CLOUD SEEDING — HOUGHTON 183 



not it is possible to increase the rainfall from a precipitating cloud 

 by seeding it. Precipitation requires vertical motion and the conse- 

 quent condensation of water vapor followed by a process which con- 

 verts the condensed water vapor into larger elements which can fall to 

 to the ground. A "precipitation efficiency" might be defined as the 

 ratio of the rainfall to the mass of water vapor in the rising current. 

 An efficiency of 100 percent could be achieved only if all the water 

 vapor were condensed and all the condensate were converted to pre- 

 cipitation. The "precipitation efficiency" can be considered as the 

 product of a "condensation efficiency" and the efficiency with which 

 the condensate is converted to precipitation elements. The condensa- 

 tion efficiency is determined primarily by how far the water vapor 

 is lifted. The efficiency of conversion of the condensate to precipita- 

 tion depends on the temperature at which the ice nuclei operate and 

 on their number. The condensate which is not converted to precipi- 

 tation finds its way into the downward circulation required by con- 

 tinuity and is evaporated. Seeding provides ice nuclei which are 

 active at a lower elevation (higher temperature) than the natural 

 ice nuclei. If there is an insufficient number of natural ice nuclei 

 which are active at or above the lowest temperature in the cloud, 

 seeding might increase the precipitation by reducing the amount of 

 condensate which would otherwise be lost in the descending branch 

 of the circulation (increase in the efficiency or conversion). On 

 the other hand, if there is a sufficient number of natural ice nuclei 

 which are active at temperatures found within the cloud, seeding 

 may reduce the precipitation by causing the precipitation mechanism 

 to operate at a lower level in the cloud where less of the water vapor 

 will have been condensed (decrease of the condensation efficiency). 

 Finally, if the splintering mechanism is a regular and effective natural 

 means of nucleating clouds once precipitation has started, seeding 

 will have no effect since splinters of ice are the best possible ice nuclei. 

 Lack of knowledge on the number and type of natural ice nuclei and 

 on the importance of the splintering effect leaves the answer in doubt. 

 It is the opinion of the writer that a significant fraction of the con- 

 densed cloud water is often lost by evaporation in the downward 

 branches of the circulation. If true, this suggests that the precipi- 

 tation can be increased by seeding if the proper number of nuclei 

 can be introduced in the right place. 



It has been suggested that silver-iodide nuclei may be effective at 

 a considerable distance from the point of release. There is no doubt 

 that the particles can be transported for considerable distances by 

 the winds. The particles will diffuse laterally and vertically, thus 

 infecting an increasing volume of air with a corresponding decrease in 

 concentration. It is not known with certainty how long the silver- 



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