What the EF? How meteorologists rate tornadoes

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This article was published 29/07/2015 (3926 days ago), so information in it may no longer be current.

Just because a tornado is big and bad — like Monday’s in southwest Manitoba — it doesn’t mean it’s the worst.

When measuring and rating a tornado, meteorologists make those assessments based on the wind speed, which is determined by the storm damage.

So, with apologies to the metal band Megadeth, when a tornado creates a symphony of destruction, it will get a high EF rating. A tornado that swirls around farmer’s field is going to get a lower rating.

Though Monday’s tornado was large and stayed on the ground for hours, it wasn’t as powerful as feared. Environment Canada determined Wednesday the twister rated a high-end EF-2. That means wind speeds were between 180 and 220 kilometres per hour and damage was “considerable,” including torn-off roofs, snapped trees and cars lifted off the ground. An EF-5, the highest rating involves total destruction.

The tornado that hit Elie on June 22, 2007, was rated as an EF-5. Nobody was injured. Elie is 40 kilometres west of Winnipeg.

“This can be confusing because people look at pictures of a tornado and they think, ‘oh that’s an EF-5 or that’s an EF-1’ without understanding how it’s rated,” said Terri Lang, an Environment Canada meteorologist. “The surveyors from Environment Canada go out and look at the damage that’s done. Damage to bins, property, trees and how the damage is spread out. Based on how bad the damage is, they can estimate a wind speed from that damage. Most people think if it’s a big tornado, it’s got to be an EF 5, that’s not true. Big tornadoes are not necessarily strong tornadoes.”

The tornado that ripped through southwestern Manitoba on Monday night, which lasted nearly three hours and covered about 100 kilometres as it moved, was so large and violent that Regina-based storm chaser Greg Johnson called it “the iconic tornado of 2015 in North America.”

While there was serious damage to property, especially in the Tilston area, it wasn’t considered “incredible damage” on the EF scale. No injuries have been reported.

Lang said the winds of a tornado can be compared to the spinning of a figure skater. The faster the skater wants to spin, the tighter the arms are to the skater’s body. When the skater wants to slow down, the arms are extended.

“Often some of the really tight tornadoes have some of the strongest winds,” Lang said.

Lang said the EF rating stands for Enhanced Fujita scale, named for Dr. T. Theodore Fujita, who developed the original scale that represents wind estimates based on damage caused by the tornado. The EF scale has seven levels from the lowest EF-0 (winds 104-117 km/h) to ER-5 (winds higher than 321 km/h). There is an EF-no rating, which would occur if winds were higher than EF-5 and actual damage couldn’t be assessed because of widespread secondary damage from items flying around.

“The strongest tornado recorded in Canada was in Elie… and it was an EF-5. That tornado was so small, the width of it was not very big, but it did absolutely devastating damage,” Lang said.

It took a couple of days to determine the rating of the southwest Manitoba twister because there was a long damage path.

“What we’re finding with this particular one is that it went through a lot of fields. It’s hard to assess damage to crops because not very strong winds can damage crops,” she said. “It (the tornado) needs to hit buildings and trees to get a good idea of exactly how strong it was.”

She said Environment Canada surveyors assess damage by examining such things as how buildings were destroyed, what was the pattern of things thrown about, what size of trees were snapped and if things were embedded in something.

Johnson, who said he’d chased this tornado through weather patterns from Mexico to Manitoba, said this tornado impressed him because of its many different forms — it was a typical funnel cloud, it had small twisters inside it and it was also a wedge (the funnel was as wide as it was tall) — and how long it lasted.

Lang said the smaller tornadoes embedded in the bigger one, which can be seen in videos of the event, are called suction vortices.

“So you can imagine how complex the damage pattern is. Because it was on the ground for a long time, there’s a lot things to look at and that takes some time,” Lang said.

— With files from mary Agnes Welch

ashley.prest@freepress.mb.ca