EXPERIMENTAL METEOROLOGY 309 
it is often possible to have this source of water hundreds of miles 
from the arid regions where it is to be used. 
Seeding operations in high mountainous areas have excellent 
potentialities. Since most mountains rising above 8,000 to 10,000 
feet msl are higher than the dew point level of winter air masses, 
the upper levels are commonly covered with clouds. These are 
often supercooled and, except where seeded by blowing snow near 
the surface, are deficient in ice nuclei. Silver todide generators 
placed at convenient locations are often operated with ambient 
temperatures close to or even colder than o°C. Under such con- 
ditions deactivation by the temperature effect is of little or no 
concern. 
Wintertime orographic clouds rarely have the vertical thickness 
or lifetime of summer clouds. They occur for longer periods, how- 
ever, and if effectively seeded may be forced to yield much of the 
condensed cloud water in the form of snow crystals. 
Although considerable progress has been made in the study of 
snow pack increasing (10, 12), much remains to be done. By con- 
trolling the particle size of silver iodide, it should be feasible to 
control the temperature of nucleation. The smaller the particle, 
the colder the threshold temperature of nucleation. Thus, it may 
be possible to move the lower level of the snow pack area by con- 
trolling either or both the concentration of ice crystals and the 
temperature at which they form. By producing silver iodide par- 
ticles of highly uniform size, the effectiveness and efficiency of 
smoke generators might be greatly improved in the same manner 
as with our artificial fog generators developed during World War 
II. This may be achieved by effective control of the quenching 
speed of the jet of vaporized silver iodide. By delaying the intro- 
duction of cold air into the vaporized stream, the smoke particles 
will grow much larger, but they will be less numerous than when 
the vapor is rapidly diluted. It should be feasible to form mono- 
dispersed particles over the size range of 0.004 to 1.0 micron diam- 
eter. Thus, under nearly the same operating conditions the number 
of particles may be varied by a factor of 100 million. 
This is a field of research in experimental meteorology which has 
hardly been touched but it may yield rich dividends in commercial 
operations as well as basic research. 
