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A small tornado in western Nebraska.

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book cover
Tornado Alley: Monster Storms of the Great Plains
 (Howard B. Bluestein)

 

WHAT are tornadoes?

Tornadoes (or "twisters") are rapidly rotating columns of air associated with severe thunderstorms. One thunderstorm can produce many tornadoes, and a tornado can have more than one vortex. Strong wind is the primary threat from a tornado, with speeds reaching as much as 480 kph (300 mph). These high winds can quickly destroy entire buildings and in some cases, entire communities. The debris carried by such high winds can easily cause severe injury or death to people and other life. Hail is commonly associated with tornadic thunderstorms and is also capable of causing extensive damage in a very short period of time. Tornadoes can move along the surface at up to 113 kph (70 mph) and can remain on the ground for several minutes. Most tornadoes occur between the hours of 4:00 P.M. and 6:00 P.M., when the lower atmosphere is most unstable. However, many tornadoes also occur after sunset. These tornadoes can be very dangerous because they are difficult to see and people are not easily alerted to severe weather while they are sleeping.

Tornadoes can take many shapes. Storm spotters recognize three common shapes: wedges, ropes, and stove pipes. Wedges are the largest and most powerful, and therefore usually the most dangerous. These are almost always associated with supercell thunderstorms. Ropes are smaller and usually less violent; they are easily recognized by their rope-like appearance. Stove pipes have similar width from top to bottom, usually with the entire column centered over a point on the ground.

Because of their destructive power, scientific observation of tornadoes has been difficult, and they are still not fully understood. Today, tornadoes are often classified according to wind speed on the Fujita Tornado Scale. Since it is not always possible to measure wind speed exactly in such dangerous storms, levels of destruction can be used to estimate speed.

Quick Fact:
In April of 1974, the worst tornado outbreak in history occurred when 148 twisters touched down in 13 U.S. states and parts of Canada, killing 316 people.


WHERE do tornadoes occur?

Tornadoes occur in many areas around the world. However, no region experiences more tornadoes than the central United States. This Great Plains region receives an average of 1200 tornadoes per year. Besides this region, Brazil and portions of western Europe are perhaps most notably at risk. In the British Isles, an average of 20 tornadoes per year are recorded. Canada receives approximately 13 tornadoes per year.

In North America, most tornadoes occur in an area known as "Tornado Alley", which is the central Great Plains region of the United States. However, every state east of the Rocky Mountains is at moderate to high risk of tornado activity. This is due to the clash of contrasting air masses (cool, dry continental air meeting warm, moist maritime air) that frequently occurs over the eastern two-thirds of the United States. The states of Kansas and Oklahoma usually experience the most tornadoes per year. Iowa, Nebraska, Texas, Arkansas, and Missouri comprise the area which reports the second highest number of tornadoes per year. States west of the Rocky Mountains are generally considered to be in a low-risk region.


WHEN do tornadoes occur?

Most tornadoes in North America occur between March and July, with the month of May normally experiencing the greatest number of tornadoes. However, depending on location, tornadoes can occur at any time of the year. The strongest tornadoes, which usually result in the highest number of deaths and greatest destruction of property, occur between April and June. Most deaths caused by tornadoes occur in April, which is considered the beginning of the tornado season in many states in the United States. In the the British Isles, tornadoes are most common during autumn, and are much less powerful than the tornadoes in North America.


HOW do we cope with tornadoes?

In the Great Plains of the United States, tornadoes are a regular part of life. Here, most people are aware of the incredible damage tornadoes can cause and respect their awesome power. Many homes have basements or separate underground rooms (called "tornado cellars") on their property. These rooms provide considerable safety when a tornado threatens. Indeed, surviving a tornado can be as simple as taking shelter underground, provided there is adequate warning when tornadoes are expected. Although buildings can be constructed to resist the winds of the smallest tornadoes, no structure has been developed that can survive the destructive force of strong tornadoes without significant damage. Accordingly, property damage is inevitable when a tornado hits a community.

In the United States, the National Weather Service issues tornado watches when atmospheric conditions are favorable for the formation of tornadoes and tornado warnings when a tornado appears to have formed. Doppler radar has proved to be an essential tool for detecting and studying tornadoes. A network of Severe Weather Spotters, who are citizen volunteers trained by the National Weather Service, provide critical local field observations. Currently, watches and warnings are communicated to citizens by the media (radio and television), the Internet, NOAA weather radios, and outdoor alert sirens. Improving tornado warning times through better weather forecasting continues to be one of the highest priorities of the National Weather Service.


WHO has more information on tornadoes?