Cloud seeding, a form of intentional weather modification, is the attempt to change the amount or type of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud. The usual intent is to increase precipitation (rain or snow), but hail and fog suppression are also widely practiced in airports.

The most common chemicals used for cloud seeding include silver iodide and dry ice (solid carbon dioxide). Liquid propane, which expands into a gas, has also been used. This can produce ice crystals at higher temperatures than silver iodide. The use of hygroscopic materials, such as salt, is becoming more popular^{[citation needed]} after promising research^{[citation needed]}.

Seeding of clouds requires that they contain supercooled liquid water—that is, liquid water colder than zero degrees Celsius. Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing nucleation. Dry ice or propane expansion cools the air to such an extent that ice crystals can nucleate spontaneously from the vapor phase. Unlike seeding with silver iodide, this spontaneous nucleation does not require any existing droplets or particles because it produces extremely high vapor supersaturations near the seeding substance. However, the existing droplets are needed for the ice crystals to grow into large enough particles to precipitate out.

William Sanford "Bill" Nye (born November 27, 1955), popularly known as Bill Nye the Science Guy, is an American science educator, comedian, television host, actor, mechanical engineer, and scientist. He is best known as the host of the Disney/PBS children's science show Bill Nye the Science Guy (1993–1998) and for his many subsequent appearances in popular media as a science educator.

William Sanford Nye was born in Washington, D.C., the son of Jacqueline (née Jenkins; c. 1920–2000), a codebreaker during World War II, and Edwin Darby "Ned" Nye (died 1997), also a World War II veteran whose experience in a Japanese prisoner of war camp led him to become a sundial enthusiast. Nye is a fourth-generation Washington, D.C. resident on his father's side of the family. After attending Lafayette Elementary and Alice Deal Junior High in the city, he was accepted to the private Sidwell Friends School on a partial scholarship, graduating in 1973. He studied mechanical engineering at Cornell University, where one of his professors was Carl Sagan, and graduated with a Bachelor of Science in 1977. He was awarded an honorary doctorate by The Johns Hopkins University in May 2008. In May 2011, Nye was awarded an Honorary Doctor of Science degree from Willamette University where he was the keynote speaker for that year's commencement exercises.