ON THE PHYSICS OF CLOUDS AND PRECIPITATION 
No ice formed on soluble nuclei even at low temper- 
atures; it appeared that solid nuclei were required. 
Weickmann and others [55, 31, 32] also considered 
the problem from the theoretical side and showed that 
the structure of the ice crystal is so unique that it is 
extremely unlikely that any substances exist in the 
atmosphere which are truly isomorphic with ice. Weick- 
man concluded that the atmospheric ice phase is formed 
by the freezing of the liquid on solid freezing nuclei. 
He felt that the freezing nuclei were often condensation 
nuclei with a microporous or fissured surface which pro- 
moted condensation. He conceded that sodium chloride 
nuclei might act as freezing nuclei at temperatures 
below —35C. Although he observed a few nuclei on 
which ice formed below water-saturation, he preferred 
to call all of the nuclei freezing nuclei rather than 
sublimation nuclez. 
There are still many unanswered questions regarding 
the formation of ice crystals in the atmosphere, but 
some tentative conclusions can be formed. It seems 
clear that at all temperatures down to about —40C 
liquid condensate is more common than ice. Almost all 
investigators found a critical or transition temperature 
near —40C although mixed water-ice clouds have been 
reported both in the free atmosphere and in the labora- 
tory down to at least —50C. The latter measurements 
may be in error and should be checked. It should not be 
concluded without further information that —40C is a 
spontaneous freezing temperature. Since all drops or 
erystals probably form on some type of nucleus this 
may be the temperature at which the small soluble 
condensation nuclei act as freezing nuclei. The com- 
puted spontaneous freezing temperature of —70C is 
based on physical constants, the values of which are not 
accurately known. 
It is probably true that there are no atmospheric 
nuclei on which sublimation occurs at or below ice- 
saturation. It has been concluded from this that there 
are no true sublimation nuclei in the atmosphere except 
ice itself. On the other hand, ice crystals have been 
formed below water-saturation, although at consider- 
able supersaturations with respect to ice. In the opinion 
of the writer it is not proper to require that a subli- 
mation nucleus be active at ice-saturation. Many solid 
condensation nuclei do not become active until a con- 
siderable water-supersaturation is attained but they 
are still classed as condensation nuclei. Until more in- 
formation is available it would seem preferable to 
eall all nuclei on which ice forms below water-saturation 
sublimation nuclei. It is conceded that the deposition 
of the first few molecular layers on such a nucleus may 
not be in the form of ice, but little is known about this. 
There is certainly a clear physical difference between 
ice crystals formed in this way and those which are 
formed by the freezing of a liquid drop which has 
already attained cloud drop size. In the latter case it is 
evident that a freezing nucleus is involved. 
On the basis of the definitions given above, it appears 
that a few sublimation nuclei which are active at 
temperatures as high as say —10C exist in the atmos- 
phere. The failure to find these nuclei in the small ex- 
175 
pansion chambers may be attributed to their low con- 
centration. Such low concentrations are adequate and 
even requisite for the release of precipitation by the 
ice-crystal mechanism. The evidence is that the large 
number of ice crystals found in surface air at —32C and 
in cleaned air at —41C are formed on freezing nuclei 
rather than on sublimation nuclei. There may also be 
freezing nuclei in the atmosphere which are active at 
much higher temperatures than —32C. In some cases 
these may be solid condensation nuclei, or they may be 
picked up by collision after the drops are formed. 
PRECIPITATION PROCESSES 
It has been realized for some time that precipitation 
elements cannot be formed by a continuation of the 
processes of cloudy condensation, but that other physi- 
cal processes are necessary. In general, cloudy condensa- 
tion leads to the formation of a high concentration of 
small particles. The precipitation process must convert 
this multitude of small particles into a smaller number 
of much larger elements. Since the mass of a raindrop 
of 1-mm diameter is one million times that of a cloud 
drop of 10-u diameter, any proposed precipitation mech- 
anism must be capable of causing the rapid combination 
of large numbers of cloud elements. 
In a classic paper, Bergeron [4] reviewed the possible 
precipitation mechanisms and concluded that the only 
one of importance was the colloidal instability of a 
mixed water-ice cloud at: temperatures below OC. As is 
well known, the vapor pressure over water is greater 
than that over ice, at temperatures below freezing. 
The introduction of a few ice crystals into a super- 
cooled water cloud will result in the relatively rapid 
growth of the ice crystals at the expense of the super- 
cooled waterdrops. This idea was expanded and ex- 
tended by others, particularly by Findeisen [13]. Vari- 
ous theories were offered to explain the appearance of 
the necessary ice crystals in the supercooled cloud. 
Findeisen’s proposal of sublimation nuclei was once 
accepted as best explaining the observed phenomena 
but is now questioned for the reasons already discussed. 
The substitution of freezing nuclei would not alter 
Findeisen’s theory in any important respect. 
The ice-crystal theory of precipitation was widely 
accepted, since it seemed to be in accord with observa- 
tional evidence. The proponents of the theory cate- 
gorically stated that all moderate-to-heavy precipita- 
tion was initiated in this fashion and that, at most, only 
drizzle-type precipitation could fall from clouds which 
did not contain ice crystals. This conclusion was based 
largely on the observations that the precipitating clouds 
of middle latitudes extend above the freezing level and 
that much of the precipitation reaching the ground as 
rain is melted snow. The most common and apparently 
conclusive evidence is the glaciation of cumulonimbus 
prior to the release of precipitation. 
It has now been established that many low-latitude 
clouds which release moderate to heavy precipitation 
are entirely below the freezing level. This shows that 
there is another mechanism for the release of precipita- 
tion but does not invalidate the ice-crystal theory of 
