ON THE PHYSICS OF CLOUDS AND PRECIPITATION 
that the practical maximum diameter reached by con- 
densation is about 100 to 200 un. 
OBSERVATIONS OF DROP SIZE 
AND LIQUID WATER 
Methods of Measurement. Although a complete dis- 
cussion of methods of measuring cloud drop size and 
liquid-water content is beyond the scope of this article, 
some understanding of the measuring techniques is 
essential to a proper evaluation of the data. The most 
frequently used method of measuring cloud drop size 
has been the corona method. The angular diameter of 
the first- and higher-order diffraction rings observed 
around a light source is a measure of the average drop 
size. The method is theoretically unsound for drop di- 
ameters less than about 10 u. Although the possibility 
of determining the drop-size distribution by the corona 
method exists, no satisfactory technique has been de- 
veloped. The method is most applicable to the meas- 
urement of the drop size in homogeneous clouds; the 
coronas become diffuse and difficult to measure when 
the drop-size distribution is broad. 
Mean drop size and some indication of drop-size 
distribution can be obtained from the rotating multi- 
cylinder. This instrument consists of a series (often 
five) of cylinders of different diameter arranged to be 
slowly rotated with the axes of the cylinders normal to 
the wind. The collection efficiency of a cylinder is 
dependent on the air speed, the cylinder diameter, 
and the drop diameter. The numerical relations between 
these quantities and the collection efficiency have been 
determined by Langmuir and Blodgett [35]. From a 
comparison of the relative collections of the several 
cylinders the mean drop size and a measure of the 
drop-size distribution may be obtained. The liquid- 
water content (mass of liquid water in a unit volume of 
air) also may be determined from the measurements. 
This technique has been used at Mount Washington, 
New Hampshire (where it was developed by Arenberg), 
and on aircraft. It was first used only in supercooled 
clouds where the deposit is in the form of rime but 
absorbent cylinders are now used at temperatures above 
freezing. In order to obtain measurable collections the 
cylinders must be exposed to a few miles of cloud. For 
this reason the method yields only average values and 
cannot indicate rapid changes in drop size. In order 
to obtain a drop-size distribution it is necessary to 
assume the general form of the distribution curve. 
The most direct means for the measurement of drop 
size and drop-size distribution is the collection and 
photomicrography of a sample of the drops. Direct 
photography of the drops in the air has so many in- 
herent difficulties that it cannot be considered to be a 
practicable method. The usual technique is to expose a 
suitably coated slide to the windstream and take photo- 
micrographs of the collected drops. Each drop image 
must be measured individually, and from one hundred 
to several hundred drops must be so measured to secure 
a representative distribution curve. The slide surface is 
covered with a hydrophobic surface, with an oil layer, 
or it is smoked. If the oil film is used, the drops are 
171 
immersed thus retarding evaporation and preserving 
their spherical shape. On the smoked slides the drops 
leave clear areas which are related to the drop size. 
The drops on the hydrophobic surface are semiflattened 
and are subject to evaporation. Because of the finite 
size of the slides there is discrimination against the 
smaller drops. In addition, large drops tend to fracture 
on impact at high air speeds. In spite of these difficulties 
this general method is the only one which permits the 
nearly instantaneous determination of the drop-size 
distribution. 
It is clear, even from this brief discussion, that none 
of the present techniques is entirely satisfactory. It is 
not believed that improvements in any of the existing 
techniques will remedy this situation. A new approach 
to the measurement of drop size and size distribution is 
badly needed. 
Drop-Size Measurements. Kohler [30] has reported 
on the results of a large number of corona measurements 
of drop size made at a mountain observatory in 
northern Norway. Similar measurements have been 
made by others but KGéhler’s work may be taken as 
representative. Kohler made several thousand indi- 
vidual measurements in mountain fog, stratus, strato- 
cumulus, and altocumulus. The absolute range of mean 
drop diameter was about 5 to 70 u. The most frequent 
diameter was found to be about 17.6 u. He found that 
the range of the most frequent diameter was smaller for 
altocumulus and stratocumulus than for fog and stratus. 
Kohler’s measurements yield no information on the 
size distribution, although he notes that some of the 
coronas were sharper than others; the sharper coronas 
corresponded to the narrower size distributions. 
Kohler has claimed that his data show what he calls 
a “mass grouping” such that the sizes in a group 
represented by d = d)(2)"/, n= +1, 2, 3, etc., are 
predomimant. Here dp is the constant modal diameter of 
the group and d represents the diameters of the drops 
composing the group. Other investigators have reported 
similar groupings. If this result were accepted, it would 
imply that there is a rather fixed drop size resulting 
from condensation and that other sizes are formed by 
the combination of drops of this initial size. This does 
not seem reasonable on physical grounds. No such 
grouping has been found in the multicylinder or micro- 
scopic data. It seems that Kohler’s results must be 
due to some peculiarity of the corona technique and it is 
no longer believed that such a mass grouping exists. 
A large amount of data on drop size, size distribution, 
and liquid-water content in clouds has been collected 
at Mount Washington, New Hampshire [51]. All three 
of the methods described above have been utilized but 
the bulk of the data has come from the rotating multi- 
cylinder. The mean drop diameter was found to be 13 
u, which is somewhat smaller than Kohler’s 17.6 wu. 
The observed range of median diameter was about 5 
to 40 u. It should be noted that the multicylinder mean 
diameter is a volume median such that one-half of the 
water is in smaller drops and one-half is in larger drops. 
The diameter obtained from the corona method prob- 
ably corresponds to the most frequent size. With the 
