Making rain while the sun shines.

Some scientists have maintained that cloud seeding -- the practice of dispersing chemicals into cumulus clouds in order to make rain -- is about as effective as paying someone to do a rain dance. For midwestern farmers and water managers, however, cloud seeding has often seemed to offer hope in times of drought. North Dakota farmers, for instance, have had enough confidence in the procedure to target 6.6 million acres for the "store-bought" rain at the cost of 5^ an acre, a state meteorologist estimates.

Skeptics have never doubted that when silver iodide seeding agents come into contact with a cloud's very cold moisture droplets, ice crystals -- which become raindrops in warm weather -- are formed; in laboratory cloud chambers this is known to happen. But could the process work in the field -- or, perhaps more aptly in this case, in the sky -- where the real clouds roam? The skeptics doubted that seeding agents dispersed at the base of a tall cumulus cloud could wend their way 5,000 to 12,000 feet up, to the part of the cloud that contains moisture droplets.

Last month, in a collaborative effort, scientists from the North Dakota Weather Modification Board in Bismarck and the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colo., took to the clouds in order to settle things once and for all. The researchers released a tracer gas -- sulfur hexafluoride -- simultaneously with the silver iodide, and then followed in a second plane equipped with detection equipment to monitor the tracer's dispersal.

"The stuff had a bit of a climb to make," says John Flueck, a NOAA research scientist, "but our preliminary guesses are that in at least one instance the seeding agent was successful in reaching the level where the cold water is." The researchers also tested silver iodide with sodium chloride. This combination, says Flueck, "works much more quickly -- because the water in the cloud doesn't have to be as cold for it to work."