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fall to earth from the cloud. The first material to be used was dry ice, but silver iodide has become the preferred agent. A cloud is actually a mass of air in which there are multitudes of microscopic liquid water droplets too small to fall through the surrounding air. Some high-altitude clouds are composed of tiny ice crystals. So- called warm clouds exist at temperatures above 320 F. whereas super- cooled clouds, the more common type in the United States, both in winter and summer are partially or entirely at temperatures below the freezing point of water. In tiny droplets, however.. water will not start to freeze at 320 F. In fact, liquid water droplets can exist at temperatures down to _390 F. without freezing. Below this temperature, however, the water droplets cannot exist as a liquid, and ice crystals spontaneously form. Unless the temperature of a super-cooled cloud jails below _390 F" tiny foreign solid particles apparently must be present to initiate precipitation through the coalescence and freezing of the liquid droplets. These nat- urally occuring smoke and dust particles, containing mineral matter from the earth, meteorites, or the sea, serve as nuclei of condensation or freezing. Some are able to start ice crystal formation at temperatures close to 320 F., while others require much lower temperatures to be effective, depending on their composition, size and shape. After ice starts to form on these nuclei, the crystals grow by collection of liquid and vapor from the rest of the cloud, until they are large enough to lall through the air. The atmosphere always contains many types of dust at various con- centrations and altitudes.: where the air has moved long distances over land areas there usually are enough of these particles to initiate pre- cipitation if other necessary conditions prevail. A rather delicate balance of these factors must be present, for the process is apparently very sen- sitive to small changes in conditions. These natural conditions suggest the following possibility: if a cloud does not have sufficient quantities of ice. forming nuclei, but if other conditions such as temperature, air motion.. water droplet size and distri- bution are satisfactory, the artificial addition of crystallization nuclei may initiate the precipitation process. The potentialities of increasing water supplies by the introduction of such particles into suitable clouds are therefore interesting, and are the ones that are being exploited by the contemporary cloud-seeding operations. Between July 1950 and June 1956, an average 10 per cent of the total land area of the country was covered by commercial cloud-seeding opera- tions. 1\10st of these operations had the precipitation of additional moisture as their objective. Similar, but less extensive, cloud-seeding efforts have been undertaken in Europe, Australia, and South America. By 1957, fourteen states of this country had considered the subject im- portant enough to pass Jaws providing for regulation of ~\"eather modifica- i I 20