CLOUD SEEDING — ^HOUGHTON 179 



overseeding occurs anywhere it should be found along the seeding line. 

 There is no convincing evidence of overseeding in this region. 



It may be concluded that it is often possible to release some precipi- 

 tation from supercooled clouds by seeding them. The pertinent ques- 

 tion is how much precipitation can be released in this way which would 

 not fall from natural causes. If the answer to this in unfavorable, 

 it is still important to know whether the time of release of precipita- 

 tion and the total precipitation can be altered by seeding. 



Unless the seeded cloud is replenished, the upper limit to the pre- 

 cipitation is determined by the total liquid water contained in a verti- 

 cal column through the cloud. Measurements indicate that a liquid 

 water content of 1 g/m ^ is rather a high value. If all the water in a 

 cloud 10,000 feet deep of this water content were deposited as rain, 

 the total rainfall would be about one-eighth of an inch. This is 

 clearly an overestimate, since it is not likely that all the water could be 

 precipitated and some would evaporate before reaching the ground. 

 Most clouds would contain less water than has been assumed in this 

 example. It is concluded that, unless the cloud is continuously re- 

 plenished, the precipitation released by seeding will be very light 

 and the cloud will be partially or completely dissipated by the re- 

 moval of its water. Stratiform clouds have released practically no 

 precipitation when seeded, as would be expected in view of their small 

 total water content. 



THERMODYNAMIC EFFECTS 



It is next in order to consider the ways in which seeding might 

 stimulate the growth and replenishment of the cloud. The phase 

 transformation induced by the seeding releases the latent heat of fu- 

 sion and the latent heat of sublimation of the water vapor which 

 sublimes as a result of the lower equilibrium vapor pressure over 

 ice. The exact value of the resultant temperature rise of the air 

 depends on the initial temperature, the air density, and the liquid 

 water content, but for a typical case it is of the order of 1° C. It is 

 important to note that this heating occurs only in the supercooled 

 portion of the cloud and that the temperature rise ordinarily increases 

 with elevation. To be really effective the heating should occur in 

 the lower part of the cloud and below the cloud base. Unless there 

 is a temperature inversion of 1° C. or more, the heating will produce 

 accelerated vertical motion in the supercooled portion with a maxi- 

 mum near the cloud top. As a consequence, there will be horizontal 

 inflow above the freezing level. It is probable that a small pressure 

 fall will ensue with a correspondingly small horizontal inflow into 

 the column below the freezing level. None of these effects will induce 

 additional vertical motion below the freezing level since they do not 



