Tornado

A tornado is a very violent wind that blows at a great speed in a rotatory fashion, forming a tall, funnel-shaped column of air and debris. Also called twister, a tornado covers a small area, but moves from place to place in this funnel-like fashion, wreaking havoc on its path. Tornadoes come in many sizes with wind speeds between 40 mph (64 km/h) and 300 mph (480 km/h), stretching more than a mile across, and moving for dozens of miles.

A tornado or twister's intensity can be measured on a scale known as Fujita Scale, which was introduced in 1971. It consists of six ratings from F0 to F5 with damage rated as light to incredible, though sometimes an F6 category is included on the scale. Since the Fujita Scale is based on damage and not on wind speed or pressure, it is not perfect. The primary problem is that a tornado can only be measured on the Fujita Scale after it has occurred. Secondly, the tornado can not be measured if there is no damage when the tornado occurs in an area without any features to be damaged.

Tornadoes often develop from a class of thunderstorms known as supercells, which contain vortexes of air, approximately 2 to 10 km in diameter, known as mesocyclones. A tornado begins when increasing rainfall drags with it an area of quickly descending air known as the rear flank downdraft. This downdraft accelerates as it approaches the ground, and drags the supercell's rotating mesocyclone towards the ground with it. As the mesocyclone approaches the ground, a visible condensation funnel appears to descend from the base of the storm, often from a rotating wall cloud. As the funnel descends, the downdraft also reaches the ground, creating a gust front that can cause damage a good distance from the tornado. Usually, the funnel cloud becomes a tornado within minutes of the downdraft reaching the ground.

The United States has the most tornadoes of any country, about four times more than estimated in all of Europe, not including waterspouts. This is mostly due to the unique geography of the continent. North America is a relatively large continent that extends from the tropical south into arctic areas, and has no major east-west mountain range to block air flow between these two areas. The United States averages about 1,200 tornadoes per year. The Netherlands has the highest average number of recorded tornadoes per area of any country with more than 20, or 0.0013 per sq mi, annually, but most are small and cause minor damage.