Forces of nature
Winds of destruction
Tornadoes
Last Updated: Thursday, October 1, 2009 | 2:32 PM ET
CBC News
Damage in Elie, Man., the day after an F5 tornado tore a long, wide path of destruction through the community. (John Woods/Canadian Press) What causes tornadoes?
Tornadoes occur when warm, humid air meets cool, dry air. When the two meet, the cool air traps the warm air, and keeps it from rising. The trapped air is further warmed by radiant heat of the earth's surface.
Trapped, the warm air rotates horizontally and finally gains enough strength to punch through the ceiling of cool air. The cool air then begins falling quickly, forcing more warm air upward. Often, this swiftly falling air — known as a downburst — is mistaken for a tornado when it strikes the Earth's surface.
Indeed, downbursts can be just as destructive as some tornadoes, and can generate a 200-km/h blast of tree-flattening wind.
The swiftly moving air begins to spin like a top as it moves. If it spins fast enough, a funnel forms. When a funnel touches the ground, it is referred to as a tornado.
What are the signs of a tornado?
A tornado usually first appears as a rotation in a huge thundercloud, often behind heavy rain or hail. The sky often turns green, yellow or black. The sound can be deafening, like the rumble of a freight train.
How does a tornado move?
Tornadoes often wind erratically from southwest to northeast. They may last only a few minutes — or more than an hour — travelling at speeds from 20 to 90 km/h and often leaving a trail of destruction.
If a tornado looks as if it's not moving, it is either moving straight away or straight toward you, says Environment Canada.
When do tornadoes happen?
Peak months for tornadoes are June and early July, but they can happen anytime from May to September. Winter twisters are uncommon.
Tornadoes often happen in the afternoon or early evening.
How much damage does they cause?
Many twisters do little more than uproot trees and cause minor damage to barns and sheds, but the more violent tornadoes can be devastating.
The most severe tornadoes can completely level houses, and cause a path of destruction over 100 km long and over a kilometre wide.
How do we rate tornadoes?
The Fujita scale is used to rate the severity of a tornado. F0 ranks as the least intense, while F5 is the highest.
Devised in 1971 by a pioneer in tornado research at the University of Chicago, Tetsuya (Ted) Fujita, the scale is used to rate the severity of tornadoes as a measure of the damage they cause.
| Intensity | Wind speed | Type of damage |
|---|---|---|
| F0 | 64-116 km/h | Damage to trees, shingles, antennas and windows. |
| F1 | 117-180 km/h | Trees uprooted, cars overturned. |
| F2 | 181-252 km/h | Roofs blown off homes, sheds destroyed, mobile homes flipped. |
| F3 | 253-330 km/h | Walls, roofs destroyed, metal buildings collapsed, forests destroyed. |
| F4 | 331-417 km/h | Well-built homes mostly destroyed, heavy objects thrown long distances. |
| F5 | 418-508 km/h | Homes destroyed and/or blown great distances, roofs blown off larger structures, which are otherwise heavily damaged. |
What's the strongest tornado Canada has had?
The strongest documented twister in Canada's history is the one that hit the southern Manitoba community of Elie on June 22, 2007, according to Environment Canada.
It is the only officially confirmed tornado as an F5, the highest rating on the Fujita scale.
The Elie tornado cut a swath of damage up to 300 metres wide, travelled for about 5.5 kilometres and stayed on the ground for 35 minutes. Wind speeds reached 420 to 510 km/h.
No one from the community, population 550, was killed or seriously hurt. Nineteen people were left homeless.
What are the chances of death from a tornado?
According to Environment Canada, the odds of dying from a tornado are 12 million to one.
An average of 80 tornadoes strike across Canada every year, causing about two deaths and 20 injuries, Environment Canada says. Damages can run into the tens of millions of dollars.
What should you do if a tornado strikes?
According to most weather authorities, including Environment Canada, your best option in the event of a tornado is to head for your basement. If you don't have one, get in a closet or small room near the centre of the building, away from windows or doors.
Environment Canada suggests getting into the bathtub and covering yourself with a mattress. Otherwise, get underneath a sturdy piece of furniture which can help shield you from falling or flying debris.
Stay away from school gyms, rinks and other places with wide, unsupported roofs.
If you're caught outdoors or in a vehicle, find the nearest ditch and lay low with your head shielded by your arms.
If you're in a mobile home, get out and find a permanent shelter (preferably with a basement) or find a ditch. More than half of all tornado deaths occur in mobile homes, Environment Canada says.
