Cloud-like formation on NWS weather radar turns out to be migrating butterflies; image via NWS

Folks at the National Weather Service were mystified recently when a bizarre cloud formation appeared on the radar over southern Illinois and central Missouri.

The cloud-like object was moving erratically across the landscape, and changing shapes along the way.

It didn't take long to realize, however, that this wasn't a cloud at all.


Monarch butterflies are known for their long, mass migrations; photo via Wikipedia

Reads a statement on the U.S. National Weather Service Saint Louis Missouri Facebook page: "High differential reflectively values as well as low correlation coefficient values indicate these are most likely biological targets."

Translation: They were critters of some sort.

Finally, it was determined that this large, mysterious cloud could be only one thing: Monarch butterflies on their southbound migration.


Massive flutter of migrating butterflies is shaped somewhat like a butterfly; image via NWS

The flock, or flutter, was so immense that it was detected by weather instruments.

The Facebook post concludes:

"A Monarch in flight would look oblate to the radar, and flapping wings would account for the changing shape!

"NWS St. Louis wishes good luck and a safe journey to these amazing little creatures on their long journey south!"

The images themselves almost look butterfly-shaped.

Monarch butterflies are famous for the brilliant coloration and long migrations, which in some cases can span 3,000 miles and involve millions of butterflies.

The insects embark on these marathon southbound adventures in early fall in order to reach their southern destinations before the onset of deadly winter weather.

Unfortunately, Monarch butterflies are in steep decline in many areas, largely because of habitat loss.

Earlier this summer, a massive swarm of mayflies over Wisconsin appeared as rain on weather radar.

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