Dr. Bernard Vonnegut (1914-1997), Kurt Vonnegut, Jr.'s brother, was a professor of atmospheric sciences at the State University of New York at Albany, retired in 1985. A colleague of Dr. Vonnegut, Vincent J. Schaefer, discovered in an experiment during the 1940s that dry ice rapidly produced ice crystals when introduced into a cloud of supercooled water droplets.
Many clouds in our atmosphere are giant collections of supercooled water droplets that refuse to precipitate. One form that clouds precipitate is by a process called aggregation where the water droplets freeze and begin to fall through the cloud, gaining mass as they contact and acquire other water droplets on their way down. Unfortunately, water in this form will not freeze without the help of a condensation nuclei or until the temperature reaches levels around or below -40°C (-40°F), hence the designation "supercooled".
The dry ice (solid carbon dioxide) Schaefer introduced into the cloud acted as a cooling agent, instantly bringing the droplets well past the necessary -40°C (-40°F) they need to freeze causing the cloud to precipitate. This was the birth of cloud seeding, or the artificial coaxing of precipitation by introducing particles into clouds to act as condensation nuclei and/or a cooling agent.
Dr. Vonnegut soon discovered that silver iodide has a crystalline structure similar to an ice crystal and that, because of this, it acts as an effective ice nucleus at temperatures of only -4°C (25°F). Additionally, silver iodide is far easier to handle than dry ice when attempting to fly it over a cloud to seed the cloud, which is precisely a process that Dr. Vonnegut refined and that is still in use by rainmaking corporations today.
It was this process and research of cloud seeding that inspired his brother Kurt Jr. to create and write a wonderful novel about Ice-Nine.
(some people used to spell it "Ice-9")