Early one morning last week, a strange cloud appeared on the radar at the National Weather Service’s office in Duluth. Along the St. Croix river near Hinckley, Minn., millions of mayflies were emerging from the water and taking to the air.

Also called fish flies, these insects spend the first one or two years of their lives on the mucky bottom of certain lakes and rivers, eating decaying plants. Then, they crawl out of the water en masse, transform from a submarine to aerial body, and spend a few frantic hours flying and mating in whirling clouds. Then they lay their eggs in the water and die. The cycle continues.

The cloud of bugs on the St. Croix was so thick at 5:46 a.m. that the weather watchers were able to see them on their screens. The same systems that detect rain and snow also register a mass emergence like this.

Because the cloud was near the St. Croix River and because it came right around sunrise, they were able to guess it was bugs. But the meteorologists say there is another way of discerning between weather and insects. The radar provides a measurement that helps.

“One way we can narrow down these returns as biological instead of meteorological is by using Correlation Coefficient,” the agency wrote. “CC is a measure of the consistency of the shapes and sizes of targets within the radar beam. Low values of CC (the green values around 0.85) point towards biological targets such as insects or birds.”

Mayflies like these are indicators of clean water and good habitat. The insects live in the water for most of their lives, emerging to fly only for a brief time as they reproduce. They need clean water and a certain type of river bottom to live to adulthood and reproduce.

“Burrowing mayflies are sensitive to gross organic pollution and do not tolerate low levels of dissolved oxygen,” says the U.S. Fish and Wildlife Service. “Annual observations of mayfly abundance in portions of the river can serve as indicators of the relative biological health of the Upper Mississippi River.”

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