDS IN FORECASTING 
By CHARLES F. BROOKS 
Blue Hill Meteorological Observatory, Harvard University 
Introduction 
A storm may be described as precipitation sur- 
rounded by clouds and usually accompanied by wind. 
The essence of forecasting is in telling when precipita- 
tion will occur and when it will not. Since rain or snow 
does not occur without advance notice in the form of 
clouds, we conclude that clouds are a prime element in 
forecasting. 
Weather proverbs from long ago [52, 55] indicate 
attempts at spot forecasting from cloud observations. 
The professional forecaster, while less dependent upon 
scanning the local sky, cannot neglect the synoptic 
reports of cloud systems and their motions and de- 
velopment. Cloud observations supplement the more 
direct and quantitative aerological data from balloons, 
airplanes, and mountain stations, which, moreover, are 
not always available. 
The very presence of a cloud indicates condensation 
in the atmosphere and hence the occurrence of some 
cooling process. Cooling in the free air is most com- 
monly caused by ascent and expansion of air; therefore, 
a cloud usually indicates ascending air. If the cloud is 
cumuliform, the ascent is localized; if it is stratiform, 
the ascent is general, probably from broad lifting, such 
as convergence on a large scale or the upglide of one 
air layer over another. The sharpness, density, and 
form also indicate lapse rates, vertical currents, wind 
shear, and turbulence. The progressive motion provides 
the wind velocity at or immediately below the cloud 
height. Better observations and greater attention to 
interpretation of clouds and their sequence are there- 
fore urged in the interest of improved local and general 
forecasting. 
For comprehensive discussions of aerological inter- 
pretations of clouds, the reader may refer to books by 
Stiive [93], String [95], and Kahler [56], to Brooks’ 
systematic article [13], to Modller’s review of the liter- 
ature from 1939-1946 [71], and to relatively recent de- 
tailed treatments of special phases such as the studies 
of Ci! by Ludlam [62], the studies of Sc and St by 
Raethjen [78], Schwerdtfeger [89], and Neiburger [72], 
of convective phenomena by Bleeker and Andre [10], 
and of convective clouds in the tropics by Craddock 
[28-30] and Deppermann [35]. 
Cloud Observing 
Cloud observing presents many problems: separating 
the levels, observing the forms, extents, and positions, 
and determining the heights and motions. Summaries 
of methods of determining cloud heights and motions 
have been published by Middleton [68], Stiring [96, 97], 
and Hredia [43], and a brief evaluation of the accuracy 
1. The international abbreviations of cloud genera will be 
used throughout 
of different methods of determining cloud heights, by 
the International Meteorological Organization [54]. Dis- 
cussions of cloud observing have recently been pre- 
pared by Brooks [12], emphasizing the range-finder and 
dew-point methods for determining cloud heights, and 
by Fergusson [44], and Conover, Fergusson, and Brooks 
[26], emphasizing the nephoscope method of obtaining 
cloud motions. It appears that a coincidence-type of 
range finder with a base of at least 114 m is by far the 
quickest means of getting cloud heights within an 
accuracy of 5 per cent when there are sharp, well- 
lighted clouds within 15 km in any part of the sky 
other than the zenith (where the ceilometer is better), 
However, the cloud-range meter, of radar type, de- 
scribed by Moles [70], may have some advantage over 
the range finder within its limit (7000 yd), since sharp- 
ness in cloud outline and illumination are not required. 
The dew-point method is good within 8 per cent, Clay- 
ton found [24], when care is exercised to apply it only 
to clouds forming by the convection of fair-sized parcels 
of air likely to have the same temperature and dew 
point as at the observer’s station. Large deviations 
result under sea-breeze or morning-inversion conditions. 
Heights by the single-theodolite, pibal method were 
found by Rossi [82] to differ from double-theodolite 
heights by 5 per cent under stable conditions and by 
16 per cent under labile ones; Berg [3] found them to 
differ from those by airplane by 4 per cent for Ac and 
Sc, 8 per cent for Cu, and 13-20 per cent for other clouds. 
The quickest way of getting cloud motions is with 
a window-sill nephoscope consisting of a plane, black, 
horizontal mirror of eight to ten inches diameter, with- 
out markings on the mirror other than a depression at 
the center, and a peephole eyepiece through which the 
observer can watch the motion of the image of a cloud 
and follow it with a small marker. When followed for a 
standard period (or easy fraction or multiple), the 
direction and relative speed are determined with a 
single placement of a ruler [12, 26, 44]. 
Clouds in Short-Term Forecasting at a Single Station 
Application of the Diurnal Cycle to Observed Clouds. 
Early on a calm, clear morning, the occurrence of low 
clouds moving rapidly from any direction, but par- 
ticularly from northwest or north, is a good indication 
of a day with strong winds from the direction from 
which the clouds are moving. Diurnal convection will 
soon bring this wind down to the ground. 
Harly-forming, thermal-convectton Cu or Se on a 
winter morning give promise of a generally cloudy day 
with little rise in temperature, for thermal convection 
will increase as the sun becomes effective, and the 
cloud masses will spread under the inversion responsible 
for the flattening already observed. 
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