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.
Tornadoes in the United States
Tornadoes have been documented in every U.S. state (not including the non-state territories of Guam, the Virgin Islands, American Samoa, and Puerto Rico) at least once since 1950, although some regions and states are hit by tornadoes far more than others. For example, the average number of tornadoes to hit the states of Alaska, Rhode Island, and Vermont is less than one, while the state of Oklahoma receives an average of 52 tornadoes per year, and the state of Texas is hit with 126 tornadoes in an average year. The state with the most tornadoes per unit area is Florida, though most are weak tornadoes of EF0 or EF1 intensity. A number of Florida's tornadoes occur along the edge of hurricanes that strike the state. The state with the highest number of strong tornadoes per unit area is Oklahoma. States such as Oklahoma and Kansas have much lower population densities than Florida, so tornadoes may go unreported.