308 THE FUTURE OF ARID LANDS 
21 could be related to the silver iodide and dry ice seeding opera- 
tions. The writer flew into this storm in the early afternoon and 
observed that it consisted of a great mass of cumulus with heavy 
precipitation moving in a northeasterly direction. 
These observations present strong evidence that the convective 
clouds, which daily form over the mountains in the Rio Grande 
Valley in the summer, tend to serve as chimneys which lift the 
valley air to higher levels of the atmosphere. Silver iodide or any 
other material introduced into the valley region is carried by the 
converging air and effectively introduced into cloudy regions 
where they may affect the cloud structure, if still active as either 
condensation or ice nuclei, or both. 
Effectiveness of Silver lodide as a Nucleus for Ice Crystal For- 
mation 
A great deal has been written about the role of silver iodide as 
an ice crystal nucleus. Although much research has been conducted 
since Vonnegut’s original paper (30), silver iodide still remains 
the most effective foreign particle for ice crystal nucleation. Many 
different factors have been shown (4, IT, 21, 31) to affect its poten- 
tial activity. It has such complex crystal habits, however, that 
much caution is required in concluding that all or any of these 
factors cause irreversible effects. Recent studies by the writer 
show that silver iodide is most effective if it first serves as a con- 
densation nucleus in the liquid water zone of a cloud (26, 29). If 
this occurs, the particles serve as freezing nuclei at a threshold 
temperature between —4° and —s°C. If the silver iodide particle 
enters a liquid water droplet, there is the possibility that the effects 
of sunlight, low humidity, temperature, certain impurities, and 
other factors which have been cited for their deactivation effects 
may be nullified. The active program of laboratory study and 
field evaluation currently underway in many parts of the world 
may be expected to shed much light on the properties of this im- 
portant cloud seeding material within the next few years. 
Use of Silver lodide for Increase of Snow Pack 
One of the most important aspects of water supply in regions 
of submarginal rainfall is the snow pack of the high mountains. 
With improved methods of moving water by pipe line and canal, 
