Laboratory Studies -30- 



RAINDROP STUDIES 



Although many of the details are still lacking, studies conducted by 

 Project Cirrus began to provide answers to the question of how rain is 

 formed. 



In 1947, when reports were received of successful results obtained 

 by dry-ice seeding of cumulus clouds over Hawaii having a temperature 

 above the freezing point , Langmuir restudied theoretical calculations he 

 had prepared in 1944 in studies relating to work at Mt„ Washington Ob- 

 servatory. As a result he developed a theory which agreed very well with 

 the reactions reported.^ ' ' 



According to Langmuir 's theory, actively growing cumulus clouds 

 having an average drop size of 20 microns, a liquid water content exceeding 

 2.5 G/M 3 , and a vertical thickness of more than a mile are in favorable 

 state for starting a chain reaction. This could be achieved by introducing 

 water drops greater than 50 microns in diameter into the actively growing 

 part of the cloud. 



Large drops in such a cloud would fall at a greater velocity than 

 would small drops. In falling, they would overtake and collide with the 

 small drops and thereby increase in size. In time the large drops would 

 become so large that surface tension could no longer hold them together, 

 and they would break up into two or more smaller drops. These in turn would 

 grow and break up, and the number of large drops would increase in this 

 manner by a chain reaction. 



The process would not be sufficient to produce large numbers of 

 raindrops in a cloud without a vertical updraft. However, in the case of 

 clouds with suitable updraft conditions, many stages of the chain reaction 

 are carried out, resulting in the production of rain. 



This chain-reaction theory led Langmuir to postulate that cumulus 

 clouds having sufficient updrafts could be seeded with a few large water 

 drops. 



To determine the validity of several of the important phenomena in- 

 volved in this theory, various studies were initiated in the laboratory and 

 experiments conducted in the field. Blanchard devised a splendid method 

 for studying the properties of free -falling water droplets in air, using a 

 vertical wind tunnel. A series of striking stroboscopic photographs was 

 made, showing the oscillations, gyrations, pulsations, and fractures that go 

 on as water drops fall at their terminal velocity. ^' 



