An Appraisal of Cloud Seeding as a Means of Increasing Precipitation

Abstract

The results of cloud-seeding experiments are reviewed briefly and it is concluded that, taken alone, they do not constitute an adequate basis for a sound appraisal of the potentialities of the cloud seeding techniques. An attempt is made to extend and supplement the observational evidence with the aid of existing knowledge of cloud physics and synoptic meteorology. It is found that significant amounts of precipitation can fall only from clouds which are continuously replenished. It is concluded that the thermal effects of seeding are seldom sufficient to stimulate the vertical development of a cloud. Since the activation-temperatures of the artificial ice-nuclei are believed to be higher than the activation-temperatures of natural ice-nuclei there is a possibility that seeding may advance the onset of precipitation or release precipitation from clouds which do not extend to the activation-temperature of the natural ice-nuclei. It appears that the seeding of clouds which are already releasing precipitation may either decrease or increase the total precipitation; an unequivocal answer is not possible due to the lack of pertinent information regarding natural precipitation processes. On the basis of the analysis of the physical processes and general knowledge of synoptic meteorology it is concluded that it is unlikely that cloud seeding will cause large-scale effects. Further research on cloud physics and cloud seeding is recommended.