CLOUD SEEDING — HOUGHTON 177 



due to poor observing conditions or to tlie choice of a cloud which 

 either was not sufficiently supercooled or was already composed of ice 

 crystals. Nucleation of stratiform clouds often leads to the produc- 

 tion of "valleys" or "canyons" and occasionally to the formation of 

 holes through which the ground may be seen. Holes seem to occur 

 most often when the cloud deck is relatively thin. The effect of the 

 seeding is observed to progress laterally from the seeding line, often 

 reaching a width of 1 to 2 miles in 30 to 50 minutes. Precipitation 

 in the form of virga is a common observation, but only rarely does 

 any precipitation reach the surface from stratiform clouds. 



The observational evidence on the effects of seeding cumulus clouds 

 is extremely varied. Again there is evidence that seeding converts the 

 supercooled portions to ice crystals, but observation is much more 

 difficult. It appears that some cumulus clouds dissipate when seeded, 

 some are little affected, and, occasionally, accelerated vertical develop- 

 ment results. Reports on precipitation range from none to very heavy. 

 It is extremely difficult to say whether the precipitation results from 

 seeding or from natural processes independent of the seeding. Obser- 

 vations made in a few cases of the elapsed time between seeding and the 

 appearance of the precipitation are reasonably consistent, suggesting 

 a cause-and-effect relationship. The data suggest that small cumulus 

 clouds are usually partially or completely dissipated by seeding. It 

 appears that the seeding of more active cumulus clouds often results in 

 virga or light precipitation and, vely occasionally, in heavier pre- 

 cipitation. Claims have been made that the seeding of large cumulus 

 clouds under proper conditions may lead to the development of wide- 

 spread precipitation. Very few cases of this type have been reported. 



Langmuir (4) has suggested that "warm" clouds, in which the ice 

 nuclei can have no effect, may be induced to precipitate by seeding 

 them with water. The injected water drops grow as they fall by 

 colliding with the cloud drops. If the cloud is deep and wet with 

 strong updraf ts the drops may grow to such size that they will rupture. 

 The broken portions might then be carried up in the updraft so that 

 the process would become self-sustaining. The conditions requisite 

 for the operation of this process are likely to be those that lead to 

 natural precipitation. A few tests of water seeding have been made 

 but results are still inconclusive. There is little question that injected 

 water drops will grow by collision and fall out as rain but there is 

 reason to doubt that the continuing process visualized by Langmuir 

 can ojjerate except in clouds that are ready to release precipitation 

 from natural causes. Because of the lack of adequate tests of this 

 method and the more widespread interest in seeding with ice nuclei, 

 no further discussion of the water-seeding process will be undertaken 

 here. 



