Until the late 1940's, the only men who attempted to "make rain" were Indian medicine men and "rain doctors" who claimed to work by magic. Today, scientists know much about why and how nature produces rain and snow. Under certain special conditions, scientists can cause rain to fall on a small area.
Scientists can make rain only if clouds are present. The clouds must be almost ready to produce rain or snow. With these conditions, scientists cause the water drops or ice crystals in the clouds to collect and fall to the ground. Sometimes this is easy, but sometimes it is impossible. The fluffy cumulus clouds that often form over mountains in winter and spring consist of water droplets cooled to below their normal freezing point. Scientists drop crystals of dry ice (solid carbon dioxide) into the clouds from an airplane. This causes the water drops in the clouds to change into large ice crystals and snowflakes, which fall as snow.
Under favorable conditions, rain or snow can be produced in the same kind of clouds by putting silver iodide into the clouds from below. Scientists produce silver iodide smoke by means of special generators on the ground. They allow die smoke to rise into the clouds. The tiny crystals of silver iodide act as nuclei (tiny centers) for the ice crystals to grow on. Other methods of rain making have been used, based on the same principles. Scientific rain making can be done only with special kinds of clouds and under special conditions.
