224 
solid precipitation types are a product of the variations 
in the physical nature of their environment. Let us 
consider the factors which initiate the formation of 
such particles in the atmosphere. At least five different 
mechanisms are of importance. 
1. Freezing Nuclei. It has been shown experimentally 
[85] that bulk water may be cooled to at least —38.5C. 
Under carefully controlled laboratory conditions, which 
avoid seeding of the water by contact with frosted 
surfaces, water nearly always supercools to at least 
—5C to —10C [4]. In streams and ponds, however, there 
are always some freezing nuclei which initiate freezing 
in water at a temperature of only a few hundredths of 
a degree below 0C. However, the concentration of such 
particles seems to be relatively low, rarely exceeding 
1 X 10° m™*. The thin dises which form on such nuclei 
in flowing water are responsible for the formation of 
frazil ice [33]. 
Little evidence has been found in our studies to 
suggest that freezing nuclei are important in the forma- 
tion of snow crystals in the atmosphere. The centers of 
snow crystals rarely show them to be formed on a 
frozen cloud droplet except when the crystals have 
apparently formed at very low temperatures. The size 
of cloud droplets, which on the average range between 
8 and 25 yw, makes such observations quite feasible. 
However, the failure to find frozen droplets in the 
centers of snow crystals may be explamed by some 
recent studies in my laboratory which show that cloud 
droplets condensed on silver iodide particles in warm 
air tend to assume the form of perfect hexagonal plates 
and columns as they freeze at a temperature of —4C. 
2. Sublimation Nuclei. The most important mecha- 
nism responsible for the znztzation of snow-crystal forma- 
tion in the lower portion of the free atmosphere involves 
sublimation nuclei. These are very small air-borne parti- 
cles upon which ice forms by the condensation of water 
molecules directly from the vapor phase. 
Despite the claims of some workers [6, 40] who believe 
sublimation nuclei are extremely rare, experiments show 
that small particles of silicates and minerals common to 
desert and volcanic sources readily serve as active 
centers for snow-crystal formation. Figure 6 illustrates 
the temperature dependence of the effectiveness of 
different types of such particles when they are placed 
as an aerosol im cold air having sufficient moisture to 
be supersaturated with respect to ice. 
Sublimation nuclei should not be confused with con- 
densation nuclei. The latter are foreign particles, such 
as those produced by the evaporation of sea spray 
[9, 41], the burning of combustible waste, the par- 
ticulate effluent from industrial processes, and the 
smoke of forest fires. Water molecules condense on 
such particles to form liquid droplets. Concentrations 
of condensation nuclei range from 2 X 10° salt particles 
per cubic meter in the air over the sea to concentrations 
of 1 X 10” particles per cubic meter in the vicinity of 
‘industrial regions [11]. While the relative concentration 
of condensation nuclei is important when related to 
the stability and persistence of liquid-water clouds, 
since it is an important factor m determining the 
CLOUD PHYSICS 
particle size, no evidence is known which relates the 
ordinary condensation nuclei to the formation of ice 
crystals. 
A three-year study of the concentration of sublima- 
tion nuclei in air passing over the summit of Mount 
TEMPERATURE (°C) 
-20 -10 (0) 
OTE: ACTIVITY DUE TO TEMPERATURE OF -78C; ANY— 
HING COLDER THAN -39C PRODUCES SIMILAR EFFECT 
ae | 
————s 
=—— 
—— 
-40 -30 
GARBON DIOXIDE 
SILVER [ODIDE 
LOAM — RUGBY, N. DAK. 
CLAY -GUILDERLAND , N.Y. S—= 
LOESS - HANFORD, WASH. 
LOAM -BRUEL,WIS. 
= 
wee 
SOIL- WOLF, PT- MONT. __| i 
SOIL- BAGGS, WYO. = 
SAND-AGATE,GOLO. _| — 
LOAM-GCOEUR DALENE,!IDAHO 
ASH-GRATER LAKE, OREG. 
KYANITE Ac 
ASH, PARIGUTIN, MEX, = 
DUST-PHOENIX , ARIZ. 
~ MARL= RAVENA,N.Y. 
BENTONITE — MONT. 
SOlL- NEV) = 
ee 
DIATOMS >= 
SPORES 
LOAM=— OAKLEY, KANS. — 
KAOLIN-GA. SS THRESHOLD OF 
ACTIVITY 
= 
ie SS ae — 
[eae ae COMPLETE ACTIVITY 
Fic. 6.—The temperature dependence of foreign-particle 
ice nuclei. 
Washington (elevation 6288 ft) using the cold-chamber 
method [29] showed that the variation in concentration 
of such nuclei ranged from none observable to ten mil- 
lion per cubic meter [380]. Table I illustrates the range 
in concentration observed in the 8137 observations 
TasBLE J. RaNGm IN CONCENTRATION OF Ick NUCLEI AT 
Mount WASHINGTON 
JANUARY 1, 1948-January 1, 1950 
Number of 
observations 
Approximate number of nuclei 
per cubic meter of air 
1 X 10°5 X 10? 4062 
5 X 107-5 X 10° 3388 
5 X 105-1 X 107 687 
which were made up to January 1, 1951. Such studies 
are continuing at Mount Washington and are being 
supplemented by similar observations at Socorro, New 
Mexico. A study of the synoptic situation existing when 
the levels of nuclei were unusually high suggests that 
they might be dust particles from the Northwest and 
