A cloud is a large mass of water droplets and ice crystals. Precipitation normally occurs in a cloud only when ice crystals grow large enough to fall to Earth as rain, snow, hail, or some other form of precipitation. When conditions do not favor the growth of ice crystals, moisture remains suspended in the clouds, and precipitation does not occur.
The general goal of cloud seeding is to find some way of converting the supercooled droplets of liquid water in a cloud to ice crystals. Supercooled water is water that remains in a liquid state even below its freezing point. The two substances most commonly used to transform water droplets to ice crystals are dry ice (solid carbon dioxide) and silver iodide.
The ability of dry ice to trigger the condensation of supercooled water droplets was discovered accidentally in 1946 by Schaefer. He had planned to use a block of dry ice to cool a container of moist air, but discovered that the dry ice actually initiated the formation of ice crystals in the container. Shortly after Schaefer's research, the ability of silver iodide to produce similar results was also discovered.
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