For the past week or so one of our weekly guests on WTVY This Morning’s “Talk of the Town” has been asking me to blog about the principles of Doppler Radar. Well, I’ve been putting it off primarily due to laziness, but since we use radar so much, (and because we talk to this particular guest tomorrow) I decided to buckle and explain.
The Doppler Effect is based on a pulse frequency. In weather we use a WSR, Weather Surveillance Radar, to measure intensity, direction and movement of different weather phenomena such as rain, sleet, hail, snow. When a Doppler signal is sent it is searching for something to bounce back off of. The frequency changes when something moves closer or farther away from an object.
An example of this would be an ambulance siren, it sounds different moving away from an object than it does moving towards it, because it sends different sound signals as it changes direction.
*** An analogy would be pitcher throwing one ball every second in a person’s direction (a frequency of 1 ball per second). Assuming that the balls travel at a constant velocity, if the pitcher is stationary, the man will catch one ball every second. However, if the pitcher is jogging towards the man, he will catch balls more frequently because the balls will be less spaced out (the frequency increases). The inverse is true if the pitcher is moving away from the man; he will catch balls less frequently due to the pitcher’s backward motion (the frequency decreases). If the pitcher were to move at an angle but with the same speed, the variation of the frequency at which the receiver would catch the ball would be less as the distance between the two would change more slowly.
Note that, from the point of view of the pitcher, the frequency remains constant (whether he’s throwing balls or transmitting microwaves). Since with electromagnetic radiation like microwaves frequency is inversely proportional to wavelength, the wavelength of the waves is also affected. Thus, the relative difference in velocity between a source and an observer is what gives rise to the doppler effect. *** Taken from Wikipedia.
With weather, signals are bouncing off particles and sending back a particular frequency. This is measured in different colors on radar.
- magenta: 65 dBZ (extremely heavy precipitation, possible hail)
- red: 52 dBZ
- yellow: 36 dBZ
- green: 20 dBZ (light precipitation)
Strong returns (red or magenta) may indicate not only heavy rain but also thunderstorms, hail, strong winds, or tornadoes, but they need to be interpreted carefully, for reasons described later in this article.
Radar is not only used to measure intensity, direction and speed…but also to recognize specific weather “signatures” such as hook echos, which indicate rotation and are often a precursor to tornadic activity.