Strange circles on weather radar explained

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HUNTSVILLE, Ala. — Three strange circles showed up on a weather radar in Huntsville, Ala., on Sunday morning — and before people begin assuming it’s aliens, a meteorologist in Alabama has an explanation.

“I noticed a curious feature on the radar this morning. Within the span of 60 minutes, a few “donut rings” developed over Rainsville, Winchester, Huntsville and west Decatur,” meteorologist 

Edwards said there was no reported rain across the area this morning despite what was shown on the radar.

So what caused the rings?

Edwards writes:

“Purple Martins — which are migratory birds — have been reported in the Rainsville area recently, and given the proximity of one of the ‘rings’ in Rainsville, the radar imagery is indicating the location and movement of Purple Martin roosts departing their nests this morning.”

According to research published in the Wildlife Society Bulletin, weather radar can detect about 80 percent of the roost departures that occur in its surveillance region, and purple martins in particular tend to depart about 40 minutes before sunrise (in this case, 5:10 a.m. as indicated by the radar imagery. Sunday’s sunrise was 5:57 a.m.).

Their departure resembles an annulus, or ring, that can spread as far as 30 to 60 miles from the original roosting site.

Using radar data, the researchers also determined the purple martins can reach flight speeds as high as 30 mph.

In addition to birds, weather radars can also detect bats, bugs, buildings, trees and even trucks as they head down the highway. This is why you can see “green” on a radar, even when no rain is available to produce a signal.

Radar energy will “bounce” off of any object present in the atmosphere, rain or shine.

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