Tornados, cyclones and hurricanes



Tornado (Latin tonare, "to thunder"): violent whirling wind, characteristically accompanied by a funnel-shaped cloud extending down from a cumulonimbus cloud. Commonly known as a twister or cyclone, a tornado can be a few meters to about a kilometer wide where it touches the ground, with an average width of a few hundred meters. It can move over land for distances ranging from short hops to many kilometers, causing great damage wherever it descends. The funnel is made visible by the dust sucked up and by condensation of water droplets in the center of the funnel. The same condensation process makes visible the generally weaker sea-going tornadoes, called waterspouts that occur most frequently in tropic waters. Most tornadoes spin counterclockwise in the northern hemisphere and clockwise in the southern, but occasional tornados reverses this behavior.
The exact mechanisms that cause a tornado to form are still not fully understood, but the funnels are always associated with violent motions in the atmosphere, including strong updrafts and the passage of fronts. They develop within low-pressure areas of high winds; the speed of the funnel winds themselves is often placed at more than 480 km/h (more than 300 mph), although speeds of more than 800 km/h (500 mph) have been estimated for extremely strong storms. Damage to property hit by a tornado results both from these winds and from the extremely reduced pressure in the center of the funnel, which causes structures to explode when they are not sufficiently ventilated to adjust rapidly to the pressure difference. The pressure reduction is in keeping with Bernoulli's principle, which states that pressure is reduced as velocity increases.
Tornadoes are most common and strongest in temperate latitudes, and in the U.S. they tend to form most frequently in the early spring; the "tornado season" shifts toward later months with increasing latitude. The number of funnels observed each year c