Issue 42
April 16, 2004

 Any Questions?

Environment Canada's knowledgeable specialists answer questions about wildlife, air pollution, water, weather, climate change and other aspects of the environment.

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What causes a tornado to form, how long does the average tornado last and how is it tracked? Stephanie A. Smith, Reading, PA

There are three ways a tornado can form – from a particular pattern of winds in the lower atmosphere (supercell tornadoes), by overrunning a vortex that already exists in the atmosphere (a landspout), or by making a vortex from a strong wind downburst (a gust tornado or gustnado). The supercell tornado is the biggest and strongest, but the landspout is one of the most common. The gust tornado is a bit of a mystery especially as they frequently occur at night when they are hard to see.

Supercell Tornado

The supercell tornado forms when the winds in the lower atmosphere are in the right configuration and get pulled up into a thunderstorm, where they cause it to begin rotating. The "right" configuration is what is known as a veering wind profile – say southerly winds at the surface, southwest winds at 2000 feet and west winds at 8000 feet – a clockwise turning of the wind in other words. Of course you need the ingredients for a thunderstorm first – instability, moisture (the fuel) and some sort of trigger (a cold front is a good trigger, but there are many others).

These are pretty common ingredients whereas tornadoes are pretty rare, so there must be something else – and there is. Recent research suggests that the cold air that drops out of a thunderstorm while it is raining wraps around the back of the larger thunderstorm rotation (called a mesocyclone) and forms the actual tornado. The mesocyclone provides the environment that allows this second mechanism to build up into the thunderstorm and become a tornado.

Landspout Tornado

As for landspouts, moving air very frequently has big and little vortices, in much the same way that a rushing stream has little whirlpools that are carried along in the flow. These vortices are often found on boundaries between two air masses – along a weak front, or at the junction where two different wind streams collide. If a thunderstorm passes over one of these pre-existing vortices, the vortex can be pulled up into the storm and a weak tornado can form.

Gust Tornado

Gust tornadoes are formed on the edges of a strong downburst wind flowing out of a thunderstorm, usually one with heavy rain. The rushing wind forms vortices along its edges (think about squirting a hose into a swimming pool – the jet of water will have little whirlpools on its edges). These vortices can form little tornadoes under the right conditions but are usually not connected to the clouds above. In many respects gust tornadoes are more closely related to dust devils than to supercell tornadoes. We often have reports of damaging straight-line winds where a gust tornado was also spotted and it sometimes becomes very difficult to separate the two events from each other.

Average Duration

The length of time a tornado lasts is highly variable – from seconds to hours. Five minutes is typical, but no detailed statistics have been collected.

Tracking Tornadoes

Environment Canada's weather radar is a prime tool for detecting heavy precipitation, rapidly developing cloud systems and other signatures of severe thunderstorms, some of which can spawn tornadoes. Weather radars cover the areas inhabited by 95 per cent of Canada's population. Each radar scans a radius of about 325 km, although the best range for detecting tornadoes is between 30 and 120 km. May to September are the prime tornado months, with the peak season in June and early July. The afternoon and early evening are the peak times. Twisters are rare in winter.

Radar tracks thunderstorms but cannot usually see the actual tornado unless it is very close. Mobile research radars have been able to see the rotation and the hollow core at the centre of the tornado but this is ordinarily not visible to conventional weather radars.

All across the country, thousands of volunteers help Environment Canada meteorologists watch the skies for severe weather. When they spot tornadoes or other severe weather events, they can report their observations to the Storm Prediction Centres. These reports are critical in issuing severe weather watches and warnings, and confirming observations made by radar and satellite. Forecasters usually contact observers near large thunderstorms to ask for information about the storm as it passes. Ham radio operators can become members of CANWARN, an Environment Canada program that provides specialized severe weather training to interested amateurs so that forecasters can be provided with more timely information about threats from severe thunderstorms.

Related Links:

• Tornado Fact Sheet
• Summer Severe Weather
• Severe Weather Safety Tips
• Weather Watches, warnings and Advisories
• Weather Watchers
• CANWARN