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What causes a tornado?

The Story

"Good weather is boring," says Bob McDonald (future host of CBC Radio's Quirks and Quarks, but in this 1978 clip he's an employee of the Ontario Science Centre). It's the destructive "big weather" phenomena that are interesting but seldom understood. From the vortex of water going down your bathtub drain, to a funnel cloud as wide as a four-lane highway, McDonald explains the science of extreme storms. 

Medium: Radio
Program: IDEAS
Broadcast Date: Jan. 10, 1978
Guest(s): Bob McDonald
Host: Russ Germain
Duration: 8:54

Did You know?

• As with hurricanes, most tornadoes in the Northern Hemisphere spin in a counterclockwise motion, clockwise in the Southern Hemisphere.

• Contrary to popular belief, the same is not true of water going down a bathtub drain. The Coriolis force caused by the earth's rotation is too small to influence this. The direction of spin in a bath or sink is determined by the tub's surface and the configuration of taps and drains.

• Tornadoes typically move erratically in a northeasterly direction, lasting between a few minutes and a few hours. They usually occur between May and September, peaking in June and early July. Most occur in the afternoon or early evening.

• In Canada, tornadoes are measured using the Fujita scale, developed by American researcher Tetsuya Theodore Fujita in 1971. There are six categories of tornadoes, based on wind speed:
- F0 ("light"; winds 64-116 km/h): shingles, trees, windows and antennae damaged
- F1 ("moderate"; winds 117-180 km/h): trees uprooted and cars overturned
- F2 ("considerable"; winds 181-252 km/h): roofs blown off houses, mobile homes overturned
- F3 ("severe"; winds 253-330 km/h): walls and roofs destroyed, metal buildings collapsed
- F4 ("devastating"; winds 331-417 km/h): homes mostly destroyed, heavy objects hurled
- F5 ("incredible"; winds 418-509 km/h): homes levelled or blown away, large buildings damaged

• Most tornadoes in Canada are classified as weak; either F0 or F1. About 24 per cent are F2 tornadoes, while six per cent are F3 tornadoes. Only two per cent reach F4. There has been only one confirmed F5 tornado in Canadian history. It hit Elie, Man. on June 22, 2007. (Some researchers believe two others may have occurred in Saskatchewan, but were unconfirmed.)
• There were 50 F5 tornadoes in the United States between 1953 and 1999.

• There are many other types of weather phenomena related to tornadoes. These include:
- Wall clouds: part of a cloud hanging beneath a thundercloud, caused by an updraft.
- Funnel clouds: a tornado that doesn't touch down on the ground.
- Landspouts: weak, short-lived tornadoes with narrow funnels.
- Waterspouts: slender tornadoes over water that often collapse when they hit shore.
- Gustnadoes: swirls of dust or debris appearing along a storm front.
- Dust devils: hot air rising in small spinning columns. They appear only in fair weather.


Deadly Skies: Canada's Most Destructive Tornadoes more