METEOROLOGICAL ASPECTS OF AIRCRAFT ICING 1203 
knowledge appears to be inadequate concerns the con- 
ditions under which icescrystals form in clouds. Meteor- 
ologists have been keenly interested in this problem 
since the advent of the Bergeron-Findeisen theories of 
precipitation. Thus far, however, no adequate explana- 
tion is available for the observed facts that certain 
types of clouds usually contain ice erystals while other 
types do not, nor is there a satisfactory explanation of 
the mechanism of the formation of ice crystals in 
liquid clouds. 
The discovery by Schaefer [18] that liquid clouds 
could be converted to ice crystals by seeding with dry 
ice has resulted in investigations of the effects of cloud 
seeding by various agencies. At present there is a wide 
difference of opinion among investigators in this field, 
but there is good reason to expect that these experi- 
ments will eventually make an important contribution 
to a fuller understanding of the formation of ice crystals 
in clouds. 
The modifications of clouds that have been accom- 
plished by seeding suggest the possibility that this 
technique may provide a practicable means of reducing 
the icing hazard in limited areas, for example in the 
vicinity of busy airports. Further experiments will be 
required to establish the practical possibilities and lim- 
itations of seeding as a means of locally controlling 
icing conditions. 
The data now available on liquid-water concentra- 
tion and drop size in clouds, while sufficient to define 
in general terms the ranges of values usually encoun- 
tered, are still inadequate to determine the frequency 
and severity of icing conditions to be expected in 
various areas under the influence of various synoptic 
situations. It is expected that if and when automatic 
recording instruments come into general use, a large 
body of statistical data will be accumulated which can 
be analyzed from a synoptic-climatological standpoint 
to provide information which may be of considerable 
value to airplane operators and practicing meteorolo- 
gists. 
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