Doppler Radar Map for New Jersey
This is the latest Doppler Radar Map for New Jersey from
the National Weather Service. This image is generated at the National
Weather Service's Mount Holly, New Jersey station by NEXRAD.
NEXRAD (Next Generation Radar) obtains weather information
(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 them to issue
tornado warnings with more advanced notice.
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