Last month, meteorologists at the U.S. National Weather Service picked up some strange-looking, slow-moving clouds on their radar images. Even stranger, the “clouds” appeared on a bright, clear day.
Looming over St. Louis, Missouri, the clouds were spotted moving south over southern Illinois and central Missouri.
“It was a completely clear day—there were not even high clouds—and so we were all kind of scratching our heads saying ‘what are we looking at here?’” Laura Kanofsky, a meteorologist at the NWS St. Louis office told The Washington Post. “It was not ground clutter, or mid-level clouds, which we can sometimes get.”
Now, after studying the radar echoes, meteorologists have come up with an explanation for the bizarre mass: butterflies.
On their Facebook page, they wrote: “High differential reflectivity values as well as low correlation coefficient values indicate these are most likely biological targets. High differential reflectivity indicates these are oblate targets, and low correlation coefficient means the targets are changing shape.”
In non-scientific speak, the “biological targets” mean migrating Monarch butterflies and “the changing shape” refers to the butterflies’ fluttering wings as they fly to Mexico.
Every year, hundreds of millions of Monarch butterflies from the eastern American states migrate more than 5000-kilometers to a small area in Mexico. The Canadian Monarch butterfly population east of the Rocky Mountains migrates to Mexico as well, while the butterflies of the west travel to California.
And although the radar images suggest a huge mass of butterflies migrating—which is hopeful sign of their health, considering the population of Monarch butterflies has severely declined due to climate change, deforestation, and the use of GMO-filled crops—Kanfosky says the images can be deceiving.
“In dry conditions, the radar is very sensitive to something like insects. It doesn’t take a whole lot of insects to create a high return.”
Adding to the mystery, the mass even resembles a very rudimentary outline of a butterfly. (Or rather, it resembles a “butterfly” that your three-year-old would paint using only their hands.)
“It was not something we were expecting,” said Kanfosky. “It was really interesting.”