This is the VOA Special English ENVIRONMENT REPORT.
Doppler radar is an increasingly important tool to study weather. It is named after a physical effect first reported by an Austrian scientist, Christian Doppler. In eighteen-forty-two, he described how movement seems to influence the rate at which energy waves are produced.
Doppler found that the number of sound waves from a moving object would increase as the object came closer to an observer. The frequency would decrease as the object moved away. This became known as the Doppler effect.
You may have experienced it, for example, as a train goes by. As the train moves closer, the sound -- or pitch -- seems higher. As the train moves away, the pitch seems lower.
The number of sound waves that reach your ear in a given amount of time influences what you hear. In this case, the train moves a lot slower than the sound waves it produces. The waves that move out in the direction of travel get pushed together. So, to the observer, the frequency increases.
Behind the train, sound waves become spread out, so the frequency decreases. In reality, the sound of the train stayed the same. This effect happens with light and radio waves, too.
The ideas described by Christian Doppler are important to modern science. For example, they help scientists estimate the age of stars and the distance from Earth.
Doppler radar can tell more about storm systems than older radars could. Scientists hope to continue to improve the technology to warn people about severe storms. These can form suddenly -- like the tornadoes that tear across parts of the United States.
Weather scientists use radar systems to send out radio waves at moving objects, such as snowfall or raindrops in a storm. The radio waves hit the objects and return to the receiver. The period of time in between helps to show the storm's position and strength. But Doppler radar also measures changes in the frequency of the radio waves. This shows the direction and speed of winds. A computer combines all the measurements with a map, so scientists can follow the storm.
In recent years, information from Doppler radar has come into widespread use in the United States. Scientists say this has helped to improve the reporting of severe weather.
This Special English ENVIRONMENT REPORT was written by George Grow.