Pouring salt on stingy clouds.

At the end of the last century, an editorial in the Hartford Courant quipped, "Everybody talks about the weather, but nobody does anything about it." That statement remained true until the 1940s, when rain seekers started seeding clouds in hopes of generating precipitation. After half a century, meteorologists still regard this widespread practice with skepticism because no experiment has proved it effective. Now, scientists at the National Center for Atmospheric Research in Boulder, Colo., have launched a major project to determine whether seeding clouds can actually work.

In a 4-year experiment conducted in Mexico, NCAR investigators and their Mexican colleagues are using pyrotechnic flares on aircraft to seed clouds with salt. The moisture-loving salt, a mix of sodium chloride and potassium chloride, prompts water vapor to condense into droplets that, as they grow bigger, fall to the ground. This technique has shown promise in trials over South Africa, where scientists report a 30 to 60 percent increase in precipitation from seeded versus unseeded clouds.

The problem with most previous experiments has been their inability to distinguish natural precipitation from that generated by cloud seeding. In the Mexico study, researchers plan to track the effects of seeding by using radar and lasers to measure droplet size within the clouds before and after applications of salt.