What were those ‘rings’ on the radar this morning?

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I look for this curious feature on the radar every morning:

Within the span of 60 minutes, a few “donut rings” developed over Winchester as well as the Arsenal in western Huntsville.

Rain does not make this pattern, and usually the Valley is free of rain when I see it.

So what causes these rings?

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 prior to 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 50 to 100 kilometers, or 30 to 60 miles, from the original roosting site.

Using radar data, the researchers also determined that the purple martins can reach flight speeds as high as 13.4 meters per second, or roughly 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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