Tornadoes, sometimes called twisters or cyclones, are violent rotating columns of air that make contact with the ground during strong thunderstorms (▼).

Tornadoes occur worldwide, but the United States has the highest number of yearly tornadoes—every year, about 1200 tornadoes hit the United States. Tornadoes rank high among nature’s most destructive forces, and their sporadic occurrence and violent winds cause many deaths each year (▼).

Pressures within some tornadoes have been estimated to be as much as  percent lower than pressures immediately outside the tornado. Drawn by the much lower pressure in the center of the vortex, air near the ground rushes into the tornado from all directions. As the air streams inward, it spirals upward around the core until it eventually merges with the airflow of the parent thunderstorm deep in the cumulonimbus tower. Because of the tremendous pressure gradient associated with a strong tornado, maximum winds can sometimes approach 480 kilometers (300 miles) per hour.

A tornado may consist of a single vortex, but within many stronger tornadoes are smaller whirls called suction vortices that rotate within the main vortex (▼). Suction vortices have diameters of only about 10 meters (33 feet) and rotate very rapidly. This structure accounts for occasional observations of virtually total destruction of one building while another one, just 10 meters (33 feet) away, suffers little damage.

Tornado Development and Occurrence

Tornadoes can form in any situation that produces severe weather, including cold fronts, squall lines, and tropical cyclones (hurricanes). Usually, the most intense tornadoes are those that form in association with huge thunderstorms called supercells. An important precondition linked to tornado formation in severe thunderstorms is the development of a mesocyclone. A mesocyclone is a vertical cylinder of rotating air, typically about 3 to 10 kilometers (2 to 6 miles) across, that develops in the updraft of a severe thunderstorm. The formation of this large vortex often precedes tornado formation by 30 minutes or so.

We have learned that in order for a thunderstorm to form, there must be moisture, instability, and a lifting mechanism; but to get a storm that produces a strong tornado, there needs to be one additional condition—wind shear. Wind shear is a change in wind speed and/or direction with height. Mesocyclone formation depends on the presence of wind shear. Moving upward from the surface, winds change direction from southerly to westerly, and wind speed increases. The speed wind shear (that is, stronger winds aloft and weaker winds near the surface) produces a rolling motion about a horizontal axis, as shown in ▼A. If conditions are right, strong updrafts in the storm tilt the horizontally rotating air to a nearly vertical alignment (▼B,C). This produces the initial rotation within the cloud interior.

At first, the mesocyclone is wider, shorter, and more slowly rotating than will be the case in later stages. Subsequently, the mesocyclone is stretched vertically and narrowed horizontally, causing wind speeds to accelerate in an inward vortex (just as spinning ice skaters accelerate by pulling in their arms). Next, the narrowing column of rotating air stretches downward until a portion of the cloud protrudes below the cloud base to produce a very dark, slowly rotating wall cloud. Finally, a slender and rapidly spinning vortex emerges from the base of the wall cloud to form a funnel cloud. If the funnel cloud makes contact with the surface, it is then classified as a tornado.

The formation of a mesocyclone does not necessarily mean that tornado formation will follow. Only about half of all mesocyclones produce tornadoes. Forecasters cannot determine in advance which mesocyclones will spawn tornadoes.

Tornado Climatology

Recall that severe thunderstorms—and hence tornadoes—are most often spawned along the cold front of a midlatitude cyclone or in association with supercell thunderstorms. Such severe storms are most likely to form during the spring, when the air masses associated with midlatitude cyclones are most likely to have greatly contrasting conditions. Continental polar air from Canada may still be very cold and dry, whereas maritime tropical air from the Gulf of America (Gulf of Mexico) is warm, humid, and unstable. The greater the contrast, the more intense the storm tends to be.

These two contrasting air masses are most likely to meet in the central United States because there is no significant natural barrier separating the center of the country from the Arctic or the Gulf of America (Gulf of Mexico). Consequently, this region generates more tornadoes than any other part of the country or, in fact, the world. The following figure (▼), which depicts the average annual tornado incidence in the United States over a 27-year period, readily substantiates this fact.

On average, about 1200 tornadoes are reported annually in the United States. However, the actual number that occurs from one year to the next varies greatly. During the period from 2000 to 2022, for example, yearly totals ranged from a low of 886 tornadoes in 2014 to a high of 1817 tornadoes in 2004. Tornadoes occur during every month of the year. April through June is the period of greatest tornado frequency in the United States, and December and January are the months of lowest activity (refer to the graph inset in ▲).

The preceding paragraphs described tornado climatology. Recall from the discussion of weather and climate earlier in this class that climate provides a statistical perspective of atmospheric behavior and that “Climate is what you expect, but weather is what you get.” However, as the saying warns, weather events do not always occur when statistical probabilities suggest. The following figure (▼) provides an example from November 2021.

Tornado Destruction and Loss of Life

The potential for tornado destruction depends largely on the strength of the winds generated by the storm. Because tornadoes generate the strongest winds in nature, they have accomplished many seemingly impossible tasks, such as the ones shown in ▼. Although it may seem impossible for winds to cause some of the extensive damage attributed to tornadoes, tests in engineering facilities have repeatedly demonstrated that winds in excess of 320 kilometers (200 miles) per hour are capable of incredible feats.

Most tornado losses are associated with the rare storms that strike urban areas or devastate entire small communities. The amount of destruction caused by such storms depends to a significant degree (but not completely) on the strength of the winds. A wide spectrum of tornado strengths, sizes, and durations are observed. The commonly used guide to tornado intensity is the Enhanced Fujita Scale, or EF-scale for short (▼). Because tornado winds cannot be measured directly, a rating on the EF-scale is determined by assessing the worst damage produced by a storm. Although widely used, the EF-scale is not perfect. Unlike the original Fujita scale, the EF-scale takes into consideration the structural integrity of buildings, but it does not do a good job of classifying tornadoes that pass through regions with no structures because there is no damage to assess. These tornadoes get rated as weak, if at all, no matter how strong the wind speed.

Although the greatest part of tornado damage is caused by violent winds, most tornado injuries and deaths result from flying debris. The proportion of tornadoes that result in loss of life is small. In most years, slightly fewer than 2 percent of all reported tornadoes in the United States are “killers.” Although the percentage of tornadoes resulting in death is small, each tornado is potentially lethal. When tornado fatalities and storm intensities are compared, the results are quite interesting: The majority (63 percent) of tornadoes are weak (EF-0 and EF-1), and the number of storms decreases as tornado intensity increases. The distribution of tornado fatalities, however, is just the opposite. Although only 2 percent of tornadoes are classified as violent (EF-4 and EF-5), they account for nearly 70 percent of tornado deaths.

Tornado Forecasting

Because severe thunderstorms and tornadoes are small and relatively short-lived phenomena, they are among the most difficult weather features to forecast precisely. Nevertheless, the prediction, detection, and monitoring of such storms are some of the most important services provided by professional meteorologists. Both the timely issuance and dissemination of watches and warnings are critical to the protection of life and property.

The Storm Prediction Center (SPC), located in Norman, Oklahoma, is part of the National Weather Service (NWS) and the National Centers for Environmental Prediction (NCEP). The mission of the SPC is to provide timely and accurate forecasts and watches for severe thunderstorms and tornadoes, as well as lightning, wildfires, and winter weather.

Convective outlooks are issued several times daily to provide descriptions of the type, coverage, and intensity of expected severe weather. Day 1 outlooks identify the areas that are likely to be affected by severe thunderstorms during the next 6 to 30 hours, and Day 2 outlooks extend the forecast through the following day. Other outlooks focus on longer timescales, 3-8 days into the future. Many local NWS field offices also issue severe weather outlooks that provide more local descriptions of the severe weather potential for the next 12 to 24 hours.

Tornado Watches and Warnings

Tornado watches alert the public to the possibility of tornadoes over a specified area for a particular time interval. Watches serve to fine-tune forecast areas already identified in severe weather outlooks. A typical watch covers an area of about 65,000 square kilometers (25,000 square miles) for a 4- to 8- period. A tornado watch is an important part of the tornado alert system because it sets in motion the procedures necessary to deal adequately with detection, tracking, warning, and response. Watches are generally reserved for organized severe weather events where the tornado threat will affect at least 26,000 square kilometers (10,000 square miles) and/or persist for at least 3 hours. Watches typically are not issued when the threat is thought to be isolated and/or short-lived.

Whereas a tornado watch is designed to alert people to the possibility of tornadoes, a tornado warning is issued by local offices of the NWS when a tornado has actually been sighted in an area or is indicated by weather radar. It warns of a high probability of imminent danger. Warnings are issued for much smaller areas than are watches, usually covering portions of a county or counties. In addition, they are in effect for much shorter periods, typically 30 to 60 minutes. Because a tornado warning may be based on an actual sighting, warnings are occasionally issued after a tornado has already developed. However, most warnings are issued 10 to 15 minutes prior to tornado formation, based on Doppler radar data and/or spotter reports of funnel clouds.

If the direction and the approximate speed of a storm are known, an estimate of its most probable path can be made. Because tornadoes often move erratically, the warning area is fan-shaped downwind from the point where the tornado has been spotted. Improved forecasts and advances in technology have contributed to a significant decline in tornado deaths over the past 50 years.

Doppler Radar

Many of the difficulties that once limited the accuracy of tornado warnings have been reduced or eliminated by an advancement in radar technology called Doppler radar (▼). Doppler radar not only performs the same tasks as conventional radar but also has the ability to detect motion directly. Doppler radar can detect the initial formation and subsequent development of a mesocyclone, the intense rotating wind system in the lower part of a thunderstorm that frequently precedes tornado development. Almost all mesocyclones produce damaging hail, severe winds, or tornadoes. Those that produce tornadoes (about 50 percent) can sometimes be distinguished by their stronger wind speeds and their sharper gradients of wind speeds. The tornado itself is too small to be detected directly by the radar.

It should also be pointed out that not all tornado-bearing storms have clear-cut radar signatures and that other storms can give false signatures. Detection, therefore, is sometimes a subjective process, and a given display could be interpreted in several ways. Consequently, trained on-the-ground observers, called storm spotters, continue to be an important part of the warning system.

The benefits of Doppler radar are many. As a research tool, it not only provides data on the formation of tornadoes but also helps meteorologists gain new insights into thunderstorm development, the structure and dynamics of hurricanes, and air-turbulence hazards that plague aircraft. As a practical tool for tornado detection, Doppler radar significantly improves our ability to track thunderstorms and issue warnings.

• A tornado is a violent windstorm that takes the form of a rotating column of air called a vortex that extends downward from a cumulonimbus cloud. Many strong tornadoes contain smaller internal vortices. Because of the tremendous pressure gradient associated with a strong tornado, maximum winds can approach 480 kilometers (300 miles) per hour.

• Tornadoes are most often spawned along the cold front of a midlatitude cyclone or in association with a supercell thunderstorm. Tornadoes also form in association with tropical cyclones (hurricanes). In the United States, April through June is the period of greatest tornado activity, but tornadoes can occur during any month of the year.

• Most tornado damage is caused by the tremendously strong winds. One commonly used guide to tornado intensity is the Enhanced Fujita Scale (EF-scale). A rating on the EF-scale is determined by assessing damage produced by the storm.

• Because severe thunderstorms and tornadoes are small and short-lived phenomena, they are among the most difficult weather features to forecast precisely. When weather conditions favor the formation of tornadoes, a tornado watch is issued. The National Weather Service issues a tornado warning when a tornado has been sighted in an area or is indicated on Doppler radar.

• Doppler radar: In addition to performing the tasks of conventional radar, a new generation of weather radar that can detect motion directly and hence greatly improve tornado and severe storm warnings.

• Enhanced Fujita (EF) Scale: Also referred to as the EF-scale, a scale originally developed by T. Theodore Fujita for classifying the severity of a tornado, based on the correlation of wind speed with the degree of destruction.

• mesocyclone: A vertical cylinder of cyclonically rotating air (3 to 10 kilometers in diameter) that develops in the updraft of a severe thunderstorm and that often precedes the development of damaging hail or tornadoes.

• tornadoes: Small, very intense cyclonic storms with exceedingly high winds, most often produced along cold fronts in conjunction with severe thunderstorms.

• tornado warning: An alert issued when a tornado has actually been sighted in an area or is indicated by radar.

• tornado watch: An alert issued for areas of about 65,000 square kilometers (25,000 square miles), indicating that conditions are such that tornadoes may develop; it is intended to caution people to the possibility of tornadoes.

• wind shear: A change in the speed and/or direction of wind with height. Mesocyclone formation depends on the presence of wind shear.