Radar systems gather information by bouncing radio waves off of objects. Weather forecasters examine rainfall and other weather events using a special kind of radar called Doppler radar. A high-powered antenna rotates and sends out pulses of radio waves. The pulses bounce off the falling rain and return to the radar source. By measuring the time between pulses and the amount of time it takes these radio echoes to come back, the radar system can calculate the distance and direction of the rain. Doppler radar also measures changes in the radio waves, which indicate wind speed and direction. A computer combines the radar information with a map, so we can tell where rain is falling. Special software lets the system focus in on small areas, allowing forecasters to examine weather in towns and neighborhoods.
The Doppler Effect
The Doppler effect is a physical phenomenon named after Austrian physicist Christian Doppler. You have experienced the Doppler effect if you've listened to the sound of a fast-moving object, such as a car or train, as it passes you. The pitch of the sound is higher as the object making the sound comes toward you. The pitch drops as the object passes you and moves away. The Doppler effect occurs because the sound waves are squeezed together as the object moves toward you, then stretched farther apart as the object moves away. This effect doesn't just happen with sound waves. Light and radio waves also show the Doppler effect. Doppler radar systems use this principle to tell whether rain is moving closer or going farther away.