Severe weather season is upon us and as thunderstorms pop-up more frequently in the forecast, the chance for tornadoes increases as well.
The first tornado of the season was confirmed in Fergus, Ontario on April 27th. Meanwhile, parts of the southern U.S. already faced a deadly tornado outbreak during the month of April. While the destruction from these powerful storms is clear, rating the damage intensity can be murky.
Dr. T. Theodore Fujita first introduced the Fujita Scale in the early 1970's. He wanted to use something that could categorize each tornado by intensity and area.
“It's mainly for meteorologists to look at damage and decide what the intensity of damage is on the Fujita Scale,” explains Dave Sills, Severe Weather Scientist with Environment Canada.
The Fujita or F-Scale measures wind and its damage. The scale is divided into six categories with F0 being the weakest and F5 being the strongest:
“So once we know what the damage intensity is, we can estimate the wind speeds associated with that damage,” says Sills.
The Fujita Scale is still used for measuring storms across Canada, but in the United States, scientists have adopted the Enhanced Fujita Scale to try and address some of the issues with the original F-Scale.
When Fujita first implemented the damage scale, he was simply taking his best guess at what the wind speeds would be.
“And so the reason that the Enhanced Fujita Scale was created was to try to better correlate the wind speed with the damage rating. They actually got a committee of experts together, came up with 28 damage indicators, and decided what the damage to wind speed relationship would be for different structures,” notes Sills.
The damage indicators range from farms and residences, commercial and retail structures, schools, professional buildings, metal buildings, towers and poles and vegetation like softwood and hardwood trees.
Sills adds that the EF-Scale is more in line with the kind of winds that engineers would expect with the damage that occurs. Based on engineering science, the more narrow ranges of the new EF-Scale are more reflective of the actual wind speeds, especially for strong and violent tornadoes.
“The range on the F-Scale goes from 60 km/h (F0) to 510 km/h (F5), quite a wide range. Whereas the EF-Scale starts at 105 km/h and an EF5 is greater than 320 km/h so it's open ended at the top. An EF5 would probably be rated somewhere between 320 km/h maybe up to 400 km/h, but that's about it. So it doesn't get anywhere near 500 km/h like the F-Scale,” explains Sills. “The EF- Scale still preserves the damage ratings, it just changes the wind speeds.”
Although Canada hasn't taken on the enhanced rating system just yet, Sills says it will be used here eventually.
“They came up with the new wind speeds based on building standards down in the U.S., which are sometimes quite different from what we have in Canada. So the approach here is to wait and see what experiences the U.S. has with it and we can evaluate while they're doing that and get the bugs worked out of it before we adopt it here.”