![]() As mentioned above, it is measured in decibels of reflectivity (dBz). Reflectivity is a measure of the energy returned by the atmosphere to the radar. The energy returned to the radar is processed by the radar software into three types of data: reflectivity Doppler velocity and spectrum width. This provides a 3-dimensional picture of the reflectors above and around the radar location. To sample the entire atmosphere within the range of the radar, the radar antenna is rotated through all 360 degrees of the compass, and at various elevation angles starting at 0.5 degrees above the horizontal. Combining these two pieces of information provides the location of the reflecting precipitation. By knowing the direction that the radar beam is pointing when the pulse is emitted, it knows the relative direction of the reflector from the radar location. If the radar's computer knows when the pulse of energy was transmitted and the speed it travels, it can calculate the distance to the reflector by using the time it takes the energy to travel to the precipitation and to return to the radar. More about this later.įig 1: Schematic of a Radar Beam and the Energy Returned by Precipitation. The strenght of the reflected energy is measured in decibels of reflectivity, or dBz, and is a measure of the intensity of the precipitation. Snow is the weakest reflector, rain a very good reflector, and wet hail the best reflector. Only a very small fraction of the emitted energy returns to the radar. ![]() As the energy travels away from the radar, it is reflected off precipitation. In fact, a weather radar listens for a considerably longer time period than it transmits. It sends out a pulse of energy for a very short period of time, typically from 1 to 4 microseconds, and then listens for that energy to return to the antenna. By the time the radar beam is 150 miles away from the radar site, it is sampling the atmosphere 10 to 12 thousand feet about the Earth's surface.Ī weather radar does not continuously transmit energy. This phenomenom is referred to as beam refraction. (Some local weather radars operate with a 5 cm wavelength.) When the radar beam is aimed horizontally, the atmosphere bends the beam upward relative to the Earth's surface. This energy travels at the speed of light and has a wavelength around 10 cm. Most radars use a parabolic dish to focus the energy along a narrow beam. Weather radar is essentially a radio transmitter that sends out a pulse of energy that interacts with the precipitation in the atmosphere. This led to operational weather radar in the early 1950s. After the war surplus radars helped spur research efforts to apply radar technology to weather detection. The development and use of radar expanded tremendously during the war when weather was seen as unwanted "noise" that interferred with the radar's primary purpose, the detection of aircraft and ships. The term was officially coined during World War II (1939-1941) even though the concept of radar was discovered and demonstrated prior to the war. The word, RADAR, is an acronym that stands for RAdio Detection And Ranging. Detailed interpretation of these displays can be complex and is beyond the scope of this web page. The purpose of this web page is to provide some basic background information to help non-meteorologists understand what is shown on radar displays. With the proliferation of radar imagery on the Internet and television/cable, it is important that non-meteorologists (and even some so-called "television meteorologists") have an idea of what a radar senses and displays.
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