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4 CLOUD SEEDING 1.4 SCIENTIFIC BASIS FOR CLOUD SEEDING Scientific weather modification grew from initial experiments in the late 1940s under the direction of Nobel Laureate Irving Langmuir at the General Electric (G.B.) Laboratories in Schenectady, New York. A strong stimulus was the 1946 seeding field test by Vincent Schaefer in which dry ice was dropped into a stratocumulus cloud. Very quickly the cloud, consisting of supercooled water droplets, was transformed into a swarm of ice crystals that grew and fell from its base, leaving behind a distinct hole in the cloud. If naturally occurring ice-forming nuclei had been of sufficient concentration in this cloud, they would have transformed its supercooled liquid droplets into ice crystals themselves. The dry ice pellets dropped into the cloud counteracted the dearth in natural nuclei by artificially creating ice embryos. The formation of these ice crystals takes place in the vapor phase and does not depend on either foreign nuclei or on the droplets in the supercooled cloud. The new frozen particles then mix through the cloud, growing at the expense of the supercooled cloud droplets, and thus enhancing the precipitation process within either a non-precipitating or precipitating cloud. Later, Vincent Schaefer and Bernard Vonnegut of the G.E. Laboratories discovered how to produce ice embryos by introducing a swarm of minute silver iodide (AgI) smoke particles into a supercooled cloud. Their structure is similar to that of ice, and water deposits on them to form ice. Subsequently, other investigators discovered other nucleating agents. At present, AgI produced in complexes with other chemicals remains the chief agent in use, although dry ice and some organics are occasionally used. Scientific studies of nucleation led to questions about the details of how nature produces precipitation. The field data had indicated that natural precipitation efficiencies were often low. Many supercooled cloud droplets are formed in updraft regions that occur in cumulus clouds or on the upwind slopes of mountain ranges that are not completely trans- formed into precipitation size particles capable of reaching the ground. They either blowout the top and sides of the cumulus clouds or blow over the mountain crest to the lee side and evaporate. Introduction of artificial ice-forming nuclei into these clouds could decrease these losses, thus enhancing precipitation. Furthermore, it was found that under some conditions, an increase in updraft speed and in cloud development oc- curred with artificial nucleation. This could lead to more total condensa- tion, and therefore, to more precipitation. It is now recognized that clouds suitable for seeding are supercooled, relatively free of ice at critical times in their evolutions, and have appreciable natural forcing, and that clouds not meeting these criteria have little chance of producing precipitation and thus cannot be usefully seeded for this purpose. Technologies and computer power to measure and numerically model cloud processes and the dispersion of seeding material, and thus evaluate cloud seeding po- tentials and effects have improved dramatically in recent years. How-