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The United States has the most tornadoes of any country, as well as the strongest and most violent tornadoes. This is mostly due to the unique geography of the continent and the size of the country. North America is a large continent that extends from the tropics north to arctic areas, and has no major east-west mountain range to block air flow between these two areas. In the middle latitudes, where most tornadoes of the world occur, the Rocky Mountains block moisture and buckle the atmospheric flow, forcing drier air at mid-levels of the troposphere due to downslope winds, and causing cyclogenesis downstream to the east of the mountains. Downslope winds off the Rockies force the formation of a dry line when the flow aloft is strong, while the Gulf of Mexico fuels abundant low-level moisture. This unique topography allows for frequent collisions of warm and cold air, the conditions that breed strong, long-lived storms throughout the year. A large portion of these tornadoes form in an area of the central United States known as Tornado Alley.〔 This area extends into Canada, particularly Ontario and the Prairie Provinces. Strong tornadoes also occur in northern Mexico. The United States averaged 1,274 tornadoes per year in the last decade. Canada reports nearly 100 annually in the southern regions.〔http://www.noaanews.noaa.gov/2011_tornado_information.html〕 However, the UK probably has most tornadoes per area per year, 0.14 per 1000 km².〔http://www.dandantheweatherman.com/Bereklauw/dutchtns.htm〕 The Netherlands have a similar number of tornadoes per area. In absolute number of events, ignoring area, the UK experiences more tornadoes (excluding waterspouts) than any other European country. Other areas of the world that have frequent tornadoes include significant portions of Europe, South Africa, and parts of Argentina, Uruguay, and southern Brazil, along with New Zealand, and far eastern Asia. The severity of tornadoes is commonly measured by the Enhanced Fujita Scale, which scales tornado intensity from EF0 to EF5 by wind speed and the amount of damage they do to human environments. These judgments are always made after the tornado has dissipated and its damage trail is carefully studied by weather professionals. Tornadoes are most common in spring and least common in winter. Since autumn and spring are transitional periods (warm to cool and vice versa) there are more chances of cooler air meeting with warmer air, resulting in thunderstorms. Tornadoes are focused in the right poleward section of landfalling tropical cyclones, which tend to occur in the late summer and autumn. Tornadoes can also be spawned as a result of eyewall mesovortices, which persist until landfall. However, favorable conditions can occur any time of the year. Tornado occurrence is highly dependent on the time of day, because of solar heating. Worldwide, most tornadoes occur in the late afternoon, between 3 pm and 7 pm local time, with a peak near 5 pm. Destructive tornadoes can occur at any time of day, but are most common in the afternoon and early evening. However, the Gainesville Tornado of 1936, one of the deadliest tornadoes in history, occurred at 8:30 am local time, but this is very unusual.〔 == Geography == The United States has the most tornadoes of any country. Many of these form in an area of the central (with some definitions including Southern) United States known as Tornado Alley. This area extends into Canada, particularly the Prairie Provinces and Ontario; however, activity in Canada is less frequent and intense than that of the US. A large region of South America is characterized by storms that reach the level of supercells and produce intense hailstorms, floods, and tornadoes during the spring, summer, and early fall. The recently appointed as the Tornado Corridor (South America) is considered as the second largest in the world in terms of the formation of extreme weather events. It covers most of central Argentina, southern Paraguay, southeastern Brazil, and Uruguay. Bangladesh and surrounding areas of eastern India suffer from a couple tornadoes annually of similar severity to stronger tornadoes in the US. These occur with a greater recurrence interval (although over a smaller region), and tend to be under-reported due to the scarcity of media coverage of a developing country. The annual human death toll from tornadoes in Bangladesh is estimated at about 179 deaths per year, which is much greater than in the US. This is likely due to the density of population, poor quality of construction, lack of tornado safety knowledge and warnings, and other factors. Other areas of the world that have frequent strong tornadoes include Germany, the Czech Republic, Slovakia, Italy, Spain, China, and the Philippines. Australia, France, Russia, areas of the Middle East, Japan, and parts of Mexico have a history of multiple damaging tornado events. == Tornadoes in the USA == The United States averaged 1,274 tornadoes per year in the last decade. April 2011 saw the most tornadoes ever recorded for any month in the US National Weather Service's history, 875; the previous record was 542 in one month.〔 It has more tornadoes yearly than any other country and reports more violent (F4 and F5) tornadoes than anywhere else. Tornadoes are common in many states but are most common to the west of the Appalachian Mountains and to the east of the Rockies. The Atlantic seaboard states – North Carolina, South Carolina, Georgia and Virginia – are also very vulnerable, as well as Florida. The areas most vulnerable to tornadoes are the Southern Plains and Florida, though most Florida tornadoes are relatively weak. The Southern USA is one of the worst affected regions in terms of casualties. Tornado reports 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tornado climatology」の詳細全文を読む スポンサード リンク
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