Monday, April 21, 2008

Doppler shift

The Doppler effect was first applied to the apparent shifting frequencies of a train whistle as it approaches and then passes. When the sound is approaching, the waves are pushed together and the wavelength is shorter, and the frequency is higher, and the whistle is heard as a higher pitch. Then, where the waves are stretched, the wavelength is longer, and the frequency is less. So as a train departs, its whistle will sound at a lower pitch. That's the Doppler shift.
The Doppler shift works with light waves as well as sound waves. For example, astronomers have concluded that the universe is expanding because of the shift to longer wavelengths, or red shift, by distant galaxies. That shift indicates that these galaxies are moving away from the earth.

Doppler radar detects precipitation and measures the speed of falling precipitation.
When microwaves are used, the Doppler shift will indicate whether raindrops are moving away from the radar or toward it. That motion is called radial velocity.
These outgoing or incoming motions are color-coded on the Doppler radar so when a sharp change in direction takes place, the colors contrast sharply. That contrast indicates that a severe storm is present, even a tornado. That wind difference is called wind shear. You can demonstrate this easily by putting a pen between your two hands. Move your left hand in one direction and your right hand in the other. What happens to the pen? It will rotate. That rotation can be detected on the radar screen by looking at the color shift of the radial velocity. A strong shift is a tip-off that some severe weather is happening. The warning can be issued as much as 20 minutes before a tornado touchdown in a distance of less than 70 km from the radar; because of the great uncertainty of this forecast and due to the high impact of such a warning the Romanian Met service (and European met services in general) do not regulary issue these warnings.
(Doppler information from infoplease - web site)

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