What is Doppler radar?

A Doppler radar is a device that has a
radio transceiver and receiver. The transceiver beams radiation at a particular
frequency. When that radiation strikes a distant object, it reflects like light from a
shinny object. The receiver then detects a small portion of the returning radiation.
When the object is moving, the motion alters the frequency of the reflected wave. This
change is detected at the receiver. The Doppler effect is commonly heard when a vehicle
with a siren approaches, passes and recedes from an observer. The received frequency
(pitch) is increased during the approach, it is identical at the instant of passing by,
and it is decreased during the recession.

Weathermen often
report on the Doppler radar image that is available from satellites. In this case, the
same technology is being applied by radars to examine the motion if individual drops of
precipitation.