Can You Guess What Caused 'Doughnut' Shape on Weather Radar? Hint: There's a Perfectly Reasonable Explanation

Just as the sun was about to rise Tuesday morning in northwestern Indiana, a cluster showed up on weather radar in the Cedar Lake area, catching the eye of some weather watchers.

It was a "donut-shaped" ring, as WBBM-TV described it. But we aren't without an explanation for what probably caused it.

A "roost ring" is caused when a large amount of birds rise from their roost site at the same time in the morning and reach altitudes high enough to be picked up by weather radar. (Image source: US National Weather Service Chicago Illinois/Facebook)

The image is most likely a mass of birds getting going for the day. When this shows up on weather radar, it's called a "roost ring."

WBBM speculated that this particular roost ring was caused by purple martins in the area as they make their migration in the late summer to early fall.

The National Weather Service out of Wilmington, Ohio, has an in-depth post explaining more about roost rings (emphasis added):

Just before sunrise on numerous mornings since mid-July, a curious doughnut pattern has appeared over several locations on NWS Wilmington, OH Doppler radar imagery. This feature, known as a "roost ring," occurs when the radar beam detects thousands of birds simultaneously taking off from their roosting site around dawn to forage for insects.

Leading up to fall migration, a number of bird species are known to gather at large communal roosting sites, which are often detected by NWS Doppler radar. The observed roost rings in recent weeks have most likely been caused by purple martins, which congregate in enormous colonies between mid-July and early August, once their fledging period has ended. By late August or early September, the martins begin their migration south, and then the roosting activity of other bird species nears its peak.

The unique doughnut pattern of these roost rings is the result of the martins 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. Purple martins typically return to the same roosting sites in the evening, which are usually situated near bodies of water. As a result, the roost rings are often observed in the same locations on radar over the course of several mornings.

NWS reported that the reverse — the birds returning at night to roosting sites — does not usually show up on radar. It's not necessarily because they're returning at slightly different times, which could be the cause in part, but because atmospheric interference, which is not usually present in the morning hours, impacts how the radar picks up images, according to NWS.

These "roost rings" are not the only non-weather phenomenon we've seen picked up on weather radar lately. See what a large mayfly hatch looked like on radar, as well as the mess it caused on the streets in Wisconsin.