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Tuesday's funnel cloud as seen from Canton. Photo: Danielle Tracy
Tuesday's funnel cloud as seen from Canton. Photo: Danielle Tracy

Not every funnel cloud is a tornado

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Towering black clouds gave a dramatic warning of the storm sweeping toward Potsdam mid-afternoon Tuesday. Most alarming was a funnel-shaped cloud reported by eyewitnesses, and captured in photographs. Say "funnel" and lots of us think: "tornado."

But not all funnel clouds are tornadoes, even when they're apparently dropping down from a big, black cloud.

Martha Foley got an alternative explanation and another name, derecho, from Aileen O'Donogue, who teaches climatology as well as astronomy at St. Lawrence University. Aileen was on the ground watching as the storm came toward Potsdam. (Note: we'd love a high resolution picture of Tuesday's funnel-shaped cloud. E-mail images to:

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Martha Foley
News and Public Affairs Director

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Aileen O'Donogue, a professor of astronomy and climatology at St. Lawrence University, was in Potsdam when the storm arrived. She said that she never saw a funnel.

“I was in the northwest corner of Potsdam, and what I saw was this black cloud. And then white clouds started billowing up in front of it, low to the ground,” said O’Donogue. “It turns out that what I was seeing is was what’s called an arcus cloud.”

According to O’Donogue, moisture condenses high in thunder storms and falls as rain. When it condenses, it warms and dries the air. When the water evaporates, it cools and puts more moisture back into the air. This cool air is heavier and denser than the surrounding air and it drops out of the thunder storm.

“We’ve all had a thunder storm coming at us and felt that cool breeze from the thunder storm. That’s that cool air that’s dropped down from within the thunder storm,” said O’Donogue. This cool dry air falls down and goes under the moist, humid air that was originally at the surface which forces it to rise. Then, the humid air begins to condense. O’Donogue said, “So that’s the billowing clouds I saw. It’s really complicated! And also, as that air is going aloft, it pulls in air from behind the storm that then gets sucked into updrafts.”

O’Donogue says that the funnel-shaped cloud that people saw in Potsdam was a derecho. “You get, kind of, the formation of what can look like a funnel cloud. I’d really like to see a high resolution image of that photograph,” said O’Donogue. “A tornado is going to have rotation around a vertical axis. The derecho has, it’s not really rotation. But it’s stuff coming down out of the thunder storm and then going back up. So it’s a little bit a rotation around a horizontal axis.”

‘Derecho’ is a term coined in 1888 by Gustavus Hinrichs at the University of Iowa. Hinrichs came up with the name in response to articles by John Finley, who was in charge of the weather service at that time. Finley conducted several studies about tornados and called all of these events tornados. Hinrichs had his own group of weather observers and begged to differ.

 “As well as seeing this arcus cloud, and I didn’t know what it was when I was watching it, I was just fascinating and trying to decide if I should go; I was in a car repair shop,” said O’Donogue. “But the other thing I noticed about this event was that all the trees and the debris, all blew generally eastward. If it had been a tornado, it would have been – it’s like a bomb dropped, and it’s in all directions.”

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