Tornado

A tornado is an intense rotary storm of small diameter (tens or hundreds of metres), characterized by at least one vortex reaching the Earth's surface. The vortex is usually visible as a funnel cloud associated with a THUNDERSTORM, although on occasions it is invisible. In either case, damage results at ground level. A tornado may be composed of a single funnel, several funnels occurring simultaneously, one or more funnels reforming one after the other or various combinations of these states.

A tornado's life cycle typically consists of 3 stages. It starts when a funnel-shaped protuberance (tuba) develops beneath a rotating section of the southwest flank of a thunderstorm. The tuba elongates downwards from the cloud and is enveloped by a rotating sleeve (annulus) that develops upwards from the ground. The full merging of the 2 constitutes the mature tornado vortex. This stage is followed by the third or degenerating phase, when the tuba rises back towards the cloud base and disappears. Cases where the tuba alone develops are called "funnels aloft" and are not tornadoes. The rotation of tornadoes is almost always cyclonic (clockwise in the northern hemisphere, anticlockwise in the southern hemisphere, when the circling clouds are viewed from below).

Tornadoes occur in all parts of Canada except those with an arctic CLIMATE. They are relatively frequent in the interior, from New Brunswick to the Rocky Mountains, and are most common in southern Ontario (which experiences an average of 21 per season), followed by southern Manitoba. In Canada, the tornado season begins as early as March and ends as late as October, generally lasting 107 days in the interior and about 60 days elsewhere. Activity peaks in late June and early July. The average tornado causes a damage swath with median values of 6.2 km in length, 83 m in width and 0.6 km² in area. A tornado is most likely to occur in the afternoon, 3-7 pm local standard time, and to approach from the west or southwest.

Tornadoes range in intensity from very weak (winds from 64 km/h) to devastating (winds up to 509 km/h), on a scale of 0 to 5 devised by T.T. Fujita. More than 90% of Canadian tornadoes can be categorized as weak, F0 to F1 on the Fujita scale. Tornadoes ranked F0 are called light and are characterized by winds of 64-116 km/h, which cause damage to roofs and trees; those ranked F1 are classified as moderate, with winds 117-180 km/h causing damage such as cars being overturned, carports torn apart and trees uprooted. Severe tornadoes recorded in Canada include the Regina tornado of 30 June 1912, which killed 28, injured hundreds and demolished much of the downtown area, which was rated as F4 (devastating, with winds of 331-417 km/h). It was rivalled by the Edmonton tornado of 31 July 1987, which took 27 lives, injured more than 200, left more than 400 homeless and caused damage along a 40 km track estimated at more than $250 million, "the greatest single storm loss in Canadian history." The severity of this tornado is unprecedented in Alberta.

The first officially documented F5 tornado in Canada struck Elie, Manitoba, 22 June 2007. An F5 tornado is classified as incredible and has winds of 418-509 km/h. It is strong enough to lift strong frame houses off their foundations, hurl automobile-sized objects more than 100 m through the air, strip bark off trees and cause considerable damage to reinforced-concrete structures. The Elie tornado was on the ground for 35 minutes and travelled approximately 5.5 km, causing damage along a swath 300 m wide. Winds reached 420-510 km/h at the tornado's most intense point. Fortunately, there were no fatalities or serious injuries.

Canada ranks second in the world for the occurrence of tornadoes, averaging 80-100 tornadoes annually, compared to an average of 1000-1200 in the United States. The incidence of tornadoes in Canada is probably higher than that, but Canada has large, unmonitored and sparsely populated areas where tornadoes may go unreported.

Another example of an F4 tornado occurred as part of a major tornado outbreak (in this case a family of tornadoes moving along 8 well-defined tracks) that swept across southern Ontario 31 May 1985, causing an estimated $100 million in property damage. This particular tornado devastated the southern section of Barrie and was responsible for 8 of the 12 deaths that occurred during the outbreak. Fortunately less than 1% of all tornadoes fall into the F4 category; none of F5 intensity are known to have occurred in Canada.

Author MICHAEL J. NEWARK

Links to Other Sites
Is Your Family Prepared?
This site offers useful tips and guidelines for preparing and responding to earthquakes and other extreme natural events. Click on the "hazards poster" on the right to download a map of Canadian locations prone to various natural events. Includes brief notes about each type of event. From Public Safety Canada.

Sky Watchers
A guide to interactive meteorology activities for students and teachers from Environment Canada.

Fujita Scale
Background information about the Fujita scale, in which the level of damage caused by a tornado is used as a measure of its severity. From Environment Canada.

Top Weather Events of the 20th Century
Check out the top weather events of the 20th Century from Environment Canada.

Inside a supercell tornado
Watch a video depicting the damage caused by violent tornadoes and learn about updrafts, vortices, and funnel clouds at this CBC website.