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 create different damage depending on how fast their wind speeds are. The Fujita Scale classifies a tornado by its wind speeds. For example, if the tornado's wind speeds are low, then it is classified as a F0 tornado. If the tornado's wind speeds are about 200 miles per hour or at 200 miles per hour, then it is classified as a F5 tornado. In the United States, tornadoes have been known to form at high and low intensities. The probability of a high intensity or violent tornado differs by location across the country. For example, due to the high frequency of tornadoes in the "Tornado Alley" area mentioned before, a more violent tornado would be more likely to form there due to the strength of the thunderstorms produced by the two bordering air masses. The states with the highest number of F5 and EF5 rated tornadoes since data was available in 1950 are Alabama and Oklahoma, each with seven tornadoes. Iowa, Kansas, and Texas each are tied for second most with six. The state with the highest number of F5 and EF5 tornadoes per square mile, however, was Iowa. The state with the most number of tornadoes classified as "violent", or F4 and F5, is Kentucky, and the state with the highest average intensity ranking for tornadoes is Alabama.