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A tornado is a violent column of rotating air with an average diameter of 160 ft (50 m) and wind speeds up to 318 mph (512 km/h) or greater. Tornadoes occur in practically every country, but are most common in the United States. Tornadoes that cross, or form, over water are known as waterspouts. Many waterspouts, however, are smaller and less intense than land-based tornadoes and are more similar to a rotating wind commonly known as a dust devil.

Historical Background and Scientific Foundations

The United States has the highest annual frequency of tornadoes (approximately 900 per year) because the physical geography characteristics of the central region of the country are ideal for producing the meteorological conditions necessary for tornado development. This region is often referred to as Tornado Alley, and extends northward into Canada. Some of the other more common places around the world to see tornadoes are Australia (approximately 15 per year), New Zealand, Bangladesh, Japan, South Africa, Argentina, Ireland and the United Kingdom, Western Europe, and Russia.

Tornadoes are cyclonic (low-pressure) weather systems that form through a complicated process that is not well understood. The key components of tornado formation are thought to be a horizontal cylinder of rotating air known as a mesocyclone, and a severe weather system such as a thunderstorm. The mesocyclone is produced by wind shear, which is basically friction between winds flowing at different rates of speed. As the thunderstorm develops, it rises and interacts with the mesocyclone, which tilts and becomes a vertical column of rotating air. The term “funnel cloud” is used to describe the appearance of the mesocyclone as it extends outward from the bottom of the thunderstorm and reaches for the ground. If the funnel cloud actually touches the ground, then it is officially classified as a tornado.


DOPPLER RADAR: The weather radar system that uses the Doppler shift of radio waves to detect air motion that can result in tornadoes and precipitation, as previously developed weather radar systems do. It can also measure the speed and direction of rain and ice, as well as detect the formation of tornadoes sooner than older radars.

FUJITA SCALE: A scale of six categories that rates tornado wind speed based upon the observed destruction of the storm.

JET STREAM: Rivers of high-speed air in the atmosphere. Jet streams form along the boundaries of global air masses where there is a significant difference in atmospheric temperature. The jet streams may be several hundred miles across and 1-2 mi (1.6-3.2 km) deep at an altitude of 8–12 mi (13-19 km). They generally move west to east, and are strongest in the winter with core wind speeds as high as 250 mph (402 km/h). Changes in the jet stream indicate changes in the motion of the atmosphere and weather.

MESOCYCLONE: A horizontal cylinder of rotating air produced by wind shear, which is basically friction between winds flowing at different rates of speed. A mesocyclone can tilt and become a vertical column of rotating air known as a funnel cloud and extend from the bottom of a thunderstorm. If the funnel cloud then reaches the ground it becomes known as a tornado.

Although tornadoes develop throughout the year in the United States, they are most common during the spring season months (April to July) when weather conditions are most favorable. The northward migration of the polar jet stream during this period allows warm, moist air from the Gulf of Mexico to flow into the central regions of the country, where it mixes with cold, dry air. This interaction occurs when a cold front associated with a mid-latitude cyclone migrating eastward along the path of the polar jet moves into the central United States and forces the warm, moist air upward. These unstable atmospheric conditions lead to thunderstorm formation, which often leads to the development of a tornado.

The Fujita Scale was developed in the 1950s for measuring tornado intensity. The categories range from F0 (<72 mph, or <116 km/h) to F5 (261–318 mph, or 419–512 km/h). It is extremely difficult to obtain wind-speed measurements during a tornado, so the classification is typically based on post-storm evaluations of physical damage caused by the storm. For example, roof damage might be classified as an F1, while a roof torn completely off might be classified as an F2. A refined version of the scale was recently developed by the National Weather Service and titled the Enhanced Fujita Scale.

Impacts and Issues

Tornado activity in the United States increased throughout the 1900s, especially after the 1950s. This trend is a product of population growth and technology, and should not be confused with global warming. Population was sparsely distributed throughout the central United States in the early 1900s, so there were fewer people to observe and report tornadoes. Military research during World War II (1939–1945) produced radar, which was subsequently adapted for monitoring weather conditions. Doppler Radar is currently one of the most important tools in modern tornado detection.

Although tornadoes are most commonly associated with thunderstorms, they also develop during hurricanes and are often a major contributor to the amount of property damage, deaths, and injuries that occur during a hurricane landfall. One of the potential outcomes of global warming is an increase in hurricane frequency, which could lead to an increase in tornado activity during landfalls. However, the influence of global warming on hurricane activity is a highly debated topic in the scientific community, so it is unclear how tornado activity will be influenced by global warming.

More specifically, global warming is expected to increase sea surface temperatures in the tropical regions of the world, which will increase the amount of energy in the oceans available for hurricane formation. The result of this process could be an increase in the annual frequency of hurricanes, and an increase in the intensity (higher wind speeds). The process of hurricane formation, like tornado formation, is extremely complicated, so the positive influence of warmer ocean temperatures could be countered by the typical factors that disrupt formation resulting in no net gain in frequency. A more likely outcome is an increase in hurricane intensity, which would potentially provide more energy for thunderstorm and tornado formation at landfall; however, this too is a complicated process with no clear consensus among the scientific community. These types of problems complicate the process of understanding how global warming will influence tornado activity in the future.

See Also Extreme Weather; Jet Stream; Meteorology.



Bluestein, Howard B. Tornado Alley: Monster Storms of the Great Plains. New York: Oxford University Press, 1999.

Burt, Christopher C. Extreme Weather. New York: W.W. Norton, 2004.

Grazulius, Thomas P. Nature's Ultimate Windstorm. Norman, OK: University of Oklahoma Press, 2001.

Web Sites

National Weather Center, 2007. <> (accessed December 9, 2007).

“Tornadoes: Nature's Most Violent Storms.” National Severe Storms Laboratory, National Oceanic and Atmospheric Association.<http://www.nssl.noaa gov/edu/safety/tornadoguide.html> (accessed December 9, 2007).

Philip L. Chaney