Preliminary experiments have been made on seeding natural supercooled clouds with silver iodide smokes. It is believed that in many cases positive effects were observed. In a majority of the experiments it is impossible to prove beyond doubt that the effects are the result of the seeding. However, certain of these experiments demonstrate conclusively that modification of natural clouds with silver iodide smokes can be achieved. Areas of supercooled ground fog several hundred feet in diameter have been changed to ice at a temperature of − 5°C by small scale releases of silver iodide smoke from the ground. Similarly areas up to a mile in diameter have been filled with small ice crystals by releasing the smoke at − 20°C when the air is supersaturated with respect to ice.
On December 21, 1948, a supercooled stratus cloud layer approximately 1,000 feet thick at a temperature of − 10° was seeded from an airplane with silver iodide smoke produced by dropping three or four pounds of burning charcoal which had been impregnated with one percent by weight of silver iodide. For purposes of comparison, and in order to definitely establish the position of the seeding, dry ice seedings were made about three miles away on either side. The results of the silver iodide seeding are clearly visible in photographs taken from the airplane. About six square miles of the supercooled cloud layer were transformed into ice crystals as the result of seeding with somewhat less than one ounce of silver iodide.