Cloud seeding
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Cloud seeding is the attempt to change the amount or type of precipitation that falls out of clouds or the structure of clouds by dispersing substances into the air which allow water droplets or ice crystals to form more easily. The most common chemicals used for cloud seeding include silver iodide and dry ice (frozen carbon dioxide).
These chemicals may be dispersed by aircraft or by dispersion devices located on the ground. For example, silver iodide flares will be ignited as an aircraft flies through a cloud. When released by devices on the ground, air currents may pull the fine particles up into the air. These chemicals provide a nucleus for moisture in the cloud to form around (heterogeneous nucleation), which in turn will usually cause the precipitation to increase from the clouds or cause the clouds to become less dense.
While cloud seeding has shown to be effective in reducing the amount of cloud cover, it is more controversial whether cloud seeding increases the amount of precipitation from a cloud. Part of the problem is that it is impossible to know how much precipitation would have occurred had the cloud not been 'seeded'.
One scientist associated with the invention of cloud seeding is Irving Langmuir. The first attempt at cloud seeding was in the state of Massachusetts in 1946. A plane 'seeded' a cloud with crushed dry ice; snow began falling out of that cloud. Noted atmospheric scientist Bernard Vonnegut (brother of novelist Kurt Vonnegut) is credited with discovering the potential of silver iodide for use in cloud seeding. This property is related to a good match in lattice constant between the two types of crystal; the crystallography of ice later played a role in Kurt Vonnegut's novel Cat's Cradle.
An attempt by the US to modify hurricanes in the Atlantic basin using cloud seeding in the 1950's was called Project Stormfury. Only a few hurricanes were tested with cloud seeding because of the strict rules that were set by the scientists of Project Stormfury. It was unclear whether this project was successful; hurricanes appeared to change in structure slightly, but it appeared to only be a temporary effect. The fear that cloud seeding could potentially change the course or power of hurricanes and negatively affect people in the storms path stopped the project.
Today, cloud seeding is used to increase precipitation in areas experiencing drought, to reduce the size of hailstones that form in thunderstorms, and to reduce the amount of fog in and around airports. Cloud seeding is also occasionally used by major ski resorts to induce snow fall.