cloud seeding

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A technique of stimulating or enhancing precipitation by distributing dry ice crystals or silver iodide particles over developing storm clouds in a specific area of the atmosphere.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The act of firing munitions containing silver iodide at passing clouds to make them more likely to produce rain.