Radar bounces radio waves off water particles in clouds.
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Weather Radar
Radar is important to weather forecasters because it can detect rain and hail in clouds. The radar emits radio waves in all directions. When a radio wave hits water or ice particles, some of the energy is backscattered to the radar. A receiver picks up the returning energy and calculates how many particles there are (by how much energy is returned) and how far away the particles are (by how long it took for the energy to return).
The next generation of weather radar, NEXRAD, uses Doppler technology. It is also called WSR--88D, which is short for Weather Surveillance Radar--1988 Doppler. 1988 was the year they were developed to replace the previous radar, WSR-57 (which still used vacuum tube technology). The new radars can detect a doppler-shift in the returning radio waves and uses this shift to measure the speed of the particles toward or away from the radar. Meteorologists know that the wind must be pushing the particles. Using the doppler radar, they can measure the wind speed and direction in a cloud, which helps detect possible tornadoes.
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