SNOW AND ITS RELATIONSHIP TO EXPERIMENTAL METEOROLOGY 
By VINCENT J. SCHAEFER 
General Electric Research Laboratory, Schenectady, New York 
Snow in its many forms has been the subject of 
observation, conjecture, and scientific discussion for 
many centuries. It has long been recognized that a 
better understanding of the formation of snow in the 
atmosphere would eventually explain some of the little- 
known but important meteorological processes related 
to the development of precipitation. 
The occurrence of supercooled clouds in the free 
atmosphere is one of the most common of meteoro- 
logical phenomena, even in many parts of the tropics. 
The importance of such clouds as the source of much 
heavy precipitation has been pointed out by Bergeron 
[2]. The differential in vapor pressure between water 
and ice at all temperatures below OC permits the 
rapid growth of snow particles at the expense of the 
liquid cloud droplets. This process is of basic im- 
portance in the formation of snow and is a primary 
mechanism in the science of experimental meteorology. 
Types of Solid Precipitation 
Over the years, many attempts [25] have been made 
to devise a classification system for describing the 
observed forms of solid precipitation. Most of these 
have been either too elaborate for easy use or have 
failed to mclude important forms. 
During the course of snowstorm studies in 1944, an 
effort was made to devise a simple system which might 
be used in the field under adverse weather conditions. 
Revisions of this system were made as extensive field 
experience demonstrated the need. In the fall of 1949, 
an effort was made to pool the experience of workers 
concerned with this problem in Switzerland, Japan, 
Canada, and the United States. The chart shown in 
Fig. 1 illustrates the classification decided upon and 
the code and types proposed for international use. 
Although subject to further revision, it is believed that 
the types shown on this chart include most of the basic 
forms which occur in the atmosphere throughout the 
world. 
As may be expected, there is an almost infinite 
variation in the forms of the basic types of this solid 
' precipitation. These differences may be so minor as to 
be visible only under high-power magnification or great 
enough to be easily seen by the unaided eye. Typical 
variations in structure and relative size of the plate- 
type crystal are illustrated in Fig. 2. 
Nakaya [16], in his ice-crystal experiments, showed 
that the crystal habit of snow may be due entirely to 
the temperature of the environment and the moisture 
supply available. It is quite likely that most crystals 
in the free atmosphere grow as they do because of these 
environmental conditions. 
It should be pointed out, however, that the habit of 
221 
erystals may also be modified by an entirely different 
mechanism—the blocking of the growth on certain 
crystal faces by the adsorption of surface-active chemi- 
cals [27]. Figure 3 illustrates the effects which may be 
mduced by traces of an impurity in the air where 
crystals grow. Further research to understand these 
effects better is under way in the General Electric 
Research Laboratory. 
GRAPHIC 
SYMBOL 
_TYPIGAL FORMS TYPE 
FTE 
y PLATES 
STELLARS © 
COLUMNS 
NEEDLES ! 
SPATIAL 
DENDRITES | 
CAPPED | 
GOLUMNS 
IRREGULAR |} 
CRYSTALS 
GRAUPEL 
SLEET | 
HAIL 
Fra. 1.—Types of solid precipitation. 
The Use of Replica Techniques for Studying Snow. In 
1941 a method was devised by the writer [18, 19] for 
making permanent replicas of snow crystals. The tech- 
nique emcases a snow or frost crystal within a thin 
plastic film which, as it forms, makes an exact three- 
dimensional impression of the surface features of the 
erystal. The replica solution, consisting of one to three 
parts of the synthetic plastic polyvinyl formal dissolved 
in 100 parts of ethylene dichloride, readily wets an ice 
surface. By capillarity and surface activity it rapidly 
covers any ice crystal which comes in contact with it. 
