Tornado climatology

These reports have been gathered by the National Climatic Data Center (NCDC), based in Asheville, North Carolina.

The high frequency of tornadoes in North America is largely due to geography, as moisture from the Gulf of Mexico is easily advected into the midcontinent with few topographic barriers in the way.

The Rocky Mountains block Pacific-sourced moisture and buckle the atmospheric flow, forcing the drier air to mid-levels of the troposphere.

[b] Downsloping 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.

South America's tornado corridor region is favorable for tornadoes and severe thunderstorms due to the large size of the Pampas Plain where the cold, dry air from Patagonia, the Andes, and Antarctica collides with warm, moist air from areas of Brazil, northern Argentina, and Paraguay.

Located in the heart of Europe, between the Alps and the Apennines, the collision between cold and warm air here during the summer months easily permits the growth of very strong thunderstorms.

[7] A large region of South America is also characterized by storms that reach the level of supercells and produce intense hailstorms, floods, and tornadoes during the spring, summer, and early fall.

The region recently dubbed as the Tornado Corridor [es] (South America) is considered the second largest in the world in terms of the formation of extreme weather events.

[8] Other areas of the world that have frequent strong tornadoes include Germany, the Czech Republic, Slovakia, Italy, Spain, China, and the Philippines.

Australia, France, Russia, areas of the Middle East, and Japan have a history of multiple damaging tornado events.

[9] Various government agencies and published academic studies have kept track of the number of tornadoes based on geographic region.

In October 2011, two people were killed and nearly 200 were injured after a tornado formed, near Ficksburg in the Free State; more than 1,000 shacks and houses were flattened.

These occur during the onset of the rainy season when tumultuous winds accompanied by sheets of rain as well as spectacular thunder and lightning batter the coast.

[29] The tornado generating storms were often welcomed by colonial settlers in the region since they dissipated extreme heat and humidity during the last days of the dry season.

Tornadoes are often embedded in the African squall lines,[30][31] but they damage crops, and diminish any beneficial effect of its rains.

Storms producing multiple tornadoes and hail struck a densely populated area of farms and factories near the city of Yancheng in Jiangsu province, about 800 kilometers (500 miles) south of Beijing, China.

[33][34] Throughout China, an estimated 100 tornadoes may occur per year with a few exceeding F2 in intensity, with activity most prevalent in eastern regions.

They are most common from June to August, especially in Central Europe and the Po Valley of Italy, and rarest from January to March.

Since 1900, deadly tornadoes have occurred in Austria, Belgium, Cyprus, Czech Republic,[38][39][40] Finland, France,[41][42] Germany,[43] Italy,[44][45] Lithuania, Estonia, Malta, the Netherlands, Poland, Ukraine, Portugal, Romania, Russia, and the UK.

On average 62 are reported per year, but this number is expected to in fact be higher due to undetected tornadoes in large expanses of underpopulated areas.

South America has its own tornado alley, comprising areas in central and northern Argentina, southern and southeast Brazil, Uruguay, and part of Paraguay.

[citation needed] Argentina has areas with high tornadic activity, and the strongest tornadoes in the southern hemisphere,[citation needed] such as the F5 in San Justo, Argentina, 105 km (65 mi) north of the city of Santa Fe, with winds that exceeded 400 kilometres per hour (250 mph).

In Brazil, one of the most remarkable events occurred on 24 May 2005 when an F3(EF3) multiple-vortex tornado[53] struck the industrial district of the city of Indaiatuba in the state of São Paulo.

[14] The seasonal transition during autumn and spring promotes the development of extratropical cyclones and frontal systems that support strong convective storms.

[59] The reason for the peak period for tornado formation in North America being skewed toward spring has much to do with temperature patterns in the U.S. Tornadoes often form when cool, polar air traveling southeastward from the Rocky Mountains overrides warm, moist, unstable Gulf of Mexico air in the eastern states.

Since both warm and cold major weather patterns collide in Spring, the conflict between the two air masses tends to the formation of tornado producing storms and supercells.

[63][64][65][66] Due to this being the time of peak atmospheric heating, and thus the maximum available energy for storms; some researchers, including Howard B. Bluestein of the University of Oklahoma, have referred to this phenomenon as "five o'clock magic."

The highest incidence of violent tornadoes seems to shift from the Southeastern United States to the southern Great Plains every few decades.