What causes tornadoes?

Environment Canada describes tornadoes as a vortex of violently circulating winds that extends from the base of a cloud to the Earth's surface. The strongest tornadoes often develop with storms that have a large area of strong wind rotation — called a mesocyclone — within the storm cloud.

The mesocyclone is created by strong updrafts in the storm and wind shear, which causes the air near the surface, whether ground or water, to spin horizontally — like a rolling pin, Environment Canada says. This spin then tilts to a more or less vertical axis and is stretched by strong updrafts, which intensifies the rotation. Strong downdrafts that wrap around the rear of the storm help to bring the rotation down to the Earth's surface. A tornado can form at the boundary between the strong updrafts and downdrafts.

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 travel from southwest to northeast. They may last only a few minutes or more than an hour and can be nearly stationary or travel at speeds of more than 100 kilometres an hour, often leaving a trail of destruction.

If a tornado looks as if it's not moving, it may either moving straight away or straight toward you, says Environment Canada.

When do tornadoes happen?

Peak months for tornadoes are June and July, but they can happen anytime from March to October. Winter tornadoes are very rare.

Tornadoes often happen in the afternoon or early evening.

How much damage do they cause?

Many tornadoes 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 level houses and cause a path of destruction more than 100 kilometres long and a kilometre wide.

How do we rate tornadoes?

The strength of a tornado in Canada is measured by the Fujita scale, which estimates wind speed inside a tornado based on the damage that's observed. The scale ranges from F0, for a tornado that pushes over shallow-rooted trees and causes some damage to chimneys and signs, to F5, when houses are lifted off their foundations, vehicles are thrown at least 100 metres, and trees are uprooted and carried large distances.

The scale was devised in 1971 by a pioneer in tornado research at the University of Chicago, Tetsuya (Ted) Fujita.

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 tornado 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 F5 tornado, 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?

On average, about 60 tornadoes strike across Canada every year, causing one death, about 20 injuries and tens of millions dollars of damage, says Environment Canada, using data gathered between 1980 and 2009.

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.