United States has the most tornadoes of any country. It also records the strongest and deadliest tornadoes “The United States has the most tornadoes of any country, and most strong and violent tornadoes. This is mostly due to the unique geography of the continent, and of course to the size of the country. 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 cyclogenesis downstream to the east of the mountains. Downsloped 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, along with Canada reporting nearly 100 annually in the southern regions. However, the UK probably has most tornadoes per area per year, 0.14 per 1000 km². Also the Netherlands have relatively many tornadoes per area. Also in absolute number of events, ignoring area, the UK experiences more tornadoes (excluding waterspouts) than any other European country.
Tornadoes kill an average of 52 people per year in Bangladesh, the most in the world. This is due to their high population density, poor quality of construction, lack of tornado safety knowledge, as well as other factors. Other areas of the world that have frequent tornadoes include South Africa, parts of Argentina, Uruguay, and southern Brazil, as well as portions of Europe, Australia and 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 judgements 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.[clarification needed] 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. 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. The Gainesville Tornado of 1936, one of the deadliest tornadoes in history, occurred at 8:30 am local time”