What is usedfi

It depends on the goal. In cold clouds with snow potential, silver iodide is the most common seeding agent, although dry ice and compressed liquid propane or carbon dioxide can be used too. Those materials encourage water in the clouds to freeze. In warm clouds with the potential for rain, materials that attract water like salt, urea and ammonium nitrate can be used.

Do those materials end up in the precipitationfi

Yes, but in trace amounts. For example, snow prompted by silver iodide is usually spread over a large area, and the typical concentration is 0.1 micrograms per liter -- well below public health guidelines of 50 micrograms per liter.

Does it steal snow from other areasfi

Studies say no. There is evidence that precipitation increases as far as 100 miles downwind of the target area, but there's been no demonstration of decreased precipitation. One explanation is that clouds passing over a mountain barrier lose only about 10 percent of their moisture to precipitation; if seeding increases that 10 percent to 20 percent, that's only another 1 percent to 2 percent of the overall moisture available.

Doesn't it alter naturefi

Yes, but so does driving a car. In fact, studies in the United States and abroad have found that pollution from large metro areas is actually decreasing precipitation in downwind areas. In some areas, cloud seeding is aimed at compensating for those decreases.

Does it workfi

It depends on who's answering the question. Several companies do the work commercially and can point to studies and analyses demonstrating their results. On the research side, people say that commercial operators aren't likely to say too much that's bad about their methods -- but they also acknowledge that the basic principles are sound, and that the results obtained so far suggest that seeding efforts are having some impact.

Sources: Weather Modification Association; Utah Division of Water Resources; Weather Enhancement Technologies International; National Academy of Sciences

This story appeared in The Daily Herald on page A3.