A tornado or ‘twister’ is a funnel‐shaped, violently rotating vortex extending downwards from the cumulonimbus cloud in which it forms. Tornadoes are always linked to a parent cumulonimbus cloud, and are therefore intimately associated with violent thunderstorms.
Strictly speaking, to be classed as a tornado, the vortex of rapidly spinning air must be in contact with the ground. If the vortex does not make contact with the surface it is referred to as a ‘funnel cloud’. Sometimes flying debris may be visible even if there is no obvious funnel cloud, indicating that a tornado is present. The centre of a tornado is characterized by extreme low pressure, which sucks up dust to form a dark‐grey funnel rising into the sky. Around this funnel of rising air are very strong winds capable of destroying crops and buildings. Tornadoes occur in deep, low‐pressure areas, associated with fronts or other instabilities.
Tornadoes are classified using a scale devised by Japanese- American meteorologist Tetsuya (Theodore) Fujita (1920–98) in the late 1960s (see table). This allocates a tornado to one of five categories (F0−F5), according to average wind speeds. Tornado winds cannot be measured directly, so a rating on the Fujita scale is determined by an assessment of the worst storm damage.
The damage signature of an F4 tornado, for example, is the levelling of well‐constructed houses, structures with weak foundations blown some distance, and cars thrown into the air to accompany other large airborne ‘missiles’. An F5 level tornado will lift strong frame houses from their foundations, break them up and carry them considerable distances. Automobiles will be picked up and carried through the air for distances in excess of 100 m (330 feet).
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