Why the Radar Showed It Was Raining When It Wasn't

Brian Donegan
Published: August 10, 2018

It happens on occasion: you open up The Weather Channel app, look at the radar, and it shows rain over your town, but when you look out the window, no precipitation is falling from the sky. What's going on?

Under certain weather conditions, the radar will display false echoes that look like rain or snow. Most commonly, this occurs early in the morning after a cool night with warmer air just a few hundred feet above the Earth's surface.

When the radar beam hits that layer of warm air aloft, it can be reflected back down toward the ground in a process called "ducting," as the graphic below illustrates.

Certain weather conditions may cause the radar beam to be reflected toward the ground, a phenomenon called "ducting."

A recent example of false radar echoes from ducting occurred Thursday morning in central Illinois, the National Weather Service office in Lincoln, Illinois, pointed out in a tweet.

Temperatures near the ground were 61 degrees but rose to 73 degrees at 600 feet in elevation. Once the radar beam from the Lincoln Doppler radar hit that layer of warm air aloft, it reflected toward the ground, causing non-existent rain showers to appear on the radar imagery.

Thursday afternoon, the Sioux Falls, South Dakota, Doppler radar was also showing false echoes. Much of the radar imagery was shaded green, usually indicative of a steady rain falling across the area, but there weren't even any clouds in the sky.

Ducting was also the culprit here, but not from a layer of warm air higher in the atmosphere.

This time, the radar beam was bouncing off smoke in the atmosphere originating from the Western wildfires.

(MORE: Wildfire Smoke Detected in Majority of U.S. States)

"Radar bounces energy off anything in the atmosphere, and smoke is definitely adding some additional returns to what would normally be a clear image," the Sioux Falls NWS office said in a tweet.

Birds and bugs can also contribute to false radar echoes, as can planes taking off or landing at a nearby airport, if they get in the way of the radar beam.

So the next time you open up the radar on your smartphone and it shows rain overhead, make sure to take a peek outside before canceling your morning run.

Brian Donegan is a meteorologist at weather.com. Follow him on FacebookTwitter and Instagram.


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