At a Glance
- A phenomenon called "ducting" can cause the radar beam to be reflected toward the ground, showing non-existent precipitation.
- Ducting can occur in the early-morning hours if the surface temperature is cooler than the air aloft.
- Birds and bugs flying into the path of the radar beam may also produce false radar echoes.
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 outthe 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.
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.
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Ducting was also the culprit here, but not from a layer of warm air higherin 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 onFacebook,Twitterand Instagram.
