Abstract
The physical hypotheses for the cloud seeding experiments carried out by Colorado State University in the Colorado Rockies (Climax I and II and Wolf Creek Pass) in the 1960's are critically examined. Airborne measurements over the Rockies have shown that the concentrations of ice particles in the natural clouds are often much greater than originally assumed; consequently, the conditions under which it might be possible to increase precipitation by artificial seeding are probably much more limited than previously supposed. There appears to be no firm evidence to support the contention that 500 mb temperatures are a good measure of cloud-top temperatures over the Rockies. Finally, examination of a more extensive data set than previously used fails to substantiate the claim that precipitation over the Rockies decreases as 500 mb temperatures increase above certain values.
