Cirrus and Stratus Studies -36- 



probably be some multiple of the effective number of condensation nuclei. 

 Since these conditions for the ice -crystal formation are of a marginal na- 

 ture, the variability and often unique appearance of true and false cirrus 

 clouds may be closely related to these spontaneous crystal formation 

 phenomena. 



Eased on this reasoning, Schaefer concluded that it is likely that the 

 concentration of supercooled water droplets at the transition temperature 

 of -39°C is of primary importance in the formation of cirrus crystals. ^^' 



Langmuir, analyzing the behavior of cumulus clouds, described an 

 action which he called cirrus -pumping. This occurs when, with few or no 

 nuclei present, the cloud rises to great heights. If it rises to a height when 

 the temperature gets down to -39°C or thereabouts, minute ice crystals are 

 formed in great numbers, almost instantaneously. These come into contact 

 with the supercooled water droplets in the cloud and immediately cause them 

 to freeze. This, in turn, liberates a large amount of heat simultaneously over 

 the whole top of the cloud, and this upper part rises still further, forming a 

 cirrus crown shaped something like a pancake. 



The pancake grows in dimension and gets thinner, and it sometimes drifts 

 gradually off to one side, so that it assumes the general appearance of an 

 anvil- -a type of cloud characteristic of the tropics. One large cloud of this 

 type, said Langmuir, might sometimes produce cirrus clouds which would 

 spread over 10,000 square miles. Outside of the tropics, they may often occur 

 during the summer in semi-arid regions such as New Mexico, Arizona, or 

 Idaho y 8[ 



Height, Temperature, etc . Some observations were made by the project 

 of the height of cirrus clouds and their temperatures. 



STRATUS CLOUDS 



Much more attention was paid to stratus clouds. The flight test of 

 December 20, 1946, for example, was conducted when the sky was com- 

 pletely overcast, and it produced snow.^) j n the flight test of March 6, 

 1947, now under the auspices of Project Cirrus, seeding was conducted 

 on stratus clouds. Looking down on the cloud, it was observed, first, that 

 a deep groove had been produced along the top of the seeded area, and snow 

 fell. Soon the sky cleared up in a spectacular fashion, so that there was a 

 cloudless area 20 miles long and 5 miles wide where the seeding had taken 

 place, although there were no other breaks in the overcast in any direc- 

 tion/ ' Further tests on stratus clouds produced similar results. 



