ST. PETERSBURG, Fla. -- When a unique feature pops up on Doppler radar and there's no significant precipitation around, what could possibly be going on?

10News meteorologist Grant Gilmore noticed something strange earlier in the week, and a viewer tweeted at the station about a similar occurrence happening the next day, Tuesday, Feb. 13.

Check it out for yourself -- a green "dot" appears in southwest Polk County and spreads out in almost a perfect circle (click or tap here if you can't see the image):


The viewer who contacted 10News wondered if something was just released into the atmosphere at the area's Mosaic Lithia plant.

After some consulting with the National Weather Service in Ruskin, the radar feature isn't a chemical but something more natural: likely a lot of birds! These so-called "roost rings" occasionally appear on Doppler radar sites across the country.

As the weather service explains, the doughnut pattern appears as a "result of result of birds departing their roosting site in all directions, roughly in equal densities. As they travel further from their roosting site and reach higher altitudes, they are detected by radar until they either rise above or drop below the radar beam."

When rain appears on radar, it'll show on a sliding scale of green to yellow to red -- generally, the more vivid the color, the larger the size and shape of the object it's detecting. Large hail, for example, typically is indicated by red.

When birds are thrown into the mix, they are -- of course -- much larger than the average raindrop. You can see around 6:50 a.m. an area of yellow just northwest of Fort Meade, indicating not just the presence of large birds but many in a concentrated area.

The "ring" dissipates as the birds spread out and go under or above the radar beam.

There's one forecast that'll hold true today and, hopefully, many days on: there are no birds forecast to fall out of the sky.

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