228 
persistence, or dissipation of such clouds is governed 
chiefly by evaporation and nucleation. 
Evaporation may affect the life cycle of a supercooled 
cloud in two ways: (1) the cloud droplets may evapo- 
rate into air which is unsaturated with respect to water, 
or (2) they may evaporate and the water molecules 
condense on an ice nucleus. Hither process is a very 
rapid one and for most purposes may be considered to 
be imstantaneous. Nucleation under special conditions 
may cause a direct freezing of the supercooled cloud 
droplets when the concentration of ice nuclei is very 
high. It is unlikely that this latter process occurs very 
often in the atmosphere except at the spontaneous 
nucleation temperature of —39C. The tops of large 
cumulus clouds and the imitial stages of some cirrus 
clouds show abundant evidence of this process. It is 
CLOUD PHYSICS 
scattered snow particles. Initial growth develops from 
the vapor phase, but as soon as the crystals become 
sufficiently large to move faster than the surrounding 
supercooled cloud droplets, they sweep up cloud drop- 
lets which freeze as they touch the snow crystal. Grau- 
pel, hail, some forms of ice needles, and all other rimed 
crystals depend on supercooled cloud droplets for their 
formation. The presence of such crystals in the atmos- 
phere is evidence of a low concentration of ice nuclei in 
the air mass and the absence of a chain reaction. 
Most cumulus clouds supercool above the OC iso- 
therm and persist for unpredictable periods. If the 
moist air forming them carries with it certain types of 
air-borne dust which have the proper structure to serve 
as potential ice nuclei, they may rise only three or four 
thousand feet above the freezing level before shifting 
4 
Fig. 12—Typical air-to-ground electric currents which often occur during convective type snow storms. 
probably due to this mechanism that the optical proper- 
ties of clouds at the —39C level remain unchanged. 
The freezing of cloud droplets does not alter the shape 
of the particle, and although high concentrations of 
small ice crystals probably form nearby they imme- 
diately disappear by evaporation since their vapor 
pressure is higher than that of the larger frozen cloud 
droplets. It is very likely that compound crystals like 
those in Fig. 10 form on cloud droplets frozen by this 
process. 
When supercooled clouds are warmer than —39C 
and the air containing them is moist enough to prevent 
evaporation, several different processes control their 
persistence. If the cloud is of a stratiform type with 
few or no ice nuclei present, a freezing drizzle may 
develop and fall from the base of the cloud. If a few 
ice nuclei are present in the cloud, snow crystals form 
on them, grow rapidly, and fall out of the cloud as 
over to snow. At times, however, they may go nearly or 
all the way to the —39C level without enough snow 
crystals forming to initiate a chain reaction effect. It 
is this type of cloud which may develop into a storm 
producing lightning or hail, or both. At other times, 
the cloud may be completely dissipated imto false cirrus 
at high altitudes without producing any precipitation. 
Orographic clouds also have a strong tendency to 
become supercooled. Such places as the summit of 
Mount Washington have extensive icing storms during 
much of the year. Because of the very large vertical 
component of the wind caused by the mountain barrier, 
coexistence of supercooled cloud droplets and ice 
crystals is more likely than in most clouds in the free 
air, since condensation occurs faster than the evapora- 
tion-diffusion process can transport the moisture to the 
ice crystals. It is likely that orographie clouds in which 
