172 
usual type of size-distribution curve the volume-median 
diameter is larger than the most-frequent diameter. 
As already stated, an indication of the breadth of the 
drop-size distribution can be obtained from the multi- 
cylinder data. The general form of the distribution 
curve must be assumed. The evidence available sug- 
gests that the majority of drop-size-distribution curves 
are of the general form assumed although there are 
occasional curves with multiple maxima. For conven- 
ience, nine standard volume-distribution curves have 
been adopted, identified by the letters A through J 
(I is omitted). The A distribution corresponds to com- 
plete uniformity and the succeeding letters to dis- 
tributions of Increasing breadth as shown in Table I. 
Tas_eE I. STANDARD VOLUME-DISTRIBUTION CuRVES USED IN 
THE MuLricyLINDER MrrHop 
Per cent of | Ratio of diameter of group to volume-median diameter 
liquid water for each distribution 
in each 
group A B G D E F G H J 
5 1.0 256 |e | ere | | eS | (1) 
10 1.0 SPI) aI) PA) aE i ill 74) alld) 
20 1.0 84, .77) .71) 65} =.59} =.54) 50) + .42 
30 1.0 | 1.00) 1.00) 1.00; 1.00} 1.00) 1.00) 1.00) 1.00 
20 1.0 | 1.17) 1.26} 1.37) 1.48} 1.60) 1.73) 1.91) 2.22 
10 1.0 | 1.32) 1.51} 1.74) 2.00] 2.30) 2.64) 3.04) 4.01 
5 1.0 | 1.49) 1.81] 2.22) 2.71) 3.30) 4.02} 4.93) 7.34 
As will be seen from Table I, each distribution is 
made up of seven different drop diameters each repre- 
senting the percentage of the total water indicated in the 
first column. The drop sizes are represented as the 
ratios of the drop diameter to the volume-median 
diameter so that the distributions may be applied to 
any volume-median diameter. The frequency of oc- 
currence of the nine drop-size-distribution types at 
Mount Washington for the months November 1946 
through May 1947 is indicated im Table II. 
TasuE II. OccuRRENCE OF DRop-S1zE-DISTRIBUTION CURVES 
By Type at Mount WasuineTon, N. H. 
Distribution curve 
Number of 
occurrences..... 
241 | 96 | 47 | 21] 18 | 5 | 2 | 4 
“I 
The predominance of narrow size distributions is 
striking. This is probably due in part to the high 
frequency of cloud-cap conditions which do not favor 
the nonuniform rates of lift apparently required to 
produce broad size distributions. For this reason Table 
II cannot be taken as typical of the clouds of the free 
atmosphere. It is also apparent from Table II that a 
significant number of broad distributions occur (# 
through J) which certainly cannot be explained on the 
basis of uniform lift. 
Multicylinder observations were also used to compute 
the liquid-water content. For the winter season 1945— 
46 the mean liquid-water content was 0.472 g m=. 
The most frequent value was 0.24 g m~4 and the range 
was 0 to 1.44 gm-*. There is a tendency for the higher 
liquid-water contents to be associated with higher tem- 
CLOUD PHYSICS 
peratures. There is a similar tendency for the drop size 
to merease with the temperature. 
An extensive series of in-flight measurements of the 
liquid-water content and drop size of supercooled clouds 
has been made by the National Advisory Committee 
for Aeronautics and reported by Lewis and collabo- 
rators [37, 38, 39]. The principal instruments used were 
the rotating multicylinder, a rotating disc icing-rate 
meter, and a fixed cylinder which gives a measure of 
the maximum drop size. The measurements were made 
during three winter seasons and in both the eastern 
and the western portions of the United States. Although 
identification of the cloud type was made in each case 
it was found that, in general, the data did not warrant 
a more detailed classification than the distinction be- 
tween cumuliform and stratiform clouds. For three 
winter seasons the average volume-median drop di- 
ameter was found to be 20.5 » for cumuliform clouds 
and 14.7 » for stratiform clouds. The range of volume- 
median diameters was 3 to 56 » for cumulus clouds 
and 3 to 50 » for stratiform clouds. Nearly 50 per cent 
of the size distributions as determined from the rotating 
multicylinder were type A. Evidence is presented that 
the size-distribution data are unreliable and the authors 
feel that this technique is not applicable to in-flight 
measurements. By comparison of the simultaneous ob- 
servations of volume-median diameter and maximum 
diameter they concluded that the clouds are more 
homogeneous than is indicated by the rotating multi- 
cylinder. 
The liquid-water contents reported by Lewis and 
collaborators [37, 38, 39] ranged from 0.02 to 2.0 g m~* 
for cumuliform clouds and from 0.01 to 0.7 g m~$ for 
stratiform clouds. The average values for the three 
winter seasons were found to be 0.51 g m~* for cumuli- 
form clouds and 0.134 g m~ for stratiform clouds. 
In considering these data it must be remembered 
that they are winter values and that the temperatures 
were always below freezing and usually markedly below. 
The data presented show that both the drop size and 
the liquid-water content tend to mereasé with the 
temperature. The mean drop diameter obtained from 
these in-flight measurements is significantly larger than 
the Mount Washington mean. Again, this is probably 
due to the prevalence on the mountain of cloud caps 
which are presumed to contain smaller drops as a result 
of the rapid lifting. There is no significant difference in 
the liquid-water observations in flight and on Mount 
Washington. 
Diem [10] has reported the most extensive set of 
drop-size-distribution data from the free atmosphere. 
His measurements were made from aircraft by exposing 
a small oil-covered slide to the air stream for about 
1g sec. The slides were photomicrographed within a 
minute of collection. The slides undoubtedly discrimin- 
ated against the smaller drops. Diem states that the 
collection was satisfactory down to a diameter of 3 u 
but there is reason to believe that the discrimimatory 
effect started at a somewhat larger diameter. Diem 
gives the most frequent drop diameters for six cloud 
types (Table III). Fair weather cumulus, altostratus 
