Better Rainmaking

The art of rainmaking got a bad setback more than a year ago when the Air Force and the Weather Bureau spewed quantities of dry ice into juicy Ohio clouds and produced hardly any rain (TIME, Dec. 6, 1948). But Nobel Prizewinner Irving Langmuir, leading backer of scientific rainmaking, is notably hard to discourage. Last week he told a Manhattan meeting of the American Meteorological Society and the Institute of Aeronautical Sciences about more successful experiments in New Mexico.

Rainmaking, Dr. Langmuir explained, is not a matter of dumping dry ice into any likely-looking cloud. It works only when conditions are absolutely right. Even then the job can be botched by using too much dry ice. An overdose may turn the cloud into ice particles so small that they never fall as rain.

The New Mexico experiments were done by Project Cirrus (sponsored by the Army Signal Corps and the Office of Naval Research in consultation with General Electric Research Laboratory), and the experimenters tried a more subtle method.

Reluctant Clouds. In arid regions like New Mexico, Langmuir explained, big cumulus clouds often rise high in the air without dropping any rain. In such cases, the air does not contain enough natural nuclei (suitable dust particles) for moisture to condense upon. The warm air from over a sun-heated plain boils upward vigorously, but the moisture in it does not condense until the cold upper levels are reached. Then it condenses suddenly into very small ice particles that drift off at about 35,000 feet, leaving the ground dry, its inhabitants disappointed.

The way to milk rain from these reluctant clouds, said Langmuir, is to seed the air with particles of silver iodide, on which moisture condenses easily. When enough nuclei are present, snowflakes form on them at a comparatively low level. This condensation releases heat, which makes the air rise faster. The resulting turbulence tears the snowflakes apart. The fragments grow into larger flakes, releasing still more heat. The meteorological "chain reaction" turns the cloud into a violent thunderstorm that dashes torrents of rain on the ground below.

Chain Reaction. Last July 21, said Langmuir, the Albuquerque weather forecast predicted no substantial amounts of rain. But at 5:30 a.m. Project Cirrus' ground generator (a gadget for releasing silver iodide smoke) started a day-long run. About 8:30 a.m. a big cloud formed down wind from the generator. At 9:57, a chain reaction started inside it, filling the cloud with raindrops that showed on a radar screen. Lightning flashed, thunder rolled, and heavy rain fell over a large area. Later thunderstorms near by watered other parts of New Mexico.

Only 300 grams (about 2/3 Ib.) of silver iodide were used on that day, and Dr. Langmuir offered elaborate mathematical proof that this small amount brought down 320 billion gallons of rain, enough to fill all of New York City's reservoirs. He thinks that the iodide particles, drifting eastward, caused a long streak of rain through southern Colorado and Kansas. "It is very important," he concluded, "that regular tests on certain selected days of each week be carried out throughout the year using amounts of seeding agents just sufficient to obtain conclusive statistical data as to their effectiveness in producing rain."

The Weather Bureau, which has tried silver iodide on its own, is still skeptical. But Bureau Chief Dr. F. W. Reichelderfer agreed with Langmuir that careful tests should be made and the results scrutinized by disinterested scientists.

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