METEOROLOGICAL ASPECTS OF AIRCRAFT ICING 
and stratocumulus, and altocumulus and altostratus- 
altocumulus. This last group includes all middle cloud 
types containing liquid water since typical altostratus 
clouds are composed of ice crystals. 
It can be seen from Table III that the average and 
extreme values of liquid-water concentration in cumu- 
lus clouds are much greater than in other cloud types. 
This is due to the much greater vertical extent of 
cumulus clouds. A compensating factor is their limited 
horizontal extent. It is noted that the concentration 
of liquid water is considerably lower in altocumulus 
and altocumulus-altostratus than in stratus and strato- 
cumulus clouds. The difference is due to the lower tem- 
perature and smaller vertical thickness of altocumulus 
and also to the frequent occurrence of ice crystals in 
altocumulus when associated with altostratus. 
The Size of Cloud Drops 
Important inferences concerning the sizes of cloud 
drops to be expected from various conditions of cloud 
formation can be drawn from a theoretical analysis of 
the mechanism of cloud-droplet formation and growth. 
A detailed and quantitative treatment of the physics of 
cloud-drop formation has recently been given by How- 
ell [8]. According to this study, for the case of uni- 
formly lifted air, ‘‘the numerical concentration of drops 
is determined primarily by the rate of cooling during 
the initial stage of condensation. It depends, as a rule, 
only slightly on the concentration of condensation nu- 
clei. With continued uniform cooling, the drop concen- 
tration diminishes slightly toward a fixed constant 
value.” It is also shown that uniform lifting leads to a 
rather uniform drop-size distribution. The effect of 
evaporation, owing either to warming in descending 
portions of a cloud or to entrainment of dry air, is a 
decrease in the concentration of drops, since the smaller 
drops evaporate most rapidly. On the basis of these 
considerations, the highest concentration of drops would 
be expected in rapidly formed cumulus clouds and the 
lowest concentrations in altocumulus clouds formed 
slowly by convergence in stable air. 
Actual measurements of cloud-drop diameter made 
in flight indicate wide variations for clouds of all types. 
Frequency distributions of observed values of mean- 
effective drop diameter are given in Table IV for three 
principal cloud types and two geographical areas. The 
classification on the basis of location was made be- 
cause it was observed that the largest values of drop 
size were found mostly along the Pacific Coast. The 
data in Table IV are from the sources listed previously 
for Table III. Representative values of drop concen- 
tration were calculated from the median values of 
liquid-water concentration and drop diameter and are 
also included in Table IV. 
It can be seen from an examination of Table IV that 
there is a slight variation of drop size with cloud type 
and a greater variation with geographic location. The 
occurrence of larger drops in the Pacific Coast region 
is believed to be due to the prevalence of maritime air 
masses with little pollution from combustion sources, 
and hence a very low concentration of effective nuclei. 
1201 
The small average drop concentration in altocumulus 
clouds was to be expected from condensation theory. 
Larger values for stratocumulus and cumulus clouds 
are believed to be due to greater turbulence and hence 
TaBLE IV. OBsERVED FREQUENCY OF VARIOUS VALUES OF 
Mean-EFrrectiveE Drop DIAMETER IN CLOUDS 
Other areas in the 
Pacific Coast region United States 
Mean-effective drop 
we AG | st, Sc | Cu,cb| A%_| st, sc | Cu, co 
4 , Sc | Cu, 4 > > 
Rea) 60 obs. 200 obs. enn 267 one 110 obs. 
B % % % % % % 
0- 9 8 5 5 20 32 6 
10-14 22 36 19 32 43 31 
15-19 28 25 25 30 16 35 
20-24 22 17 28 12 6 20 
25-29 7 7 15 5 2 5 
>29 13 10 8 1 1 3 
B B B B BL Xe 
Lower quartile....... 13.5 | 12.5 | 14.5 | 10 9 13 
Median ee 18 16 19.5 | 14 11 16 
Upper quartile....... 23 22 24 18 14.5 | 20 
cms cms cms cm cms cms 
Representative drop 
concentration......| 35 100 90 75 |320 {160 
more rapid lifting at the time of condensation. The 
fact that cumulus clouds have smaller drop concentra- 
tion than stratocumulus may be due to the greater 
effectiveness of such modifying factors as entrainment 
and precipitation, which promote evaporation and thus 
reduce the drop concentration. 
The Problem of Forecasting Icing Conditions 
For the purposes of this discussion, it will be assumed 
that satisfactory forecasts of the type, location, thick- 
ness, altitude, and temperature of cloud masses, and 
the occurrence and type of precipitation can be made. 
It is fully realized that this assumption is frequently 
not valid, but it is made here in order to separate the 
problems peculiar to forecasting icing intensity from 
the general problem of weather forecasting. In other 
words, the problem to be treated here is that of estimat- 
ing the icing intensity to be expected within a cloud of 
known type, dimensions, and temperature. If this can 
be done satisfactorily, a forecast of icmg can be de- 
rived from any good forecast of weather and cloud con- 
ditions. However, regardless of whether satisfactory 
forecasts of cloud conditions can be made, reliable es- 
timates of the presence of icing in the currently ob- 
served and reported cloud conditions may be of con- 
siderable value. 
The importance of a consideration of cloud type in 
estimating icing conditions cannot be overemphasized, 
since entirely different icing conditions prevail in dif- 
ferent types of clouds. A brief discussion of the icing 
conditions likely to be found in clouds of various types 
is given in the following paragraphs. 
Cumulus and Cumulonimbus Clouds. Icing conditions 
in cumulus clouds are sharply limited in horizontal 
extent, highly variable, and occasionally very severe. 
