Freezing silver iodide crystals

Sometimes, too much rain falls in one location and too little rain falls in another. People have been trying to produce rain on demand for centuries. Because most clouds exist at temperatures below the freezing point of water, rain often begins when water vapor deposits on ice crystals. (When ice crystals approach the surface of Earth, they melt and fall as rain if the temperature of the air near the surface is above freezing.) Rainmakers focus on the crucial role played by ice crystals. For example, dry ice pellets can be dropped into a cloud. The cold dry ice cools the water vapor in the cloud and ice crystals form. Sometimes tiny silver iodide crystals are sprayed into a cloud to serve as artificial "ice pellets."... 

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