ON THE PHYSICS OF CLOUDS AND PRECIPITATION 
and stratocumulus show the sharpest distributions. The 
other three cloud types exhibit broad size distributions. 
It is interesting to note that, in general, the cloud types 
Tape III. Data rrom CompostrE Drop-Stze-DIstRIBUTION 
CURVES 
(After Diem [10]) 
Cloud pe fe ee ay ee 
Wensexcummllusie an. sss sce se ae: 14.5 3-40 
Fair-weather cumulus.............. 8.5 2-20 
NTAOCUMIWIUS 22.460. - sds ce eos os 7.9 2-24 
IND OSULAGUS! 446... . see see es 13.2 2-42 
SAEDIUD Sais obs 06 bOI Ca SE eee ee ee 12.9 2-42 
EMGOSMUPATSs SORES ae ea ee 10.6 2-30 
associated with precipitation have broad distributions. 
Very few of Diem’s size distributions would fall in 
types A and B of Table I. The difference between 
Diem’s data and the Mount Washington data in this 
respect is doubtless due mm part to the different methods 
of measurement, but the writer feels that much of the 
difference is real. As pointed out above, there is good 
reason to believe that the conditions of formation of the 
Mount Washington cloud cap favor a narrow drop- 
size distribution. Until more data from the free atmos- 
phere are available it seems reasonable to accept Diem’s 
data as typical rather than that from Mount Wash- 
ington. 
Drop-size measurements were made in sea fog by 
Houghton and Radford [22] on the northeast coast of 
the United States. The fog drops were collected on 
slides with a hydrophobic surface and then photomicro- 
graphed. Sampling errors occurred for drops smaller 
than about 20 » but this did not greatly affect the 
results in view of the relatively large drop size. The 
volume-median drop diameters ranged from 25 to 75 
m with an average of 45 yw. The drop-size distributions 
appear rather broad because of the large drop size but 
most of them correspond to the C distribution of Table 
I, while a few B and D distributions also occurred. 
The largest drop measured was 120 u in diameter. The 
mean liquid-water content was found to be 0.13 g m~* 
with a range of from 0.01 to 0.30 g m~*. 
The most striking feature of these results is the large 
size of fog drops as compared to cloud drops and the 
relatively small variation in the volume-median di- 
ameter and in the breadth of the size distribution. 
Coupled with the large drop size the relatively low 
liquid-water content shows that the drop concentration 
is very small. This suggests that the fogs observed 
formed on a few relatively large nuclei of condensation 
which quite possibly were sea-salt particles. Chemical 
analyses of the fog water tended to confirm this con- 
clusion. The chloride content of the water averaged 
70 parts per million and ranged from 8 to 480 parts 
per million. In fog water there appeared to be more 
sulfate ion in proportion to chloride than there was in 
sea water. This suggests the presence of some nuclei 
of industrial origin but does not prove that such prod- 
ucts served as nuclei. 
Hagemann [18] obtained drop-size distributions in 
173 
fog in Germany using an adaptation of the oil-covered 
slide technique. He found that the most frequent size 
ranged from 9- to 34-4 diameter with an average of 
15.6 ». As pointed out earlier, the volume-median 
diameter is always greater than the most-frequent di- 
ameter, so that Hagemann’s data do not differ greatly 
from those of Houghton and Radford [22]. Although 
data are not available it is to be expected that the drop 
size in urban fogs would be smaller than in fogs formed 
in relatively clean air. 
Although more data on the drop-size distribution in 
clouds of the free atmosphere are badly needed, the 
information at hand is sufficient to give a good general 
idea of the end results of natural condensation processes. 
The most important conclusion is that most clouds of 
the free atmosphere, especially those associated with 
precipitation, have drop-size distributions broader than 
is explicable by a uniform lifting process at the con- 
densation level. As already indicated, the most promis- 
ing explanation for such a broad distribution is non- 
uniform rates of lift at the condensation level combined 
with later turbulent mixing. 
THE ICE PHASE 
Supercooled Water. The regular existence of super- 
cooled water clouds in the atmosphere is now a matter 
of common knowledge. Water clouds are much more 
common than ice clouds at temperatures down to 
—10C and they have been observed down to —35C 
and possibly below. Dorsey [11] and others have shown 
that water in bulk may be supercooled from a few 
degrees to as much as 20 degrees. The temperature at 
which water freezes spontaneously is not known with 
certainty, but theoretical considerations suggest that 
it is in the neighborhood of —70C. Dorsey found that 
sealed samples of water had characteristic and repro- 
ducible freezing temperatures. The freezing temperature 
was found to be lower for the cleaner samples such as 
conductivity water than for natural water from ponds 
and puddles. Prolonged ageing, or heating to 97C, was 
found to lower the freezing temperature. Samples main- 
tained a few degrees below their characteristic freezing 
temperature remained unfrozen indefinitely. Dorsey 
concluded that his results were best explained by the 
assumption that freezing is initiated by “motes” of 
submicroscopie size. 
Rau [45] and Heverly [19] have investigated the 
freezing of supercooled water drops. Heverly reported 
that the freezing temperature was constant at —16C 
for drops larger than 400-1 diameter and decreased 
rapidly with the diameter for smaller drops with a 
suggestion of a minimum near —40C for very small 
drops. Rau repeatedly froze a group of 24 drops and 
found a distribution of freezing temperatures which was 
apparently independent of drop size. The first two or 
three freezings lowered the average freezing temper- 
ature which thereafter remained constant. Other un- 
published investigations have yielded results similar 
to Rau’s and it must be concluded that Heverly’s 
findings were in some way influenced by the experi- 
