Word: clouding
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...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...
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...
...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...
...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...
Softening Opposition. Many authorities did not agree with him. Langmuir's theories have been attacked by the U.S. Weather Bureau, by civilian and military meteorologists. In 1948 the Weather Bureau tried its own cloud-seeding experiments, dumping dry ice and silver iodide into clouds in Ohio. No significant rain fell from them. Langmuir's explanation is that the clouds were the wrong kind in the first place, and that they were greatly overseeded...