production of rain
by artificial means now generally disregarded, though it is probable that rainmaking hastens or increases rainfall from clouds suitable for natural rainfall. Interest in rainmaking has been spurred by factors including drought and the need for irrigation water. Until recent times it was thought that rain might be induced by explosions, updrafts from fires, or by giving the atmosphere a negative charge. Research during the 1930s showed that rain forms in warm clouds when larger drops of condensed water grow at the expense of smaller ones until they are big enough to fall; also that in cold clouds supercooled water below 5°F; (-15°C;) freezes into ice crystals that act as nuclei for snow. On this basis the American physical chemist Irving Langmuir and his associates carried on Project Cirrus from 1940 to 1952 to find ways to produce rain. Three methods resulted, including spraying water into warm clouds; dropping dry ice into cold clouds, where the dry ice freezes some water into ice crystals that act as natural nuclei for snow; and wafting silver iodide crystals or other similar crystals into a cold cloud from the ground or from an airplane over the cloud, with the crystals hastening the freezing of supercooled water between 27°F; (-2.8°C;) and 5°F;. Overseeding can dissipate a cloud. These techniques are only moderately successful; they cannot be relied upon in case of drought.
See Utah Water Research Laboratory, Development of Cold Cloud Seeding Technology for Use in Precipitation Management (1971); L. J. Battan, Cloud Physics and Cloud Seeding (1979).
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