Why Do Tornadoes Form?

Each year, mainly between March and July, tornadoes kill dozens of people in North America, injure many more and cause hundreds of millions of dollars’ worth of damage.

But what if we could forecast the intensity of tornado activity months, even seasons, in advance? That’s the ultimate goal of researchers at U of T Scarborough, who have developed a new model of tornado formation. “The eventual aim is to predict ahead to the following year or subsequent years about whether we’ll get above or below average tornado activity in a given area,” says Vincent Cheng, a postdoctoral fellow in UTSC’s Ecological Modelling Lab. “The first step is to fully understand what has caused these storms in the past.”

The model, developed by Cheng and professors George Arhonditsis and Bill Gough in UTSC’s Climate Lab, along with colleagues at Environment Canada, employs large-scale atmospheric variables such as those used by weather forecasters. The researchers examined variations in monthly and seasonal tornado activity over a 30-year period relative to changes in atmospheric conditions over the same time frame.

Cheng says the model explains how different conditions in the atmosphere during a thunderstorm affect the risk of a tornado. Key variables include the instability of the atmosphere and vertical wind shear, which is the change in wind speed and wind direction at different heights above the ground. It turns out there’s a much higher risk of a tornado when air is able to rise quickly, coupled with strong vertical wind shear.

Canada is second only to the United States in the number of tornadoes experienced, with more than half of them taking place in the Prairies and Northern Ontario and a third in Southern Ontario.