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Weather Talk: Radar and rain are not always the same

People sometimes become confused or frustrated when their weather app radar indicates precipitation that either is not falling, or is not falling at a rate suggested by the radar. The problem is that weather radar cannot depict precipitation perfectly or consistently because it is a remote sensing instrument. The radar beam leaves the radar site at an angle of half a degree. This means the center of the lowest scan from the Doppler radar in Mayville, N.D., is actually scanning the air at about 4,000 feet above Fargo-Moorhead and about 8,000 feet above Fergus Falls.

If the lower atmosphere is dry, significant evaporation can occur underneath the radar beam.

In addition, the type of precipitation being scanned — particularly the presence of ice at the level of the beam — can greatly affect the relative brightness of the radar echo, and therefore the precipitation display. All of this means that the colors of a radar display are a loose sliding scale of precipitation intensity.

John Wheeler

John was born in Baton Rouge, LA, and grew up near Birmingham, Alabama. As a teenager, his family moved to Madison, Wisconsin, and later to a small town in northeast Iowa. John traces his early interest in weather to the difference in climate between Alabama and Wisconsin. He is a graduate of Iowa State University with a degree in meteorology. Like any meteorologist, John is intrigued by extremes of weather, especially arctic air outbreaks and winter storms.  John has been known to say he prefers his summers to be hot but in winter, he prefers the cold.  When away from work, John enjoys long-distance running and reading.  John has been a meteorologist at WDAY since May of 1985.

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