UTILIZING WATER RESOURCES OF AIR 315 
pends upon the water content of the air. It would therefore seem 
reasonable to determine for various regions the available supply 
of moisture, primarily in the form of clouds, since they represent 
the most likely source material for rain. The frequency of occur- 
rence of clouds of various types and thickness, water content, etc., 
could be determined and estimates made of possible precipitation 
increase providing the moisture could be made to precipitate out. 
“Census” counts of clouds precipitating naturally could be made. 
In some areas, studies like these have been started and the World 
Meteorological Organization Technical Report No. 1 ‘Artificial 
Inducement of Precipitation with special reference to the arid 
and semi-arid regions of the world” (12) describes a preliminary 
survey reviewing the general atmospheric conditions necessary 
for cloud seeding operations and the likelihood of finding such 
conditions in the arid and semi-arid areas. However, in many areas 
the basic meteorological data are inadequate. To fill these gaps 
it will be necessary to find economical ways of extending the ob- 
servational network to obtain the information that will have 
direct bearing on the particular questions to be answered. 
Distribution of Condensation and Freezing Nuclei in Time and 
Space 
In addition to the usual meteorological observations, there is a 
need for more data regarding the nature and distribution of the 
particles in the atmosphere on which water droplets and ice crys- 
tals form. Studies by the previous speakers and others indicate 
there are large fluctuations in the number of these particles from 
time to time and from one location to another. Aside from the 
problems of instrumental errors, important sampling questions 
arise here regarding the representativeness of the measurements. 
How large a sample is necessary to represent adequately the aver- 
age concentration over an area? These and other questions might 
be answered by sampling experiments and statistical theory. 
Regarding the nature and source of these particles, a Japanese 
investigator (3) identified the central nucleus of natural ice crys- 
tals as belonging to the clay mineral group in about ten out of 
fifteen individual cases studied. Is this typical, or would another 
sample taken at a different time or place show something else? 
