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The Cold Does It. Langmuir, the man of theory, soon worked out the "mechanism." It was the low temperature of the dry ice, not its carbon dioxide, that did the trick. Any very cold object, e.g., a needle cooled with liquid air, served as well.
How cold is cold enough? Langmuir and Schaefer found by careful experiment that the motes form at -39° C. (38° F.). This explained some types of rain. Certain clouds rise high enough to be cooled to that temperature. Ice motes form, find their way into warmer parts of the cloud, where they grow into snowflakes and fall as snow or rain. "Why not help things along with some dry ice?" asked Langmuir & Schaefer.
One day in November 1946, Schaefer took off from Schenectady in a small airplane and directed the pilot to a fleecy cloud four miles long that was floating over nearby Massachusetts. When he reached it, he scattered into it six pounds of dry ice. Almost at once the cloud, which had been drifting along peacefully, began to writhe as if in torment. White pustules rose from its surface. In 5 minutes the whole cloud melted away, leaving a thin wraith of snow. None of the snow reached the ground (it evaporated on the way down), but the dry ice treatment had successfully broken up a cloud.
Bath in the Clouds. This dramatic feat stirred up a flurry of premature rainmaking. Barnstorming pilots took off with dry ice to knock down fleecy clouds. They did not knock down much rain. For one thing, they often picked on the wrong clouds, e.g., the stratiform (layerlike) clouds, which unless very thick do not contain enough moisture to matter. And they were inclined to overdo, choking the clouds with too much dry ice. A piece of dry ice falling through a supercooled cloud creates enormous numbers of ice nuclei. Too many falling pieces of dry ice create too many nuclei; they compete fiercely for the water in the cloud, but there isn't enough to go around. Result: a dense cloud of ice crystals too small to fall.
Dry ice, Langmuir soon decided, has other limitations. It affects a cloud only while falling through it, and the ice motes it creates must take effect immediately or they will evaporate. Dr. Bernard Vonnegut, another of Langmuir's bright protégés, was assigned the job of finding some sort of permanent, nonvolatile particles that would hang in the air long enough for ice to form on them.
Water, reasoned Vonnegut, forms hexagonal ice crystals with well-known characteristics. If another hexagonal crystal could be found with nearly the same characteristics, the water molecules in the air might be fooled into building up on it as if it were a genuine ice nucleus.
Vonnegut thumbed through fat books on crystallography. At last he spotted a promising compound: silver iodide. Its molecules do not resemble water molecules, but they build into crystals almost exactly like those of ice.
The first trial was a failure; Vonnegut's commercial silver iodide was too impure. He tried again with a few specks of pure silver iodide, which he evaporated from an electrically heated wire. At once the captive cloud in his cold chamber turned into snow. The merest smidge of the magic iodide seemed to be enough.