. Compendium of meteorology. Meteorology. .OS CM, Ui I —"-a ' #""-^{\. Fig. 5.—Reijlicas of snow from seeded clouds. (a) Stellar forms from cloud seeded with silver iodide. (6) Stellar crj'stal 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. (/) Stellar crystal 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
. Compendium of meteorology. Meteorology. .OS CM, Ui I —"-a ' #""-^{\. Fig. 5.—Reijlicas of snow from seeded clouds. (a) Stellar forms from cloud seeded with silver iodide. (6) Stellar crj'stal 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. (/) Stellar crystal 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. (i) 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 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 and
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