Why do radars sometimes show a cluster of 'rain' near them, when the rest of the area is clear...and no rain is really falling?

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by Jeff Renner

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KING5.com

Posted on December 2, 2011 at 7:36 PM

Updated Friday, Dec 2 at 8:59 PM

Why do radars sometimes show a cluster of 'rain' near them, when the rest of the area is clear...and no rain is really falling?

It's what we call 'clutter.' Keep in mind the geometry of the radar beam. It exits the radar antenna at an angle. The farther the beam moves from the radar, the higher it is above the ground. You can see this effect by holding a flashlight on a table in a dark room, turning it on and propping the front end up on a thin book. The beam encounters more 'stuff' at lower elevations...'stuff' like bugs, birds, pollutants, etc. Even a temperature inversion can cause 'false' echoes. We can filter some of those 'false echoes', but if we set the filter threshold too high, we risk missing some genuine precipitation. It also hinders our ability to discern wind speed and direction with the doppler mode of radar.

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