Radar was initially designed to detect the presence of aircraft shortly before and during World War II, however, today, radar is used for a wide array of applications. Since atmospheric moisure will reflect a small amount of radar energy, radar can be used to detect precipitation and other meteorological events.
The word "Radar" is derived from RAdio Detection And Ranging and refers to the technique of using radio waves to detect the presence and the distance of a target. Radar works by transmitting a radio signal into the air, and then listening for a returned signal. If the transmitted signal reaches a target, then most of the energy is scattered, however, some will be reflected back to the radar receiver. The presence of the target can be confirmed by the reflected signal. Since radio waves travel through air at a known velocity, the distance of the target away from the radar can be determined by measuring the amount of time between the transmission, and the reception of the reflection.
"Doppler" radar is a special type of radar that makes use of the Doppler Effect to measure not only the distance of a target, but its velocity as well. It works by comparing the frequency of the transmitted signal versus the frequency of the reflected signal. The classic example of the Doppler Effect is the sound of a train's whistle as it passes by at high speed. As the train approaches, the pitch (frequency) of the sound will rise until the train passes. After passage, the pitch will seem to be lower. By measuring the change in "pitch" Doppler radar can determine the speed of target.
There is one caveat however, the speed is relative to the radar location. For example, if an airplane is flying in a perfect circle around a radar site at the center, the radar would show the speed as zero, because the relative distance between the aircraft and the radar site would be unchanged—there would be no motion relative to the radar site. Using the train example, if an observer was standing in the center of a circle with a train track around the perimeter, the sound of the train's whistle would not change. The relative distance between the observer and the train would always be a constant. If however, the target where flying directly toward, or away from, the radar site, then the measured velocity would be the actual velocity. The velocity component of a target relative to the radar beam is referred to as the radial velocity.
"NEXRAD" is short for NEXt generation RADar and refers to the nationwide network of Doppler radar sites installed by the National Oceanographic and Atmospheric Administration (NOAA). These sites are specifically designed to provide meteorological data so the official designation is WSR-88D (WSR88D = Weather Surveillance Radar - 1988 - Doppler). The NEXRAD project was started in the late 1980s and was designed to provide comprehensive radar coverage of the United States and to replace older WSR-57 and WSR-74 radar systems. The NEXRAD system is superior to the old ones because it is much more sensitive and allows meteorologists to acquire a much wider range of data. All of the radar images on weatherTAP are produced from data acquired from one or more NEXRAD sites. Click here for more detailed information is available on the NEXRAD radar system.
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