It looked like an invasion of Cleveland — and in a way, it kind of was.

National Weather Service radar in recent days has captured some dramatic images of billions of mayflies making their annual summer hatch.

A particularly stunning June 27 hatch burst onto radar in a sudden, rapidly expanding blob over western Lake Erie. Then the winds moved and scattered the flies off the lake to the southeast.

"Radar loop from last night showing ascent and downwind drift of several billion Hexagenia #mayflies over the western basin of Lake Erie.

Yes. That is billion with a 'b,'" tweeted Phil Stepanian, a biometeorology research assistant professor at the University of Notre Dame.

That was revelatory information for Edward Verhamme, an environmental engineer with LimnoTech whose projects frequently involve the Great Lakes.

"I was on Lake Erie yesterday on a research boat … there were massive mats of dead carcasses on the surface of the water. We covered 100 miles yesterday right from Toledo to Point Pelee … great to see what the heck happened the night before," he responded to Stepanian's tweet.

The annual mayfly hatch is massive enough that it's possible to see it on weather radar pretty much every year if looking in the correct place, said meteorologist Kyle Klein at the National Weather Service's White Lake station.

A few days later, on July 1, another Lake Erie mayfly hatch burst onto the Cleveland area weather radar screen.

According to the Weather Service, the life cycle of a mayfly starts out as a larva that resides on river or lake bottoms. After three months to two years, depending on the species, they emerge as winged adults and fly in swarms to mate. Once they emerge, they only live for a few minutes to a few days, depending on the species.

Around the Great Lakes, one of the largest of the mayfly species, the Hexagenia genus, generally emerge as winged adults in late June to early July.

The adults are often attracted by lights and can fly several miles inland, commonly creating a slippery mess as they pile up and die on the pavement below the lights. At times they have been so thick on bridges that they had to be cleared with snowplows, the Weather Service noted.

As inconvenient and yucky as mayflies can be, they are actually a harbinger of good ecological news. The National Weather Service notes that mayflies are sensitive to gross organic pollution such as sewage in waterways. So the flies' thick annual presence is a sign organic pollution isn't present in large amounts.

Contact Keith Matheny: 313-222-5021 or kmatheny@freepress.com. Follow on Twitter @keithmatheny.