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| Image compliments of NOAA |
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Severe tornadoes are the most violent storms on earth. They have the highest wind speeds of any weather phenomenon, exceeding 600 km/h. Tornadoes destroy property; since 1916, they have caused more than 12,000 fatalities in North America.
Canada ranks second in the world in terms of the frequency of tornadoes, with an average of 80 tornadoes each year. In the United States, a thousand tornadoes are reported each year. Any tornado can cause injury or property damage, but two percent of the tornadoes cause more than 90 percent of the fatalities.
Tornadoes occur when hot humid air flowing north from the Gulf of Mexico collides with dry cool polar air. A tornado first appears as a rotation in a huge thunder cloud, normally following a heavy rain or hail. The sky can turn green, yellow or a very dark black. A powerful rotating (nearly always counterclockwise) column of air extends from the cloud to the ground. When the air hits the ground it look like a funnel-shaped cloud.
Tornadoes can occur at anytime of the year, but are most common in the late spring and summer months. When a tornado touches down it moves at a groundspeed of 45 km/h, on average, but can move as quickly as 100 km/h. The funnel-shaped cloud snakes a general path from southwest to northeast, but can change direction suddenly.
The Fujita scale measures the strength of tornadoes – from F0 (weak) to F5 (intense). F5 tornadoes have wind speeds in excess of 419 km/h, and they can lift homes off their foundations, and send cars and other large objects more than 100 metres through the air. Canada had its first-never F5 tornado on June 22, 2007 in Elie, Manitoba. There have been more than 50 F5 tornadoes in the United States over the past five decades.
Canada’s "tornado alleys" are found in Southern Ontario; a band through Alberta, southern Saskatchewan and Manitoba through to Thunder Bay; and, Southeastern Quebec. Tornadoes occur in most parts of the United States east of the Rocky Mountains, with the most severe tornadoes being recorded in Texas, Oklahoma and Kansas.
Tornado "watches" are issued when weather conditions could lead to the formation of tornadoes. Tornado "warnings" are issued when Doppler radar or a visual sighting has identified a tornado. Downbursts are often mistaken for tornadoes owing to the severity of damage done when one hits. The characteristics of a downburst, however, differ from those of tornadoes. For example a downburst features non-rotating air driving towards the surface of the earth at up to 200 km/h, while a tornado is composed of rotating rising air. Downburst winds affect a small area and last only a few minutes.
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