Tuesday’s violent storm was a ‘microburst’ or ‘downburst’ preceded by a ‘gust front,’ says SLU physicist
Friday, July 20, 2012 - 4:46 pm

Anna Costanzo photographed the storm billowing up next to Lawrence Avenue Elementary School as she was looking west as the storm moved east. The clouds hanging down are a shelf cloud. (This cutline has been corrected from what was originally posted).Anna Costanzo photographed the storm billowing up next to Lawrence Avenue Elementary School as she was looking west as the storm moved east. The clouds hanging down are a shelf cloud. (This cutline has been corrected from what was originally posted).

Many people who were watching the sky when the big storm hit last week swear they saw a funnel cloud, and some even got photographs. The National Weather Service said the violent winds did not come from a tornado, so what was it? We asked St. Lawrence University physicist Aileen O’Donoghue to examine the available evidence and explain it. View a more detailed version of this story with additional photos here.

By AILEEN O’DONOGHUE

The violent thunderstorm that struck Potsdam and St. Lawrence County July 17 appears to have been a downburst or a microburst preceded by a gust front.

I noticed that, with local exceptions, the debris was 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 originally 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.

Chad Hogeboon shot this photo of the approaching storm while in the parking lot of the Mohawk Bingo Palace in Hogansburg. He was looking towards the St. Lawrence River and Canada.Chad Hogeboon shot this photo of the approaching storm while in the parking lot of the Mohawk Bingo Palace in Hogansburg. He was looking towards the St. Lawrence River and Canada.A deracho is actually a cluster of downbursts and microbursts, and some events we’ve experienced, like the Adirondack “microburst” of 1995 and Labor Day storm in Syracuse of 1998, were actually derechos.

But formally, the July 17 storm, being less than 240 miles wide, was not a derecho, but a downburst (less than 2.5 miles wide) 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. 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 to many people.

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 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.

Behind the gust front, the cooler air from the downdraft is moving rapidly forward, up to 70 or 80 mph in the case of the July 17 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.”

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.

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