  A tornado is best described as a powerful, rotating column of air that reaches from the base of a thunderstorm cloud to the ground. Thunderstorms form in hot damp air that is approaching a cold front. Some thunderstorms produce strong winds, particularly ones which change direction with height. These are called wind shears. Horizontal winds that are moving in different directions at different altitudes can form horizontal columns of rotating air. This rotating air gets lifted into the thunderstorm by strong updrafts; as it rises, it stretches and tightens, causing it to spin even faster (like a figure skater who draws in her arms to rotate more quickly).
A large mass of rotating air is called a mesocyclone, or supercell thunderstorm, and can easily spawn tornadoes. Supercell thunderstorms have strong rotating updrafts with large vertical wind shears. Wind speeds in tornadoes themselves can reach 480 km/h, although only one in every fifty tornadoes have winds greater than about 320 km/h. The large hail that often precedes a tornado forms in the intense updrafts that feed the thunderstorm.   While tornadoes can occur all over the world, they are most frequent in the United States, where some 1,000 tornadoes per year touch down, killing 60 people and causing around $850 million in damage nationwide. The intensity of tornadoes is measured by the Fujita Scale. The scale is divided into levels ranging from F0 to F6, with F6 being the strongest (and rarest) tornado.
Note that the Fujita scale measures intensity - the strength of a tornado's winds, not the size of the funnel cloud. Large tornadoes can be weak, while small tornadoes can be strong.    In 1987, an F3-F4 tornado touched down in Edmonton, Alberta, spending over an hour on the ground and causing 27 deaths and $250 million in damages as it worked its way for 40 kilometres across the edge of the city at 35 km/h.   The tornado approaching Edmonton (left) and what was left of the Evergreen trailer park afterward. |
