222 
The solvent evaporates in five to ten minutes, after 
which the slide (glass, cardboard, etc.) bearmg the 
samples may be warmed above freezing. Upon melting, 
the water molecules evaporate through the thin film, 
leaving a hollow shell which refracts and scatters light 
Fig. 2.—Variation in size and structure of hexagonal plates 
(a) from low and middle type clouds, (6) from high cirrus type 
clouds, and (c) from very low altitudes, forming in clear air. 
in a manner quite similar to the optical properties of 
the original crystals. Figure 4 shows a typical replica 
of a stellar crystal. 
This technique is also very useful in making surveys 
of snowstorms since it permits the accumulation during 
the course of a storm of many samples of crystals for 
water. (b) Effect of 1 molecule of acetone to 100 molecules of 
water. (c) Effect of 1 molecule of acetone to 1000 molecules of 
water. 
subsequent study. Samples have now been obtained by 
this method in most parts of the world as well as at 
high altitudes in the atmosphere during flight studies 
with Project Cirrus airplanes. Figure 5 shows some of 
these replicas. 
A more recent technique for making replicas utilizes 
a plastic spray.! Although the solvent used in this 
1. Such as Krylon, made by Foster & Kester, Philadelphia, 
or Plastic Spray, made by the Bridgeport Brass Co., Bridge- 
port, Conn 
CLOUD PHYSICS 
spray would not work under normal conditions, since 
it would tend to dissolve the ice structure, its evapora- 
tion is so rapid that satisfactory replicas are obtainable 
if brief applications are made. This technique works 
best if the spray contamer is cooled below 0C. How- 
ever, if the spraying is carried out in air at temperatures 
below freezing, enough entrainment of cold air takes 
place so that good replicas have been made at —10C 
with the dispenser temperature at 25C. 
Solid Precipitation in the Free Atmosphere 
Precipitation in the form of snow crystals, graupel, 
sleet, or hail forms under varied conditions of tempera- 
ture, humidity, and turbulence, and in the presence of 
a variety of suitable nuclei to be described later. The 
Fig. 4.—Typical replica of a stellar snow crystal. 
moisture content of the air m which such precipitation 
may form at temperatures below OC range from more 
than 3 g m~ in supercooled water-droplet clouds to such 
small amounts that the air contains no visible cloud, 
although it is supersaturated with respect to ice. 
Ice Crystals in Cirrus Clouds. The highest clouds 
commonly found throughout the world are the cirrus 
types. Evidence is accumulating suggesting that most 
clouds of this type form at temperatures below —39C. 
At this temperature, spontaneous nucleation occurs, 
that is, foreign particles are not required to initiate the 
formation of ice crystals. 
The simple 22° halo and the more complex optical 
phenomena of cirrus clouds are generally produced by 
snow crystals of special form floating with a particular 
orientation in the air. Since the initial formation of such 
crystals may take place in clear air having a very low 
total water content, it is obvious that they must be 
very small and their growth quite slow. The crystal 
types common to these high-level clouds are the hex- 
