Scientists befuddled by huge 70-mile wide cloud of butterflies, caught on radar

National Weather Service
via National Weather Service

Facing challenges from climate change and increasing loss of habitat, butterfly populations worldwide have been affected one way or another. So it's a real wonder when you see them in large numbers, as weather scientists at the National Weather Service did last month on their radar screens. However, the meandering blob flying near Denver, Colorado on their monitors was so large -- about 70 miles across -- that they first mistook it for a group of birds.


As the Denverite reports, the staff at the Boulder meteorology office initially ruled out the possibility of an insect horde, since they “rarely produce such a coherent radar signature.” But after posting the image above to social media, the scientists got their answer: it was an enormous group of painted lady butterflies, slowly making their way from north down to warmer climes in the south.

Queerbubbles Queerbubbles /CC BY-SA 3.0

“Migrating butterflies in high quantities explains it,” they said, after coming to the conclusion that the painted lady butterflies, which travel between the central and southwestern United States and northern Mexico every autumn, showed up on the radar due to their large wings and because they were all flying in the same direction. Another big hint was the fact that the blob seemed to be following the wind; butterflies are able to fly long distances, sometimes over 200 miles in a day, thanks to the carrying power of the prevailing winds. Birds, however, tend to fly more straight toward their destination, regardless of wind direction.

Patrick CoinPatrick Coin/CC BY-SA 2.0
Patrick CoinPatrick Coin/CC BY-SA 2.0

Recognized for their orange-black markings and two large spots under its wings, the American painted lady (Vanessa virginiensis) butterflies are found in many parts of temperate North America, and their population size swells in relation to when flowers -- a main source of food -- bloom abundantly. Populations of this species have also been found in Europe, migrating to Africa during cold weather. For more, visit Denverite.

Tags: Denver | Insects

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