Cumulus Studies -41- 



'that rain had fallen from the lower part, while the top 

 of the cloud had detached itself and floated off towards 

 the northeast. 



"Shortly after seeding this cloud with 10 to 12 pellets, 

 we picked out a smaller cloud nearby whose top reached 

 about 20,000 feet and dropped one single pellet of dry ice 

 one inch cubed on this cloud. About 8 or 10 minutes later 

 we found that this whole cloud had changed to ice crystals. 

 We flew through the ice crystal cloud and verified the fact 

 that they were entirely ice crystals. You could see them 

 blowing into the cabin, and we also found that the cloud grad- 

 ually dissipated. It probably rained out from the lower part 

 of the cloud but this was down in the smoke level where we 

 could not see it, and the top of the cloud then gradually mixed 

 with the surrounding dry air which had been deprived of its 

 source of supply of moisture from below. 



"In other words, on this day we had beautiful examples of 

 two effects that can be produced by seeding with pellets of 

 dry ice. First the seeding of the top of the cloud can cause 

 the top to float off from the lower part. However, in this 

 case some of the ice crystals reach the lower part of the cloud 

 and cause rain to dissipate it. In the other seeded cloud, 

 which was much lower and reached only a few thousand feet 

 above the freezing level, the whole cloud rapidly dissipated 

 as the upper part changed to ice and the lower part rained out." 



The results of the flight of April 18 constituted for Langmuir a won- 

 derful demonstration of the effectiveness of single pellets of dry. ice for 

 modifying large cumulus clouds. Such single -pellet seeding had a number 

 of practical advantages. 



It quickly became obvious to Langmuir that the set-up for carrying 

 out cloud-seeding experiments in Honduras was unique. Silverthorne made 

 flights virtually every day, and, somewhere within a 150-mile range, clouds 

 were nearly always found suitable for seeding. Such clouds were almost 

 always orographic and associated with certain mountains. 



Many interesting experiments were conducted, and almost always 

 the clouds could be profoundly modified with single pellets of dry ice. 

 The latter part of Silverthorne's seeding operations used 10-20 pellets, 

 presumably to make sure the crystals were more uniformly distributed. 



