On Sunday night, the Doppler radar that serves the D.C.-Baltimore area detected a cloud that wasn’t made of raindrops.

The migrating birds were flying north after a long winter’s vacation around 9 p.m. on Sunday night. They were spotted on radar over northwest Charles County, heading northwest toward Fairfax County.

The birds were flying at approximately 45-50 mph, a speed that was determined using the velocity (Doppler) product on the radar images above. The Doppler component of radar measures the velocity of objects in the air — usually things like rain drops, snow or hail, but in this case birds.

Radars often pick up odd, non-precipitation echoes, including bats leaving their caves to feed in the evening, migrating monarch butterflies, and even waves of bugs along a frontal boundary.

Did you see the birds that evening?

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