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INTRODUCTION TO NEXRAD
NEXRAD (Next Generation Radar) is a special type of radar that is used for
obtaining weather information. Unlike previous radars, NEXRAD is capable of detecting not
only the existence of interesting objects in the air (precipitation, clouds, etc.)
but also their motion. This allows meteorologists to get a better idea of what is
happening inside a cloud and predict phenomenon such as wind shear and tornados with greater
accuracy.
NEXRAD was originally deployed in Oklahoma City in 1988. The technical name for NEXRAD
is WSR-88D which derives its name from Weather Surveillance
Radar Doppler, with the "88" coming from the year it was deployed.
HOW NEXRAD WORKS
NEXRAD obtains information about precipitation and wind based upon returned energy. The
radar emits a burst of energy (green). If the energy strikes an object (rain drop, bug,
bird, etc), the energy is scattered in all directions (blue). A small fraction of that
scattered energy is directed back toward the radar.
This reflected signal is then received by the radar during its listening period. Computers
analyze the strength of the returned pulse, time it took to travel to the object and back,
and phase shift of the pulse. This process of emitting a signal, listening for any returned
signal, then emitting the next signal, takes place very fast, up to around 1300 times each
second.
NEXRAD spends the vast amount of time "listening" for returning signals it sent. When the
time of all the pulses each hour are totaled (the time the radar is actually transmitting),
the radar is "on" for about 7 seconds each hour. The remaining 59 minutes and 53 seconds
are spent listening for any returned signals.
The ability to detect the "shift in the phase" of the pulse of energy makes NEXRAD a Doppler
radar. The phase of the returning signal typically changes based upon the motion of the
raindrops (or bugs, dust, etc.). This Doppler effect was named after the Austrian
physicist, Christian Doppler, who discovered it. You have most likely experienced the
"Doppler effect" around trains.
As a train passes your location, you may have noticed the pitch in the train's whistle
changing from high to low. As the train approaches, the sound waves that make up the
whistle are compressed making the pitch higher than if the train was stationary. Likewise,
as the train moves away from you, the sound waves are stretched, lowering the pitch of the
whistle. The faster the train moves, the greater the change in the whistle's pitch as it
passes your location.
The same effect takes place in the atmosphere as a pulse of energy from NEXRAD strikes
an object and is reflected back toward the radar. The radar's computers measure the phase
change of the reflected pulse of energy which then convert that change to a velocity of
the object, either toward or from the radar. Information on the movement of objects either
toward or away from the radar can be used to estimate the speed of the wind. This ability
to "see" the wind is what enables the National Weather Service to detect the formation of
tornados which, in turn, allows us to issue tornado warnings with more advanced notice.
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