Where Do Tornadoes Occur?

A tornado is a destructive windstorm characterized by a dark funnel-shaped cloud that is formed by violently rotating winds which can reach high speeds of up to 300 mph. A tornado’s diameter can range from a few feet up to several miles while its track can go as near as less than a mile or up to several hundred miles. Tornadoes will typically travel in a northeast direction, depending on the winds.

Each year about 1000 tornadoes, on average, are reported to take place; but fortunately, most of these tornadoes will not produce any significant damage to towns they hit. Some, however, can cause destruction and will cost billions of dollars in damage.

The primary portion of the U.S. where a significant number of tornadoes hits has been identified as an L-shaped region that covers the states of Iowa, Colorado, and Texas. The highest threat is in the state of Oklahoma. This area has pretty much a consistent tornado season every year which starts from April until mid-June.

Most tornadoes occur in the month of May; but, based on records, April appears to be the month with the highest fatalities from tornadoes. An average of 27 deaths occurred during April between 1950 and 1999. One of the worst tornado disasters of all-time took place on April 3 and 4, 1974 when 148 twisters struck 13 different states. There were more than 300 deaths reported.

The relation between the high frequency of strong and destructive tornadoes and its relatively consistent seasons provide scientists and weather experts a natural way to define this area as "Tornado Alley." Tornado Alley indicates the states which have the highest possibilities of having a tornado. Tornado Alley states include: Missouri, Indiana, Illinois, Arkansas, Oklahoma, Nebraska, Kansas, Colorado, and Texas.

Although tornadoes are most often reported to occur in the U.S., tornadoes can also take place in other parts of the world. Examples of other countries include Japan, Australia, United Kingdom and Uruguay. But most often, tornadoes will strike the U.S. or Canada because the conditions that promote the formation of tornadoes are more ideal in those areas.

Causes of Tornadoes

Tornadoes are frequently spawned by huge thunderstorms referred to as "supercells." These powerful and highly destructive, organized storms are formed when moist, warm air form the ground rises upward to meet the drier, cooler air.

As the rushing warm air cools down, its moisture will condense, eventually forming a huge thundercloud which sometimes can grow to as much as 50,000 feet in height. Winds at different atmospheric levels will feed the updraft and will cause the formation of the tornado's signature funnel shape.

Tornado Classifications

Until 1971, tornadoes were not classified or ranked. It was Dr. Theodore Fujita who developed a system for classifying tornadoes just by looking at the extent of its damage and giving an estimate of its wind speed. The system is called the Fujita Scale or F-Scale.

Based on the system, tornadoes can be classified into 5 categories: F-0 through F-5 with F-0 tornadoes being the mildest and F-5 tornadoes as the most destructive.

• F-0 (40-72 mph) Light damage characterized by broken tree branches, damaged chimneys, toppled shallow-rooted trees.
• F-1 (73-112 mph) Moderate damage characterized by peeled off roof surfaces, snapped tree trunks, overturned mobile homes, destroyed garages.
• F-2 (113-157 mph) Considerable damage characterized by damaged roof structures, destroyed mobile homes, airborne debris, uprooted large trees.
• F-3 (158-205 mph) Severe damage characterized by torn walls and roof structures, destroyed small buildings, most trees uprooted.
• F-4 (207-260 mph) Devastating damage characterized by destroyed well-constructed homes, structures lifted from their foundations, blown cars, airborne large debris.
• F-5 (261-318 mph) Incredible or violent damage characterized by lifted strong-framed houses from foundations, damaged concrete structures, airborne vehicle-size missiles and debarked trees.

Of the 1000 tornadoes reported every year, around 2 percent of them are classified as F4 or F5 tornadoes. That is about 20 destructive and violent tornadoes every year. Since tornadoes are only rated by the damage they cause, their strength cannot really be estimated when they do not hit any visible structures.