V - CIRRUS AND STRATUS STUDIES 



CIRRUS CLOUDS 



The significance of cirrus clouds and the role they play in various 

 weather phenomena were, of course, subjects of intense interest to Pro- 

 ject Cirrus. Various studies of and experiments with such cloud forms 

 were conducted, although more attention was paid to stratus and cumulus 

 clouds . 



A regular daily observation program was begun in 1947 to explore 

 the possibility of inducing the development of cirrus -type clouds under 

 clear sky conditions,, It was believed that supersaturation with respect 

 to ice probably occurs fairly frequently at temperatures warmer than 

 -39°C in air devoid of foreign-particle nuclei. Lacking such nuclei, a 

 considerable degree of supersaturation could develop, as is often shown 

 by the generation of so-called vapor trails behind high-flying aircraft. 



• To explore these possibilities, Falconer initiated a project in which 

 balloons carrying dry ice in open-mesh bags were released on a daily 

 schedule and followed by theodolite. Many of these produced visible trails 

 of ice crystals, and in several instances the trails were quite notice - 

 able= (75,76,77,78) 



Several seedings were also carried out from an airplane in clear 

 air, using both dry ice and silver iodide. In clear air supersaturated 

 with respect to ice, the seeding operation produced a cloud made of ice 

 crystals. The results of these operations indicated that, if the humidity 

 is low, even at temperatures below -39°C, appreciable supersaturations 

 with respect to ice can exist without the formation of ice crystals. Ice 

 crystals can then be created, however, by seeding with either dry ice or 

 silver iodide.** 'v) 



Natural Formation . In six of the Project Cirrus test flights a con- 

 siderable effort was directed toward obtaining photographic evidence of 

 the appearance of the tops of c'irrus clouds. It was found that, despite the 

 various irregularities seen from below, the top of such clouds is extremely 

 flat. 



Most meteorologists and weather students agree that a cirrus cloud 

 formation is often associated with the overrunning of cold air by a warmer 

 tongue of moist air. Whenever the moisture conditions in the warm over- 

 riding air reach saturation with respect to water and the colder air below 

 has a temperature of -39°C or colder, ice crystals will form spontaneously 

 at the inversion interface. The number of primary crystals that form will 

 depend on the concentration of condensation nuclei and ice nuclei in the 

 moist air mass. The number and size of secondary crystals that form will 



