184 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



iodide particles will retain their nucleating ability. Tlie chance 

 that the nuclei will encounter the proper conditions is evidently 

 increased by permitting them to diffuse and cover considerable areas. 

 On the other hand, this makes the task of assessing the results exceed- 

 ingly difficult. There is little to gain from this technique at least 

 until more adequate information is available on the effects of seeding 

 individual clouds under carefully determined meteorological condi- 

 tions. 



LARGE-SCALE EFFECTS 



The general conclusion reached so far is that seeding may induce 

 significant amounts of precipitation only under rather special con- 

 ditions which are very close to or identical with those that lead to 

 natural precipitation. Suppose that these conditions are met and 

 the cloud or clouds are seeded. Under the most ideal conditions the 

 seeding may result in enhanced convective activity and the release 

 of precipitation. The amount of precipitation is limited by the 

 large-scale horizontal transport of water vapor into the area. This 

 can be increased only by the creation or intensification of a circulation 

 the size of a cyclone. There is no evidence that local convective ac- 

 tivity will lead to cyclogenesis. In fact, widespread convective 

 activity often breaks out in an area characterized by flat pressure 

 gradients without any subsequent effect on the pressure distribution. 

 It must be admitted that knowledge of the causes and mechanism of 

 cyclogenesis is incomplete but there is evidence that changes in the 

 upper troposphere and stratosphere are important. It is difficult 

 to imagine any influence of seeding at these heights. This is con- 

 sistent with other evidence which strongly suggests that the location 

 of cyclogenetical areas is largely determined by the large-scale hemi- 

 spheric circulation patterns rather than by local effects. If seeding 

 increases the amount of precipitation it will also result in a proportion- 

 ate increase in the latent heat released. If a substantial increase in 

 precipitation could be produced over a large area the latent energy 

 released would doubtless have an effect on the circulation pattern 

 though not necessarily in the region of increased rainfall. Natural 

 precipitation anomalies are much larger than those which might 

 conceivably be produced by seeding, but even in such cases the effect 

 of the release of latent energy on the circulation is not clear. In 

 any discussion of a net increase of rainfall over large areas it is im- 

 portant to remember that precipitation is only one segment of the 

 hydrologic cycle. An increase in rainfall requires increased evapo- 

 ration and a greater transport of water vapor. It may be concluded 

 that there is little prospect that cloud seeding will produce large- 

 scale effects in tlie atmosphere. 



