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This is the VOA Special English Environment Report.
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| Art bystorm: a |
Doppler radar is an increasinglyimportant tool to study weather. It is named after a physical effectfirst reported by an Austrian scientist, Christian Doppler. Ineighteen-forty-two, he described how movement seems to influence therate at which energy waves are produced.
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Doppler found that the number ofsound waves from a moving object would increase as the object camecloser to an observer. The frequency would decrease as the objectmoved away. This became known as the Doppler effect.
You may have experienced it, for example, as a train goes by. Asthe train moves closer, the sound -- or pitch -- seems higher. Asthe train moves away, the pitch seems lower.
The number of sound waves that reach your ear in a given amountof time influences what you hear. In this case, the train moves alot slower than the sound waves it produces. The waves that move outin the direction of travel get pushed together. So, to the observer,the frequency increases.
Behind the train, sound waves become spread out, so the frequencydecreases. In reality, the sound of the train stayed the same. Thiseffect happens with light and radio waves, too.
The ideas described by Christian Doppler are important to modernscience. For example, they help scientists estimate the age of starsand the distance from Earth.
Doppler radar can tell more about storm systems than older radarscould. Scientists hope to continue to improve the technology to warnpeople about severe storms. These can form suddenly -- like thetornadoes that tear across parts of the United States.
Weather scientists use radar systems to send out radio waves atmoving objects, such as snowfall or raindrops in a storm. The radiowaves hit the objects and return to the receiver. The period of timein between helps to show the storm's position and strength. ButDoppler radar also measures changes in the frequency of the radiowaves. This shows the direction and speed of winds. A computercombines all the measurements with a map, so scientists can followthe storm.
In recent years, information from Doppler radar has come intowidespread use in the United States. Scientists say this has helpedto improve the reporting of severe weather.
This Special English Environment Report was written by GeorgeGrow.