Tornado
The United States has the most tornadoes of any country, nearly four times more than estimated in all of Europe, excluding waterspouts. This is mostly due to the unique geography of the continent. North America is a large continent that extends from the tropics north into 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 downsloped winds, and causing the formation of a low pressure area downwind to the east of the mountains. Increased westerly flow 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 in the southerly flow to its east. 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, although southeast Quebec, the interior of British Columbia, and western New Brunswick are also tornado-prone. Tornadoes also occur across northeastern Mexico.
List of tornadoes causing 100 or more deaths
There are many sources of uncertainty in the statistics mentioned on this page. Before the 20th century, and even until recently in third-world countries, records-keeping was spotty at best. Before the American Civil War, slave deaths were often not included in tornado death tolls. Fatalities of African-Americans in the Southern US were routinely not counted through the 1940s and in some cases into the 1950s. Most tornadoes from many decades ago had no official government report on damage or casualties, so statistics must be compiled from local newspapers, which are not always a reliable, consistent, or comprehensive source. Many death tolls were published with people still missing, or with people critically injured and likely to die later. News media, Red Cross, and other counts don't necessarily distinguish whether a death was directly caused by a tornado and can include deaths during cleanup efforts. Routine counting of fatalities began in the US in the 1950s. In Bangladesh and India, exact populations of towns were often not known, so most death figures are approximate. Individual tornado descriptions go into more detail on these uncertainties. Officials in some areas, for example in Russia (and the USSR) and parts of Europe, until recent years denied that tornadoes occur in those areas thus fatalities may not be counted as tornadic.