The widest tornado on record measured about 4.2 kilometres across
Most tornadoes are about 80 metres wide, blow under 180 kilometres per hour and fizzle within a few kilometres. The extremes are staggering: winds above 480 kilometres per hour, a 1925 twister that stayed down for 352 kilometres, and an Oklahoma storm in 2013 roughly 4.2 kilometres wide. The visible funnel is not even the tornado.
A tornado is a violently spinning column of air that touches both the ground and the base of a cumulus or cumulonimbus cloud. Strictly, the word refers to the wind vortex, not the cloud. The funnel usually appears because intense low pressure inside the spin cools the air until its vapour condenses, but a tornado can be invisible, and winds often reach the surface while the funnel still hangs aloft. If surface winds top 64 kilometres per hour, the circulation counts as a tornado even without a visible funnel reaching down.
The English word blends Spanish tronada, a thunderstorm, from a verb meaning to thunder, with tornado, meaning turned. Its straight-line relative, the derecho, takes its name from the Spanish word for straight. Meteorologists reserve cyclone for large low-pressure systems, which have no funnels, even though people sometimes use it for twisters.
Shapes and colours vary widely. Narrow stovepipes, broad wedges that look like a dark block of cloud, and thin ropes that curl as a tornado weakens are all common forms; stretching into a rope slows the winds, because angular momentum is conserved. Plains tornadoes can turn red from the soil, waterspouts white or blue, and back-lit funnels look almost black, while sunset can tint them yellow, orange or pink. Rain, dust or darkness can hide a tornado completely, making it especially dangerous.
The United States has far more tornadoes than any other country, concentrated in the central and southeastern region called Tornado Alley, but they also strike Bangladesh, Argentina, South Africa, Japan, New Zealand and much of Europe. Pulse-Doppler radar spots telltale hook echoes and debris balls, backed up by storm spotters. Strength is rated by damage on the Fujita or Enhanced Fujita scales, from EF0, which damages trees, to EF5, which can sweep buildings off their foundations, or on the TORRO scale from T0 to T11. The Tri-State Tornado of 18 March 1925 crossed Missouri, Illinois and Indiana.
Source: Tornado
