Throughout history and probably prehistory man has sought to modify weather by a variety of means. Many primitive tribes have employed witch doctors or medicine men to bring clouds and rainfall during periods of drought and to drive away rain clouds during flooding episodes. Numerous examples exist where modern man has shot cannons, fired rockets, rung bells, etc. in attempts to modify the weather (Changnon and Ivens, 1981).

It was Schaefer's (1948a) discovery in 1946 that the introduction of dry ice into a freezer containing cloud droplets cooled well below 0°C (what we call supercooled droplets) resulted in the formation of ice crystals, that launched us into the modern age of the science of weather modification. Working for the General Electric Research Laboratory under the direction of Irving Langmuir on a project investigating ways to combat aircraft icing, Schaefer learned to form a supercooled cloud by blowing moist air into a home freezer unit lined with black velvet. He noted that at temperatures as cold as –23°C, ice crystals failed to form in the cloud. Introducing a variety of substances in the cloud failed to convert the cloud to ice crystals. It was only after a piece of dry ice was lowered into the cloud that thousands of twinkling ice crystals could be seen in the light beam passing through the chamber.