Doppler Radar

If you’ve watched weather forecasts on TV, you’ve probably seen images of weather patterns in your local area, state, or even for the entire United States. Along with the weather images shown on the TV screen, you may also have seen a reference to the effect that the images are generated using Doppler radar. So exactly what is Doppler radar, how is it different from other types of radar, and why is it used for reporting weather patterns?

Doppler radar is a special type of radar that uses the Doppler effect to produce data about objects at a distance. In our discussion, the objects we are interested in are weather-related objects such as clouds, rain drops, and other weather-related occurrences. The radar does this by bouncing microwave signals off the weather-related objects and analyzing the motion of the objects based on an alteration of the frequency of the returned or reflected signal because of the working of a phenomenon known as the Doppler effect.

The Doppler effect was first described by Austrian physicist Christian Doppler who observed the apparent difference in the frequency of sound or light waves reaching an observer caused by the relative motion of the observer and the wave source. It is the kind of effect we observe when the sound of an approaching object seems to increase in pitch as it gets nearer to us and the decrease in pitch of the sound of the object as it moves away from us. A common example is the pitch of the whistle of train or the horn of a car as it moves toward and away from us. The pitch of the whistle or horn seems to increase (get higher in pitch) as the vehicle approaches us and the pitch seems to decrease (get lower in pitch) as the vehicle moves away from us. The faster the vehicle is moving, the faster the pitch changes.

It should be noted that the Doppler effect will be apparent only if there is relative motion between the object and the observer. Thus, Doppler radar is effective only if there is relative motion between the object and the radar device itself. If the object and the radar device are both moving in parallel paths at the same speed, there will be no relative motion between the two and the Doppler radar will not detect any relative motion of the object because the frequency of the reflected signal will be the same as the transmitted signal.

Doppler radar is different from other non-Doppler radars in that the speed of the object can be determined by analyzing the characteristics inherent in the frequency of the returned or reflected signal. Other radars require the calculation of the object’s speed by noting the position of the object from two successive radar pulses and the elapsed time between them.

Doppler radar is advantageous for weather forecasting because it can detect not only the location and intensity of precipitation associated with a storm but also the speed (velocity) at which precipitation is moving relative to the radar station. This information is helpful to scientists and meteorologists in detecting near-ground wind shears that are dangerous to aircraft as well as giving more general information about the location and intensity of precipitation associated with a storm, wind speed, and wind direction.

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