Radar stands for RAdio Detection And Ranging. Developed during World War II to detect enemy planes, radio waves detect the presence and distance of a target. A radio signal is sent out, and the operator listens for a returning signal. If the radio signal hits an object, most of the energy is scattered, but some will be reflected back to the receiver. By analyzing the changes in the reflected signal, the operator can determine where the object is and how far away it is.
Doppler radar makes use of the Doppler effect to determine not only the location and distance of a target, but also its speed. We have all experienced the "Doppler effect" with trains. As a train passes you, you may notice the pitch in the whistle changing from high to low. As the train approaches, the sound waves in the whistle are compressed, making the pitch higher than if the train were just sitting in the station. And as the train moves away, the sound waves are stretched, lowering the whistle's pitch. The faster the train is moving, the greater the change in pitch.
Weather radar systems send out radio waves. Objects in the air, such as rain drops, snow crystals, or even insects and dust, scatter and reflect some of the radio waves back to the receiver. Then a computer converts the "echoes" of the radio waves into pictures that show the location and intensity of the precipitation.
Doppler radar can look "inside" a thunderstorm to detect possible wind rotation. This helps weathermen predict tornadoes. A Doppler system does this by measuring the how the frequency of the radio waves (similar to the numbers on a radio dial) change as the waves return. Waves reflected by something moving away from the antenna change to a lower frequency (the left of the dial). Waves from an object moving toward the antenna change to a higher frequency (the right of the dial). The computer then uses the frequency changes to show directions and speeds of the winds.
What is NEXRAD?
NEXRAD stands for NEXt generation RADar. It refers to the nationwide network of Doppler radar sites installed by the National Oceanographic and Atmospheric Administration. The NEXRAD system is much more sensitive than older systems and allows meteorologists to obtain a much wider range of data.