Surveying the damage: How tornadoes are ranked after the sky clears

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It's officially the height of storm season on the Prairies. Severe thunderstorms, hail and, of course, tornadoes are all possible when it comes to our summer months.

In Canada, tornadoes happen every year, and though areas like Ontario and Quebec can see their fair share, the Prairie provinces are often the bull's eye, with between 14 and 18 tornadoes for each Saskatchewan and Alberta being considered normal.

Friday marked the first anniversary of the EF-2 tornado outbreak in Barrie, Ont. This month will mark 35 years since Edmonton's devastating F-4 tornado on July 31, 1987, which killed 27 people and injured hundreds more.

But this season has already been busy for storm chasers. We've seen EF-1 and EF-2 tornadoes in Alberta and a number of tornadoes causing damage in Saskatchewan.

But how do these headline-makers get those oh-so-familiar ratings? And what value does studying the past have in our forecasting futures? 

Learning from the past

Studying past events is no new concept. Researchers are constantly looking back to improve the response to severe weather events in the future. 

Francis Lavigne-Theriault surveys storm damage for the Northern Tornadoes Project out of Western University in London, Ont.

The project aims to assess and document every tornado that happens in Canada, through satellite, drone and on-the-ground observation. 

Lavigne-Theriault says that through his research, he hopes to improve the accuracy of tornado warnings.

He adds that building a complete foundation for where tornadoes have happened in the past will help future research. 

"We're trying to build Canada's true tornado climatology and we're trying to do the same thing with hail," Lavigne-Theriault said in an interview.

"And then eventually we're going to hopefully do the same thing with all kinds of severe weather hazards for sure."

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Kyle Fougere is a meteorologist with Environment and Climate Change Canada. Forecasting and warning for severe weather are a big part of his job, but he says he and his fellow meteorologists learn new skills from each tornado.

"When we look at the meteorological situation, just knowing the type of wind speed that was generated is definitely interesting because any future set up … you have something in your mind that you can look back on and compare it to."

Dealing in damage

That brings us back to the big question. How are these meteorological monsters rated? 

Tornadoes in Canada are rated according to the enhanced Fujita (EF) scale. It ranges from EF-0 to EF-5. The more damage from the storm, the higher the rating.

Each rating includes with an estimation of wind speeds during the tornado. For example, an EF-2 tornado has estimated wind speeds between 180 and 220 kilometres per hour. For an EF-5 tornado, wind speeds are estimated at 315 km/h or higher.

The EF scale, used by Environment Canada since 2013, is an improved version of the Fujita scale, developed in the 1970s by T. Theodore Fujita, a meteorologist with the University of Chicago.

Today, meteorologists track and forecast for severe weather, but Fougere said that with tornadoes, you never really know what is happening inside one of these storms.

"We don't have portable radars," he said. "So we have to look at the damage and infer the wind speeds and then assign a rating to each tornado based on that."

The first step in rating any tornado is making sure one actually happened. 

"We get a lot of reports of funnel clouds or possible tornadoes," Fougere said. "And so we have to determine whether or not in fact there was a circulation at the surface."

Another challenge to the ratings has to do with location. On the Prairies we have a lot of space. According to Fougere, that means some of these storms, although powerful, won't do enough damage to get a high rating.

"Even if it's a potent tornado and it occurred over a field, then there may not be enough damage to assign a rating to it. And so our rated default is EF-0."

Chasing after the storm has cleared

While some tornadoes can be assessed from afar, if there is enough damage, meteorologists hit the road.

"We have 32 different damage indicators — things like small barns, one or two family residences, manufactured homes, apartments, motels, and retail buildings," Fougere said.

"We have Canadian ones specifically, like electrical transmission lines, trees, churches, silos or grain bins."

Fougere says extensive engineering studies are conducted to find out what type of damage was caused by wind, and through that a rating is determined.

Lavigne-Theriault says a team from the Northern Tornadoes Project works with Environment and Climate Change Canada after the sky clears.

"We go around with our vehicles, we take drone pictures of the area and we develop orthomosaic mapping so that we can see patterns in the trees that help us determine if it's a downburst versus a tornado," he said.

Lavigne-Theriault was on scene July 7 as a tornado touched down near Bergen, Alta., 105 kilometres northwest of Calgary.

"I happened to see the tornado," he said. "I was there when it happened. So I knew that it was a tornado going into the event. That's generally not the case."

He says on-site surveying is necessary when you get into property damage, as opposed to just damage to trees.

"We need to look at how structurally sound and engineered the building is and the connections. All that stuff will affect the degree of damage and the rating of the tornado."

Lavigne-Theriault has had a fascination with severe weather since he was young. Through storm-chasing he has seen a tornado of every rating, and as a researcher, he is on site around 10 times each season.

"Last year was still active, so we were deployed a lot. I was the one that did the derecho in Ottawa as well in May."

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And Lavigne-Theriault says arriving on scene after a tornado is something he never gets used to.

"It's definitely humbling because then you see these [phenomena] and as a researcher, you're like, 'Wow, this is cool, it's a tornado.'

"But then you realize the impacts that it has on properties and lives."

Our planet is changing. So is our journalism. This story is part of a CBC News initiative entitled "Our Changing Planet" to show and explain the effects of climate change. Keep up with the latest news on our Climate and Environment page.