A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud. The word "tornado" comes from the Spanish verb "tornar", meaning "to turn."
It is spawned by a supercell thunderstorm (or sometimes as a result of a hurricane) and produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly. Many tornadoes are the tail end of a mesocyclone and they have a characteristic "hook echo" signature on a radar screen. The damage from a tornado is a result of the high wind velocity and wind-blown debris. Tornado winds range from a slow 40 mi/h (65 km/h) at the low end to a possible 300 mi/h (480 km/h) in the strongest storms. Tornado season in North America is generally March through August, although tornadoes can occur at any time of year. They tend to occur in the afternoons and evenings: over 80 percent of all tornadoes strike between noon and midnight.
Tornadoes can be nearly invisible, marked only by swirling debris at the base of the funnel. Others are composed of several mini-funnels. A tornado must by definition have both ground and cloud contact.
Tornadoes do occur throughout the world; the most tornado-prone region of the world, as measured by number of tornadoes per unit area, is the United Kingdom, especially England. However, the United States experiences by far the most tornadoes of any country, and has also suffered the most intense ones. On average, the United States experiences 100,000 thunderstorms each year, resulting in over 1,000 tornadoes and approximately 50 deaths per year. The deadliest US tornado on record is the 18 March 1925 Tri-State Tornado that went across southeastern Missouri, southern Illinois, and southern Indiana, killing 695 people. More than six tornadoes in one day is considered a tornado outbreak. The biggest tornado outbreak on record--with 148 tornadoes, including six F5 and 30 F4 tornadoes--occurred on April 3, 1974. It is dubbed The Super Outbreak. Another such significant storm system was The Palm Sunday Tornado Outbreak which affected the United States midwest on April 11, 1965.
The intensity of tornadoes is given by the Fujita - Pearson Tornado Scale (also known simply as Fujita scale). The intensity can be derived directly with high resolution Doppler radar wind speed data, or empirically derived from structural damage compared to engineering data. Also, note that intensity does not refer in any way to the size, or width, of a tornado.
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Not every thunderstorm, supercell, squal-line, or hurricane will produce a tornado. Luckily, it is very difficult for a tornado to form. It takes exactly the right combination of atmospheric variables (wind, temperature, pressure, humidity, etc) to spawn even a weak tornado. On the other hand, the instances for tornado formation are very repeatable. Those instances are governed by largely by the seasons and the immediate weather patterns.
Of all tornadoes formed in the US, F0 and F1 tornadoes account for a large percentage of occurrences. On the other end of the scale, the massively destructive F5 tornadoes account for less than 2% of all tornadoes in the US.
Even though no two tornadoes are exactly alike, they always have the same general characteristics that classify them as tornadoes. First, a tornado is a microscale rotating area of wind. A thunderstorm can rotate, but that does not mean it is a tornado. Secondly, the vortex, rotating wind, must come from a convective cloud base. Some of those are thunderstorms embedded in squal lines, supercell thunderstorms, and also not to exclude the outer fringes of landfalling hurricanes. Third, a spinning vortex of air must have a wind speed above a certain rate to be classified by the Fujita scale as a tornado.
Tornado Characteristics
No two tornadoes look exactly alike. Nor have any two tornadoes behaved exactly the same. There are true incidents of tornadoes repeatedly hitting the same town several years in a row. But forecasting the exact position a tornado will strike at a certain time is nearly impossible. Also, anywhere that convection can occur, is a place where tornadoes can be formed. See also
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Tornado is also: