Downburst wind gusts were clocked at 160 km/h
Britannia Yacht Club instrument recorded velocity
The storm that struck Ottawa on Wednesday produced a wind gust of 160 km/h at the Britannia Yacht Club, near the heart of the damage zone, Environment Canada says.
As well, it says the Britannia instrument measuring wind speed recorded a one-minute period at the storm’s height with an average wind speed of 140 km/h.
By comparison, an EF1 tornado has winds of 138 to 177 km/h.
That scale goes up to EF5, with winds greater than 322 km/h.
The information comes from WeatherFlow, a U.S. company that manufactures wind meters and which installed the Britannia device, said Environment Canada’s Peter Kimbell.
Environment Canada’s closest wind instrument is at the Central Experimental Farm, well outside the storm’s centre.
It recorded a top gust of 89 km/h.
Wednesday’s storm was a downburst of air rushing down from high in the atmosphere and bouncing sideways at ground level, Kimbell said.
“We’re talking about straightline winds that are coming as a downdraft” from thousands of feet overhead, he said.
Unlike a tornado, there’s no circular motion.
“Damage from downdrafts is much more common” than tornadoes, he said.
“The downbursts can do the same kind of thing (damage) and are caused by the descending air currents from the thunderstorm that reach the ground and then spread out and cause damage as well.”
The downburst came down on an angle — sideways from the west wind rushing through Ottawa, and also down. (The effect is sometimes called a microburst, but Kimbell says this term is somewhat out-of-date.)
Here’s how they develop:
“As the air currents rise in the thunderstorm they develop raindrops. The raindrops get larger and larger” until their weight makes them fall.
“As they fall they basically drag the air with them. And they also can fall into dry air which evaporates and cools the air. As the air becomes cooler it accelerates downward and causes these downbursts.
“It’s coming sideways and down at the same time,” and it deflects sideways when it hits the ground.”
The downbursts can do the same kind of (damage as tornadoes) and are caused by the descending air currents from the thunderstorm that reach the ground.