Tuesday's violent storm was a downburst or a microburst preceded by a gust front, says SLU physicist
Friday, July 20, 2012 - 2:54 pm


As a life-long observer and student of the sky, I stood amazed and confused in the open bay of Reliable Auto Care on Rt. 345 watching Tuesday’s storm move eastward.

As a native of Colorado alert for tornadoes, I searched the sky for rotation as the billowing clouds moved closer and higher, widening as I watched with Dennis Bond, who took a break from recharging my AC to watch with me.

Suddenly, wind from the west hit like a shock wave, blasting us with sand and gravel from the parking area, shaking the building and sending us running to close the bays and doors. We lost power almost immediately.

Ever curious, I stood in a doorway facing north still searching for rotation above when a gust caught the heavy metal door, slamming me against the opposite jam. The bruise on my arm is my only personal evidence of the storm. I am luckier than many of my neighbors.

On the opposite side of the building, Mark Paige, an employee of LaValle Transportation, photographed what appeared to be a small tornado as the storm moved into the village.

In Canton, Danielle Tracy photographed an apparent funnel cloud moving into Potsdam. Another person was quoted as having watched a funnel cloud move up the river. So a tornado was on everyone’s mind as we watched branches and entire trees fall, power poles break and roofs peel off buildings.

I tried to convince myself I had seen rotation in the clouds and waited for the roof to start lifting off the building as I watched the rain blowing in all directions. Gradually, the wind and rain subsided and I drove out into Potsdam to observe what those amazing clouds had done.

Driving south on Route 345, I found traffic stopped due to trees and power lines from the west side lying on the road. The trees on the east side were collapsed into yards and houses. Making my way to my home street of Lower Pine via Sullivan road where someone had already dragged downed trees off the road, I encountered more trees and wires blown onto the road from the west.

Driving around the village, I noticed that, with local exceptions, collections of debris were all scattered generally eastward of their sources, not in all directions as they would have been from a tornado. These were straight-line winds, not twisting winds, so I suspected we had been hit by a derecho (day-RAY-cho).

A derecho was credited as being the source of the June 29 winds knocking out power to millions in a swath of the eastern US, leaving them steaming in the heat wave.

The term "derecho" was coined by Gustavus Hinrichs of the University of Iowa in 1899 from data gathered by the hundreds of volunteer observers in a network he had developed in his work as director of the state weather service. He coined it from the Spanish word for “direct” to match the Spanish origin of tornado from the word “tronada” for thunderstorm that has been in use since at least the 1550’s.

Due to a tiff between Hinrichs and John Price, an Iowa weather service officer who studied tornados, plus the Weather Service’s decision to ban the use of tornado to avoid panic, tornado fell out of use and derecho never caught on.

Tornado was brought back into use after the Tri-State Tornado of 1925 killed 695 people. Derecho was revived by Robert Johns and William Hirt in a 1987 paper that grew out of research following Tetsuya Fujita’s (of the Fujita scale) determination that the crash of Eastern Airlines flight 66 at JFK in 1975 was due to a thunderstorm downburst.

A deracho is actually a cluster of downbursts and microbursts. Some events we’ve experienced, like the Adirondack “microburst” of 1995 and Labor Day storm in Syracuse of 1998 were actually derechos. Formally, our storm, being less than 240 miles wide, was not a derecho, but a downburst (less than 2.5 miles wide as defined by Fujita) or microburst (more than 2.5 miles wide) preceded by a gust front. A detailed assessment of the width and length of the damage will be needed to be certain.

Downdrafts begin as raindrops fall through the thunderstorm. In passing through layers of drier air, some of the drops partially evaporate, cooling the air, just as it cools wet skin. This rain-cooled air becomes denser than the warmer air surrounding it and falls as a “cold air balloon,” the opposite of a rising “hot air balloon.”

In downbursts, the cool air hitting the ground spreads in all directions. To form a gust front, the downburst spreads primarily in the direction of motion of the storm, due to wind, a “rear inflow jet” behind the storm pushing air into the back. This incoming air is part of what made the storm appear as the funnel Danielle Tracy photographed from Canton.

In Plattsbugh, my friend Gib Brown of WPTZ-tv, confirmed that they had seen a bow echo, created only by a strong rear inflow jet, in the radar image of the storm.

Below the storm, the cold, down-drafted air moves underneath warmer, more buoyant surface air on the surface, forcing it to rise. As it rises, it cools like air pushed up a mountain, causing the moisture in it to condense and form a low line of clouds known as a shelf cloud.

This is what I observed billowing up from the tree line and Anna Costanzo photographed from the elementary school parking lot as it moved in from the west. Andrew Martin, looking slightly south of east from Raymond Hall at SUNY Potsdam, photographed the gust front just as it passed. In his image, air rising ahead of the front can be seen between the buildings in the background and the edge of the shelf cloud can be seen above it.

Behind the gust front, the cooler air from the downdraft is moving rapidly forward, up to 70 or 80 mph in the case of our storm, according to Mike Muccilli of the Burlington National Weather Service office. This speed and more downdrafts behind the front can give rise to “gustnadoes”, a portmanteau of “gust tornado”, which is what Mark Paige photographed.

These differ from tornadoes in that they are not part of a vortex coming down from the cloud, but arise from the ground and are short-lived. Dust devils in the desert southwest are similar to gustnadoes.

Once the gust front passed, much of the wind was from continuing downdrafts and gusted in a variety of directions. At that point, it was just a strong thunderstorm like we’ve had many times before.

The Potsdam downburst was an exciting event that I was thrilled to see. But my heart goes out to all whose homes and businesses were damaged. It serves us well to be reminded now and then that nature can always overwhelm our constructions, however solid we build them. This magnificent planet on which we live deserves our wonder and respect.

My thanks to the photographers and North Country Public Radio that shared the images.

Aileen O’Donoghue is the Priest Associate Professor of Physics at St. Lawrence University, teaching physics, astronomy and global climate.