SNOW AND EXPERIMENTAL METEOROLOGY 
agonal plate and column and the irregular or asym- 
metric crystal. 
The ‘false cirrus” streaming from the tops of very 
high cumulonimbus clouds, the condensation trails of 
Fig. 5.—Replicas of snow crystals from seeded clouds. 
(a) Stellar forms from cloud seeded with silver iodide. (6) 
Stellar crystal from overseeded cloud after 51 min. (c) Plate 
formed in supersaturated clear air after seeding with dry ice. 
(d) Plate formed in 6 min after dry-ice seeding. (e) Plate 
formed in 10 min after dry-ice seeding. (f) Stellar erystal in 
overseeded area 45 min after dry-ice seeding. (g) Hexagonal 
column found in cirrus cloud at 27,000 ft. (h) Hexagonal 
column found in cirrus cloud at 28,000 ft. (7) Plates found in 
stratus cloud 15 sec after dry-ice seeding. 
ice crystals from airplanes, and the ice fogs occurring 
in polar regions have features similar to cirrus clouds. 
Their formation is due, apparently, to the fact that the 
air in which they form is colder than —39C. In the 
223 
case of condensation trails, this temperature occurs 
momentarily in the vortices streaming from the wings 
and propeller tips. When supercooled cloud droplets 
are present, they become frozen when contacted by the 
innumerable crystals spontaneously generated in their 
vicinity. 
A snowstorm developing from cirrus clouds often 
requires from two to six hours for the crystals to 
reach the ground. A thin ice crystal haze producing a 
22° halo thickens until the sun or moon finally appears 
as though covered by a ground-glass screen. Before 
reaching the ground, cirrus-type crystals may fall into 
lower supercooled clouds and become coated with cloud 
droplets. They sometimes serve as crystallization 
centers for the formation of stellars, spatial dendrites, 
or needles. : 
Solid Precipitation in Stratus Clouds. When air is 
stabilized by the presence of an inversion, the clouds 
which form contain considerably smaller quantities of 
liquid water than do cumulus clouds. Supercooled 
stratus clouds rarely have a liquid-water content higher 
than 1 g m~, while the average content ranges between 
0.2 and 0.4 g m-*. Ground fogs may be considered as 
a special type of stratus cloud, since they have similar 
properties. ‘ 
Depending on the properties of supercooled clouds 
previously mentioned, the precipitation types from 
stratus clouds may vary from capped columns and 
stellar crystals to sleet. A deficiency of ice nuclei in 
stratus clouds is likely to produce a fairly heavy coating 
of rime on the crystals. When a layer of warmer air 
overruns a colder stratum, light rain may form in these 
upper clouds, becoming supercooled as it falls into the 
lower clouds and reaching the earth as rain or sleet. As 
sleet forms, an icy shell first coats the raindrop and 
freezing proceeds toward the center. As the last of the 
water freezes, the expansion causes the formation of 
many strange protuberances which modify the sym- 
metry of the icy sphere. 
Precipitation in Solid Form from Cumulus Clouds. 
High values of liquid water occur only in cumulus or 
thick orographic type clouds. Highest values of con- 
densed cloud water occur when the vertical thickness 
of the cumulus exceeds 10,000 ft and the base of the 
clouds is in warm air, that is, warmer than 0C. Moist 
air forced upward by encounter with a mountain barrier 
and generally aided by convection induced by insolation 
often produces orographic clouds with features similar 
to cumulus. Such clouds may cover the upper slopes 
of a mountain or may have their bases considerably 
higher than the summit. 
Many precipitation types form within the cold part 
of cumulus clouds. These include stellars, needles, 
spatial dendrites, and irregular crystals in addition to 
graupel and hail. The erystal forms may be expected 
when suitable ice nuclei occur in abundance, while 
graupel and hail types are products of a deficiency of 
such nuclei. 
Factors Controlling the Formation of Snow 
As suggested in previous sections, and demonstrated 
in the laboratory by the classic work of Nakaya [16], 
