- These signals are then reflected back to the transmitting site, where a computer converts them into pictures that show the location and strength of the precipitation. Radio waves that are reflected off of something that is moving away from the antenna change to a lower frequency. Conversely, if an object is moving toward the antenna, the returning signal will be a higher frequency. This is because of the Doppler effect, the same principle that is used in astronomy to tell whether stars in the universe are moving toward or away from Earth.
- A computer that is part of the Doppler radar measures the frequency changes from the returning signals and shows the direction and speeds of winds blowing around the rain, snow, hail or other objects in the air that reflect the radio waves. This process is continued as the radar sends out radio signal beams in a circular pattern around it. The distance is calculated, along with the intensity and direction of movement of any rain that is falling, as well as the wind velocity. This data is then translated to radar maps that are shown on TV and on the Internet. In fact, scientists and weather forecasters use Doppler radar to analyze wind motions in storms to understand the present weather conditions and predict the weather for specific locations within a one- or two-hour time frame.
- The Doppler effect, discovered by Christian Doppler, an Austrian scientist, in 1842, is the same principle that occurs with movement of sound and light. In addition to being used to measure distance and movement of stars, the Doppler effect is the same principle that causes sound to change frequencies when the source is moving toward or away, like a vehicle with a siren that is traveling on the road at a high speed. In the 1980s and early 1990s, it was integrated into a nationwide network of radars in the United States by the National Weather Service.
