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
Most tornadoes in the United States occur east of the Rocky Mountains. The Great Plains, the Midwest, the Mississippi Valley and the southern United States are all areas that are vulnerable to tornadoes. They are relatively rare west of the Rockies and are also less frequent in the northeastern states. Tornado Alley is a colloquial term for an area particularly prone to tornadoes. There is no officially defined 'Tornado Alley' – at its broadest this area stretches from northern Texas to Canada with its core centered on Oklahoma, Kansas and northern Texas. Another highly significant region – colloquially known as Dixie Alley – is the southern United States and particularly the northern and central parts of Alabama and Mississippi. Florida is one of the most tornado-prone states. However, Florida tornadoes only rarely approach the strength of those that occur elsewhere.