Tornadoes

The typical tornado first appears as a rotation in a huge thunder cloud, behind a shroud of heavy rain or hail. The sky usually turns green, yellow or black. The tornado descends as a violently rotating funnel cloud and sounds like the rumble of a freight train or a jet and can be quite deafening. Tornadoes typically snake erratically from southwest to northeast, toppling buildings, scattering debris and tossing cars as though they were toys. A tornado can last just a few minutes or a few hours and usually leaves a wake of destruction.

In Canada, during an average year, approximately 80 tornadoes occur and, on average, cause two deaths and 20 injuries, plus tens of millions of dollars in property damage. These are the reported numbers, many more tornadoes strike unpopulated areas and go undetected.

The averages are also deceiving because the majority of twisters do little more than bend TV antennae, break windows, uproot trees, or damage weak structures such as barns and sheds. With wind speeds of less than 160 km/h, and a path 100 m wide by 2 km long, these small tornadoes cause less than three percent of all deaths.

The more violent tornadoes are the most devastating storms on earth. With winds approaching 500 km/h, they can level even the most solid structures. The path of destruction can reach 42 km long and 390 m wide. Fortunately, Canada has never seen such a storm.

Incidentally, the odds of dying from a tornado are 12 million to one.

Just as the Richter scale measures the intensity of earthquakes, the Fujita scale measures tornado strength. F0 is the least intense; F5 the most intense. The scale is named for Dr. T. Fujita, a pioneer in tornado research at the University of Chicago.

A tornado moves over the ground at speeds between 20 and 90 km/h. The path is usually southwest to northeast. The path of a tornado can be erratic and may suddenly change direction. If you see a tornado and it does not appear to be moving, it is either moving straight away or straight toward you.

Canada's "tornado alleys" are southern Ontario, Alberta, southeastern Quebec, and a band stretching from southern Saskatchewan and Manitoba through to Thunder Bay. The interior of British Columbia and western New Brunswick are also tornado zones.

Twisters are rare in winter, but May to September are the prime tornado months, with the peak season in June and early July. Most tornadoes occur in the afternoon and early evening.

Tornadoes' precursors are warm humid weather and thunderstorms that develop when cool northern air masses collide with hot air flowing north from the Gulf of Mexico. When complex patterns of updrafts and downdrafts in the atmosphere are added, part of the base of the thunder cloud begins to rotate and a tornado is born.

Most tornadoes look like a violently twisting funnel cloud, but some may look more like a large, low-lying cloud, a large rain shaft or even smoke from a fire. The shape can change before your eyes!

Exceptionally large thunderstorms can spawn multiple tornadoes, or a single tornado with a number of smaller but intense vortices rotating about a common centre.

Downbursts or "plow winds" are also associated with severe thunderstorms. They are intense concentrations of sinking air which fan out when they strike the earth's surface, producing straight winds with speeds up to 200 km/h. Like tornadoes, downbursts can cause extensive damage. In fact, damage attributed to tornadoes is often caused by a downburst. Such winds have also been known to produce a loud "roaring" noise, similar to that of a tornado.

Experts aren't sure why the centre of one thunder cloud will spin and another one won't. To better understand the dynamics, researchers at Environment Canada study images of actual tornado clouds from radar, satellites and photographs. This research will lead to more accurate forecasts and more timely warnings.