Dear Tom,
Has any progress been made in seeding clouds? How does seeding work?
— Joan Knudsen, Chicago Ridge
Dear Joan,
Cloud seeding refers to the various processes whereby material is added to a cloud to alter its natural development, usually to obtain precipitation or to increase it. Other kinds of cloud seeding attempt to dissipate the cloud (such as dispersion of fog and stratus clouds at an airport). Seeding is no longer considered to be a fringe science. It has become a mainstream tool to increase rain or snow, although its effectiveness is still a matter of academic debate. A key challenge is to determine how much precipitation has been increased in seeded clouds versus the amount of precipitation that would have occurred if nothing had been done. The most common chemicals used for cloud seeding are silver iodide, potassium iodide, dry ice (solid carbon dioxide) and hygroscopic materials such as table salt.
