Tornado 101: the science behind the might
They are the most destructive storms on the planet, but tornadoes like the twin twisters that struck Ottawa and Gatineau on Friday are also among the most limited, capable of flattening a home and leaving the neighbour next door virtually unscathed.
Southern Ontario and the Prairies are the parts of Canada most prone to tornadoes, but because the twisters tend to be short-lived and very localized, many go unseen and unreported in low-population areas. Canada averages about 60 tornadoes a year, but few are as powerful and occur so late in the year as the Dunrobin-Gatineau EF3 twister.
“At a preliminary EF3 rating, the Sept 21 Dunrobin-area tornado is only the 2nd E/F3 or higher recorded in Canada in Sept or later. The other was an F3 in Sept 1898 in the Niagara region — well south and west. The 2011 Goderich F3 was the last recorded Ontario E/ F3,” tweeted Dr. Dave Sills, an extreme-weather expert with Environment Canada and one of Canada’s foremost tornado experts.
Sills was one of 10 Environment Canada experts to tour the devastated sites over the weekend.
Tornadoes are measured on the Enhanced Fujita Scale, which rates them on a scale of zero to five, with five the most powerful. Since it’s impossible to physically measure the speed of a tornado’s winds, investigators observe and measure the damage done to estimate its size and intensity.
Eyewitnesses can describe the storm’s motion, while the direction of downed trees can show if the winds were rotating. That pattern helped investigators confirm that the second, smaller event in the Arlington Wood and Craig Henry neighbourhoods was, indeed, a tornado.
The Dunrobin-Gatineau tornado is considered a “high E/F3,” while the second twister across the west and south of Ottawa was rated E/F2.
Though tornadoes differ in intensity, they share a common genesis with a layer of warm, moist air overlain by a layer of fast-moving colder, dry air. As the air masses move past each other and the warm air rises like a hot-air balloon, the air column begins to rotate. Fed by the power of the rising warm air and the plunging cold air, the spinning can intensify to produce a tornado’s characteristic funnel cloud.
Tornadoes can occur at any time of day and in any season, but are most common in spring and summer and in afternoon and early evening.
The Dunrobin tornado cut a swath a kilometre wide that stretched 40 kilometres and possibly as far as 70 kilometres. Wind speeds in an E/F3 tornado can reach up to 260 km/h. Friday’s tornado advanced along its path at about 80 km/ h.
Powerful E/F5 storms that strike the central and southern United States can reach more than 1.5 kilometres in diameter and can persist for an hour or more.
Canada has the second highest occurrence of tornadoes in the world, behind the U.S., but tornadoes have been recorded on every continent except Antarctica.