Why Birds Make Weird Circles on Weather Radars


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Why Birds Make Weird Circles on Weather Radars

Ominous donut-shaped radar bursts are 'roost rings', not doomsday explosions.


After a swarm of mayflies large enough to appear on weather radar rose up from the Mississippi River, causing car crashes and grossing out citizens of western Wisconsin, another ring of non-weather-related blip surfaced on radar this morning. Just south of Cedar Lake, Indiana around five in the morning, a doughnut-shape exploded across radar like a firework.

It wasn't a biblical-grade swarm of flies, this time, though, nor aliens, HAARP attack, or any other apocalypse to speak of.


T'was birds. Just a huge flockin' number of birds. There's even a charmingly alliterative name for their appearance on the screen: a "roost ring."

Roost rings happen when a large group of early birds rise as one, which just goes to highlight how stupid that aphorism is. Also, as the US National Weather Service in Chicago speculated that it was a large group of purple martins, a type of swallow that plucks its insect meals right out of the air, it's safe to say that none of them were heading out to eat worms anyway.

Purple martins are known to gather in big groups before their late-summer/early-fall migration. Here's a video of a flock filling the sky in Austin, Texas, on July 25 of last year.

You might think that a flock of such proportions descending upon this, uh, parking lot would also send up a roost ring, but it's unlikely that it did. Thanks to quirks of radar, as well as the relative altitudes that purple martins travel at, roost rings, like the rooster's crow, are generally morning phenomena.

As the NWS in Wilmington, OH, explained following some intense roost rings last year, "atmospheric conditions have a big impact on the path that the radar beam travels. On a typical early morning, the beam is bent slightly downward due to an inversion in the atmosphere, and it detects objects closer to the surface more easily." But by evening, the inversion isn't usually around, and neither are doughnut-shaped bird blips.

But, hey, whoever monitors the weather on the evening shift still has giant mayfly swarms to look forward to.