One of the earliest efforts at weather modification by means of sympathetic magic. Early farmers, in an effort to encourage rain for their crops, used a variety of means to encourage rainfall, ranging from sex magic in Babylonian times to human sacrifice among the ancient Maya Indians of Central America. Magical means to induce rain fell out of favor in Christian times although medieval Christians actively sought divine intervention in the weather through prayer and the process of rainmaking declined until the 20th century.
With the coming of new scientific knowledge about the atmosphere, however, rainmaking or, at least, control of rainfall became more feasible. Research airplanes reported that clouds were made up largely of ice particles. During the 1930s and 1940s, scientists discovered that they could encourage the production of ice particles in clouds by seeding them with frozen carbon dioxide (CO2—“dry ice”) or silver iodide (Agl) crystals. The ice particles that formed around the crystals encourage the growth of snowflakes, which then melt and fall as rain.