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Tornadoes have been recorded on all continents except Antarctica. They are most common in the middle latitudes where conditions are often favorable for convective storm development. The United States has the most tornadoes of any country, as well as the strongest and most violent tornadoes. A large portion of these tornadoes form in an area of the central United States popularly known as Tornado Alley. Canada experiences the second most tornadoes. Ontario and the prairie provinces see the highest frequency. Other areas of the world that have frequent tornadoes include significant portions of Europe, South Africa, Philippines, Bangladesh, parts of Argentina, Uruguay, and southern and southeastern Brazil, northern Mexico, eastern and western Australia, New Zealand, and far eastern Asia.
Tornado reports in the U.S. have been officially collated since 1950. These reports have been gathered by the National Climatic Data Center (NCDC), based in Asheville, North Carolina. A tornado can be reported more than once, such as when a storm crosses a county line and reports are made from two counties. The severity of tornadoes is measured by the Enhanced Fujita Scale, which measures tornado intensity on a scale of EF0 to EF5 based on wind speed and intensity of destruction. The ratings are made after the tornado has dissipated and the damage trail is carefully studied by weather professionals. A series of continuous tornado outbreaks is known as a tornado outbreak sequence.