U.S. President Barack Obama made a disaster declaration for Oklahoma on Tuesday after the state was hit by a powerful tornado that killed a reported 24 people in the town of Moore.
The tornado in Oklahoma, which generated winds as strong as 320 km/h and was classified an EF-4 on the enhanced Fujita scale, fell in a geographical area of the U.S. generally known as "Tornado Alley."
Here’s a closer look at this storm-plagued region — and its counterparts in Canada.
What is "Tornado Alley"?
"Tornado Alley is a colloquial expression, there’s no formal definition of it," says Howard Bluestein, professor of meteorology at the University of Oklahoma.
He says it’s a geographical area in the U.S. that not only experiences a high frequency of tornadoes, but "fairly substantial" ones as well.
According to the website of the National Severe Storms Laboratory, which is operated by the National Oceanic and Atmospheric Administration (NOAA), Tornado Alley "is a nickname invented by the media for a broad area of relatively high tornado occurrence in the central U. S."
The science news site LiveScience reports that, over the past decade, the area covered by Tornado Alley has averaged 298 tornadoes for the month of May alone.
Where is it located?
Tornado Alley is generally described as an area running from north-central Texas, up through central Oklahoma and central Kansas into Nebraska, and curving off into Iowa, says Bluestein. Some people include parts of Minnesota and Wisconsin, as well.
Although it is not considered part of Tornado Alley, Florida also sees a high incidence of tornadoes, having averaged 66 a year between 1991 and 2010, according to the NOAA.
"Florida gets a lot, but they’re not the supercell tornadoes," says Bluestein, referring to the classification of large, destructive tornadoes that plague the central U.S.
Canada has its own Tornado Alley, says Geoff Coulson, a warning-preparedness meteorologist at Environment Canada.
"There are a couple of areas that we look at as being very active for tornadoes, as well as other forms of summer severe weather," says Coulson.
One is in southern Ontario stretching from Windsor up through London to Barrie, Coulson says. The other is the southern prairies, extending from Alberta to Manitoba.
Why are these areas prone to tornadoes?
A tornado is a fiercely churning column of air that extends from the base of a cloud to the Earth’s surface. It is formed through the mixture of warm, moist air with cooler, drier air.
Tornado Alley is the product of several meteorological factors, says Bluestein. The first is that the area is downstream of the Gulf of Mexico, which is a source of warm, moist air. The Rocky Mountains to the west, on the other hand, produce cooler, drier air.
Bluestein says that when winds from the west become superimposed over winds from the south, it can result in a change of wind direction and speed, which is called wind shear. This can result in great instability in the atmosphere.
The reason the Canadian prairies are vulnerable to tornadoes is that they get much of the warm, moist air working its way up from the American Deep South, which then combines with the drier air from the Rockies, says Coulson.
Meanwhile, southwestern Ontario is "prone to a lot of humidity in the spring and summer months," which acts as "a fuel to drive the formation of thunderstorms," says Coulson.
During the summer, lake breezes also play a role, he says. As those lake breezes move inland, they collide with cooler land air and force the air to rise, in some cases in "a fairly violent fashion."
Has climate change influenced the intensity of tornadoes in these areas?
Energy from climate change has increased the temperature of the oceans by about one degree Fahrenheit over the last 30 to 40 years, and as a result, there’s about a four- to five-per-cent increase in the amount of water vapour over the oceans, says Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo.
Tornadoes like the one in Oklahoma effectively begin with moisture coming from the Gulf of Mexico, and any additional vapour will likely affect their severity, says Trenberth.
"The estimate is that [the added vapour] produces a five to 10 per cent effect in terms of precipitation and maybe as much as that in terms of the intensity of storms. It’s a relatively small effect, but in the situation where you’ve already got strong storms from natural [weather] variability, adding a little bit more means you can break records," he says.
Environment Canada, however, is still investigating the correlation between climate change and severe weather.
"I still haven’t seen any definitive studies on that," says Coulson.
"One of the keys for us is getting a better database of historical events – how many [tornadoes] were we getting in the early part of the 20th century, how many are we getting now."