This 70-Mile-Wide Blob Over Colorado Turns Out to Be Migrating Butterflies

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There’s a well-worn cinematic trope in which some foreboding object shows up on a radar screen, defying explanation.

On Tuesday, this happened IRL in Colorado, when the National Weather Service (NWS) radar imagery picked up a 70-mile-wide unidentified blob hovering over the Denver metropolitan area. Based on the shape and behavior of the swarm, NWS at first guessed that it might be a flock of birds migrating south for the winter.

“Look at what’s flying into Denver!” tweeted the NWS Boulder office. “Radar from last hour showing what we believe to be birds. Any bird experts know what kind?”

GIF: NWS Boulder

But as eyewitness accounts poured in from Denverites, it became clear that this mass of living creatures was, in fact, composed of migrating painted lady butterflies.

For the past few weeks, Colorado locals have been noting that these gorgeous insects, which are similar in appearance to the famous monarch butterfly, are passing through the state in higher-than-average numbers as they make their annual pilgrimage to winter refuges in the southwest United States and Mexico.

Unlike birds, which often show up on Doppler radar maps, it is relatively rare for insect swarms to trigger radar stations. But it’s not unheard of, as another painted lady swarm was picked up by radar in 2014, as the insects passed through Missouri.

The yearly migration of butterflies, especially monarchs, is considered by many to be one of the most phenomenal spectacles in nature. Increasingly, it looks like these swarms are giving radar scientists something to marvel over too.

The awe-inspiring arrival of these diminutive creatures to their winter ranges, after thousands of miles of travel, has become a popular attraction for tourists.

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