http://www.dryicesource.com/dryice/rainmaker.php
EXCERPT:

Dry Ice Is A Rainmaker with Cloud Seeding

Cloud seeding with dry ice was discovered accidentally in a General Electric laboratory in New York. In 1946, GE chemist Vincent Schaefer caused a snowfall near Schenectady, NY, by dumping six pounds of dry ice from an airplane. Since then, cloud seeding has been used to create rain, and was used in China to reduce air pollution during the 2008 Olympic Games.

There have been accusations of "stealing rain" due to cloud seeding. In Vietnam it was used by the U. S. Air Force to extend the monsoon season: "Make mud, not war." Liquid carbon dioxide has been used to remove fog from airports. The Russians used cloud seeding to create rainfall to keep radioactive fallout from the Chernobyl nuclear disaster from spreading to Moscow.

Continental Carbonic Products sells carbon dioxide pellets and liquid carbon dioxide which can be used in cloud seeding operations.

Call Continental Carbonic toll-free at 800-DRY-ICE2. Visit our News Archive to find out about more ways to use dry ice.

http://en.wikipedia.org/wiki/Dry_ice