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:
In 2007, the U.S. adopted the Enhanced Fujita Scale or EF-Scale, and now, Environment Canada has done the same.
"The problem with the Fujita scale is that the winds that are associated with the damage weren't very well correlated, as far as engineering studies go," explains Sills. "The wind speeds at the high end were too high, and the wind speeds at the low end were too low. So the purpose of the EF-scale is to try to get a better correlation between the wind damage and the wind speeds."
The damage indicators for the EF-scale 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 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.”