Dopple Duty

Q. I have seen doppler radar in weather reports. What is the difference between regular radar and doppler radar?

A. Basically, radar emits a burst of radio waves. The radio waves reflects off an object and the radar antenna receives the reflection. Since radio energy travels at the speed of light, if in the time between the transmission and reception of the burst is measured, the distance to the object can be determined. The greater the distance, the longer it takes for the burst to reach the object and be reflected back to the radar unit.

If the object, such as a cloud, is not moving, the frequency of the reflected burst will be the same as the transmitted one. But if the object is moving toward the radar unit, the reflected frequency will be higher. If it is moving away from the radar unit, the frequency will be lower.

This is known as the "Doppler shift" after the name of its discoverer. The faster the object is moving toward or away from the radar unit, the greater will be the doppler shift. A doppler radar can thus display the position of a cloud, and it can indicate how the cloud is moving.

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