From Meteorologist Samantha Roberts (8/2/15 8:00 AM):
Did you know that even on bright, sunny days, we can sometimes "see things" on the radar? You wouldn't think that, right? I mean, given that we use radar to pick up precipitation.
It does happen, though. Insects, birds, chaff, smoke...we can use radar to determine the location and movement of several things. We saw this just yesterday in Baltimore and in portions of the Commonwealth.
Those circles that you see are called "roost rings," and they occur when the radar beam detects thousands of birds simultaneously taking off from their roosting site around dawn. (Most likely looking for bugs to eat!)
So, will we see the rings when the birds return to their nests? Not necessarily. Atmospheric conditions impact the path of the radar beam. Usually in the morning, the beam is bent slightly downward due to an atmospheric inversion. With the beam tilted downward, it's able to pick up on objects closer to the surface.
In the evening, an inversion is not usually present.