Tornado

Tornado (Latin, tonare, “to thunder”), in meteorology, violent whirling wind, characteristically accompanied by a funnel-shaped cloud extending down from a cumulonimbus cloud. Commonly known as a cyclone or twister, a tornado can be a few metres to about a kilometre wide where it touches the ground, with an average width of a few hundred metres.

It can move over land for distances ranging from short hops to many kilometres, causing great damage wherever it descends. The funnel is made visible by the dust that is sucked up and by condensation of water droplets in the centre of the funnel. The same condensation process makes visible the generally weaker sea-going tornadoes, called waterspouts, that occur most frequently in tropical waters. Most tornadoes spin anticlockwise in the northern hemisphere and clockwise in the southern, but occasional tornadoes reverse this.

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 updraughts 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/hr (300 mph), although speeds of more than 800 km/hr (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 centre 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. In the United States they tend to form most frequently in the early spring; the “tornado season” shifts towards later months with increasing latitude. The number of funnels observed each year can vary greatly in any given region.

Tornado Facts
- When associated with hurricanes, tornadoes are not usually accompanied by hail or a lot of lightning, clues that citizens in other parts of the country watch for.
- Tornado production can occur for days after landfall when the tropical cyclone remnants maintain an identifiable low pressure circulation.
- They can also develop at any time of the day or night during landfall. However, by 12 hours after landfall, tornadoes tend to occur mainly during daytime hours.

Fujita scale