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\.\.fQ'f:) !~ . . pressure over water is greater than that over ice, so the drop- let diminishes by evaporstion and the crystal grows by sub- limation. As the crystsl grows it becomes heavy enough to fall; as it fslls it may shatter, providing more ice nuclei to keep the process going. These crystals, as they grow and fall, may collide with one another and form still larger crystsls--snowflakes. When they reach a level where the tem- perature is warmer than O. C they melt and fall as rain or drizzle. Since all rain does not fall from clouds which contain super- cooled droplets, another theory was needed to explain "warm rain." Sometimes ane cloud droplet grows IllUch ]a rger than those around it. The physical details of the process are still obscure, but as it falls it collides with other droplets, form- ing a larger droplet. As the droplet grows larger it sweeps out a larger area of the small droplets, and eventuslly will shatter, supplying several more Isrge droplets to continue the process. These large drops ultimately fall out of the cloud as rain. Vincent J, Schaefer discovered that a tiny piece of dry ice, when dropped into a cold chamber filled with supercooled water droplets, caused the formation of millions of ice crystals. Schaefer now observed what Veraart had apparently not noted some 16 years earlier. Irving Langmuir calculsted 19at a pellet of dry ice the size of a pea would produce about 10 (ten thousand million million) ice crystals. Studies have shown that when supercooled cloud droplets are further cooled to about -40' C they change spontaneously to the solid state. Because ita temperature is much colder thsn this, dry ice chills the air around it to below -40. C, and any supercooled cloud droplets in that air will freeze. The first field test was conducted in November 1946, with definite changes observed in cloud characteristics. At about the same time (November 1946) Bernard Vannegut dis- covered that minute crystals of silver iodide, produced in the form of a smoke, acted as ice-fonDing nuclei at temperatures colder than -5. C. It is not clear just how silver iodide induces nucleation. It was originally thought that the simi- larity in crystal structure between natural ice and silver iodide was the key, but recent studies suggest that this may not be the case. Enormous numbers of nuclei (about 1015 per gram of silver iodide) can be produced by vsporizing an acetone solution of ailver iodide in a hot flame, and this suggested the possibility of a ground dispersal system, using convective currents to carry the particles into the super- cooled region of the cloud. The first large scale trials using this technique took place in June 1950. 5