Tornadoes

Tornadoes are small, violent, rotating storms that may produce devastating wind velocities of more than 400 kilometers per hour. The force of the cyclonic wind in a strong tornado can demolish even well-built structures, and people in a tornado’s path are at severe risk of physical harm and death.

Tornado Formation and Classification

Tornadoes are relatively small, localized low-pressure areas associated with powerful thunderstorms under cumulonimbus clouds. For its size, the tornado is the most violent of the whirlwinds. The "typical" tornado is 250 meters in diameter, with whirling winds of about 240 kilometers per hour. The twisting funnel cloud typically travels at about 65 kilometers per hour over the surface and lasts ten minutes, moving, in North America, along a northeasterly track. Very large, devastating tornadoes are relatively rare but have almost unbelievable destructive power. The infamous Tri-State Tornado of March 18, 1925, for example, touched down near Ellington, Missouri, at 1:00 p.m. and ripped a trail of havoc for 352 kilometers across southern Illinois, finally breaking up at 4:30 p.m. near Princeton, Indiana. The storm killed 695 people and injured 2,027. Damage, calculated in 1970 dollars, was $43 million. Compared with other tornadoes, it raced along the ground, averaging well over 100 kilometers per hour.

Tornadoes have been documented around the world. However, they are most consistently associated with fast-moving cold fronts that sweep across the midsection of the United States, drawing warm, moist, tropical air from the Gulf of Mexico. The cold front is usually associated with a strong low-pressure storm system that rotates counterclockwise as it swirls across land in the prevailing westerly wind pattern. The counterclockwise rotation of the low-pressure system brings cold air in behind, which wedges underneath the warm Gulf air that is drawn in ahead of the storm center. When heavy, cold air wedges under the less dense, warm, tropical air, the warm air is forced to rise. If the air is unstable, the cloud will accelerate upward, making a towering thundercloud. The upward surge stops only when the cloud has penetrated the excessively cold upper air. If the cloud tops can penetrate the tropopause (the border between the troposphere and the stratosphere) at an altitude above 11 kilometers, severe storms, including supercells that spawn tornadoes, are possible. These high cloud tops indicate unstable air at abnormal heights, and occasionally the unstable conditions can drive storm cloud tops to 20 kilometers or higher. A high-altitude zone of strong wind from the west, known the jet stream, tends to increase the chance of violence when associated with a storm system.

Tornadoes are classified by their maximum wind velocity, which occurs on the skin of the spinning funnel. In 1971, T. T. Fujita of the University of Chicago developed the Fujita (F) Scale, which rated tornadoes from F0 to F5, or weakest to strongest. The ratings were based on the amount of potential damage created by the speed of a three-second wind gust. In 2007, the Enhanced Fujita (EF) Scale was adopted by the United States, with Canada using the enhanced version beginning in 2013. The updated scale was based on the original Fujita scale with six categories (0–5) of damage. However, the revision better reflected recent tornado damage surveys and aligned more closely with wind-associated damage by adding additional types and kinds of buildings, levels of construction quality, and various kinds of trees and vegetation. Tornadoes consisting of wind gusts between 65 and 85 miles per hour (mph) are ranked EF-0 (with "EF" representing "enhanced Fujita"); 86–110 mph gusts are ranked EF-1; 111–135 mph gusts are ranked EF-2; 136–165 gusts are ranked EF-3; 166–200 gusts are ranked EF-4; and storms with wind gusts over 200 mph are ranked EF-5.

Multiple tornadoes can occur when the weather conditions are ideal for severe weather. One of the worst US tornado outbreaks on record was from April 25 through April 28, 2011. A total of 355 tornadoes—with 211 on April 27 alone—created catastrophic and deadly paths through the northeastern, southern, and midwestern United States. Four of the tornadoes were rated as EF5, and over 340 people were killed, with April 27 logging the most single-day, tornado-related deaths in the United States since the Tri-State Tornado that killed over seven hundred people. Another notable event was from April 3 to April 4, 1974, referred to as the Jumbo Tornado Outbreak, when a remarkable 148 tornadoes struck eleven states, centering on Kentucky. Multiple tornado outbreaks occurred in April of 2014 when 38 tornados touched down, including multiple EF4, killing 34 people and injuring 440. In 2021, a multiple tornado outbreak in December produced 66 tornados and left 56 people dead and over 500 injured. Canada also reported an abnormally high number of tornadoes during that episode. The swarm of funnels claimed 300 lives and left 5,500 injured. Clusters of tornadoes can also occur during rare storms known as derechos, which are often compared to eyeless, linear hurricanes in inland areas.

The National Oceanic and Atmospheric Administration (NOAA) reports that the United States experiences approximately 1,200 tornadoes per year. However, this is a rough estimate because official records date only to 1950, and methods of storm spotting and reporting have evolved considerably over the years.

Occurrence and Damage

The United States accounts for approximately 75 percent of all tornadoes globally. Tornadoes have been reported in each of the fifty states, but are more common east of the Rocky Mountains, especially in the plains regions and Gulf Coast. An area roughly from the city of Fort Worth in northern Texas and then north through Oklahoma, Kansas, and Nebraska has been dubbed Tornado Alley by the media because the storms develop there so consistently. Another area in the United States that is prone to tornadoes is Florida, which has a higher-than-average amount of daily thunderstorms as well as intense tropical storms that tend to produce tornadoes when the storms move ashore. They are particularly rare in Alaska, Rhode Island, and Vermont. From 1950 to 2013, for instance, only four tornadoes were reported in Alaska, and there were no tornado-related deaths. The state of Texas, on the other hand, had over 8,200 tornadoes in the same time period, with over 560 fatalities.

Conditions that favor the formation of tornadoes include broad flatlands with no obstructions to the flow of surface wind; an elevation near sea level to allow the full height of the atmosphere for the development of towering clouds; a position on a large continent where very cold air from the north can be swept into a low-pressure storm system that has access to hot, humid tropical air to the south; a southward bulge of strong jet stream currents aloft; and springtime weather patterns that provide intense low-pressure systems that can penetrate rather close to the Gulf of Mexico coast of the United States.

Although tornadoes can happen anywhere at any time of year, March through July is considered the peak season in the United States, with May being the most tornado-prone month. Winter tornadoes are mostly confined to the Gulf of Mexico coast, and the frequency moves north and swings toward Kansas as springtime progresses. By July and August, the area of tornado danger spreads through the northeastern states and into southern Canada. Oklahoma is in or near the worst areas most of the year. Most of the storms occur during the late afternoon, at the climax of daily heating, although a really violent frontal advance occasionally will generate night and early morning tornadoes.

Some of the damage caused by tornadoes results from the rapid passage of low pressure. Most houses are built to withstand downward pressure from water, snow, or wind against the structure, especially weight on the roof. When a tornado passes over a house, however, the low pressure above, countered with high pressure inside that cannot leak out quickly enough, plus wind pressure under the eaves, causes the house to appear to "explode" from within. A rapid pressure drop of 10 percent would give the pressure inside a house a lifting force of nearly 1 metric ton per square meter. The roof is lifted slightly off the supporting walls, which, in turn, fall outward. The roof then drops back onto the interior of the house or blows away.

Whirlwinds can develop multiple vortices. Around the core of the funnel cloud, it is possible for high-speed "suction spots" to develop that might have wind velocities 100 kilometers per hour faster than the average velocity of the whirling funnel. Tornado paths observed in open ground often reveal a swirling pattern of streaks or scratches in the soil. On a larger scale, these suction traces match the paths of greatest destruction along the tornado’s path. Even in the much smaller dust devils, multiple vortices have been observed. Two or more small dust columns may rotate around the perimeter of the central column of a dust devil. For example, Hurricane Celia, which struck near Corpus Christi, Texas, in 1971, approached landfall with only 145-kilometer-per-hour winds, but the core (eye) of the storm broke into multiple vortices at Corpus Christi, and the damage was more typical of winds of 250 kilometers per hour. The hurricane was expected to cause minor damage, but the center behaved more like a cluster of tornadoes and caused extensive damage, even to well-built structures.

Study of Tornadoes

Although meteorologists are quite successful at predicting the general region where tornadoes are likely to occur, they have not been so successful at measuring the wind velocities and air pressure in tornadoes. The storms are so small and so violent that it is nearly impossible to get instruments into their direct path and have the instruments survive the passage of the funnel. The few weather stations that have been hit by tornadoes were destroyed or lost power to record the data. Barographs have recorded a drop in pressure of about 1 centimeter of mercury in about thirty seconds during the passage of tornadic storms.

Many of the best measurements of tornadoes are from indirect methods, such as calculating the wind velocity required to cause the kinds of destruction observed. Close estimations of wind velocity can be calculated from steel towers that have been toppled or from railroad boxcars that have been tipped over. Reinforced concrete grain silos have been ripped apart, and walls have collapsed in enough storms to get a large collection of approximate values. Surprisingly, a fairly reliable set of data involves the penetration of straw or splinters into wood surfaces. These projectiles are so small that they are quickly accelerated up to the velocity of the wind, and they are so common that most storms will hurl vast numbers of them at fixed targets. Experiments reported in 1976 provide many examples of the penetration of straw and toothpicks into all varieties of wood, both wet and dry. The data indicate that a velocity of 30 meters per second (108 kilometers per hour) is sufficient to drive a toothpick into soft pine. Broom straws need about twice as much velocity as toothpicks to penetrate wood.

Meteorologists continue to improve their ability to forecast, locate, and track tornadoes. Space satellites, a worldwide network of manned weather stations, and sophisticated computer systems enable meteorologists to "see" weather as it develops. Balloons carry instrument packages aloft regularly from weather stations across the continental United States. The balloons give wind, humidity, temperature, and pressure data for all of the lower atmosphere where storms develop. Forecasters can determine which levels of the atmosphere are unstable and where the moist air is likely to be forced aloft with conditions that can generate violent storms. When dangerous storms begin to develop, Doppler radar can be used to detect the wind component parallel to the radar beam. Scientists can then examine the pattern of the wind field to find locations of potential tornadoes. The radar helps to spot hail formation while other instruments monitor the location and frequency of lightning. All these clues to violent storms focus attention on the parts of storms that might spawn tornadoes. Critical information can be relayed to news services, which, in turn, can warn citizens who are in danger.

Despite ongoing research and scientific advances, many questions remain about how tornadoes actually form. Studies suggest that there are many factors involved on the storm scale around and in a mesocyclone. Temperature differences in the downdraft are thought to play a major role, although models and some observations suggest they may not be necessary for tornado formation. Scientists continue to study tornadoes to better understand, and therefore predict, these dangerous storms.

Significance

Tornadoes are serious weather events that threaten life and property, so it is important to be aware of storm watches and warnings and to be prepared to find proper shelter. The National Weather Service usually can predict severe weather regions several hours in advance, but the exact location of a tornado must wait for a visual sighting or the occurrence of a tornado signature on Doppler radar. When threatening storms develop, it is wise to monitor local weather broadcasts and to keep a lookout for the characteristic funnel cloud. Often the twister is causing major damage long before the dust swirl on the ground ever connects with the descending visible funnel. The storms move erratically but typically do not alter course very much, and if the funnel appears to be heading toward an observer already outside, it might be possible to leave the area of greatest danger by moving away in a direction perpendicular to the path of the storm.

Attempting to escape a tornado, however, is often more dangerous than taking some protective measures, in part because drivers trying to flee tornadoes in cars are often involved in serious accidents resulting from panic. Conditions of traffic, congestion, and available time should be considered before attempting to run from a twister, and this should never be attempted in an urban or otherwise crowded area. If a tornado warning is declared, shelter should be sought immediately inside a sturdy building. Those in manufactured homes or offices should evacuate to stronger structures or storm shelters. Those already in sturdy buildings should try to get to lower floors or narrow, confined corridors or closets. Pre-designated safe rooms built to Federal Emergency Management Agency (FEMA) standards are ideal. Above all, windows should be avoided: Many serious injuries during tornadoes result from flying pieces of glass. Taking shelter under heavy tables or inside a sturdy tub can prevent some injury from falling beams or masonry. A deep storm cellar with a latched door provides excellent protection.

Tornadoes tend to stay on or even above the ground surface, so a depression, pit, culvert, gutter, or ditch may provide safety if caught outside. Otherwise, proper action, if a sturdy building is not available, will depend on the situation. Shelter should not be taken under an overpass or bridge. A mobile home is perhaps the worst place to seek shelter from a tornado due to its light and, consequently, weak construction. Automobiles also are not safe; cars are easily overturned and are often beaten into shapeless masses by tornadoes. However, in some situations, cars may provide better protection than remaining in the open or may provide transportation to a shelter. Flying debris causes most of the injuries in tornadoes, and shelter should include protection from tumbling containers, planks, sections of fence, branches, splinters of glass, and other items that could be ripped loose by the storm.

There is no end to the documented stories of strange phenomena caused by tornadoes. Many instances are recorded where heavy boards have punched through steel plate or metal pipes. Automobiles have been pounded and rolled into battered wads of twisted steel by the storms, and some vehicles have been thrown into upper stories of buildings. Wire fences have been ripped up and wound into prickly balls up to 16 meters in diameter. Objects such as animals, trash barrels, photographs, or blankets may be picked up in one area by a tornado and deposited incongruously many kilometers away. A survivor of a tornado near Scottsbluff, Nebraska, reported seeing a head-sized boulder whirling around his car after the funnel engulfed the auto and its two occupants. After witnessing the flying boulder, the man was hurled from his car and nearly killed, regaining consciousness in a hospital. His passenger, also ejected from the car, died. The vehicle was destroyed and deposited in a nearby field.

Principal Terms

cold front: the transition zone or zone of contact between two air masses when cold air moves into a region occupied by warmer air

Coriolis effect: a phenomenon in which, because of the planet’s rotation, an apparent force is exerted on objects in motion, causing them to deflect from their intended path to the right in the Northern Hemisphere or to the left in the Southern Hemisphere

cumulonimbus cloud: also called "thunderstorm cloud"; a very dense, tall, billowing cloud form that develops an anvil-shaped head due to high-altitude wind shear, and normally accompanied by lightning and heavy precipitation

dust devil: a rotating column of air rising above a hot ground surface, made visible by the dust it contains; it is much smaller than a tornado, having winds of less than 60 kilometers per hour, and causing little or no damage

hurricane: a huge, tropical low-pressure storm system with sustained winds in excess of 118 kilometers per hour, formed over warm ocean surface water and powered by thermodynamic heat transfer from the water

squall line: any line of vigorous thunderstorms created by a cold downdraft that spreads out ahead of a fast-moving cold front

unstable air: a condition that occurs when the air above rising air is unusually cool so that the rising air becomes relatively warmer and accelerates upward

vortex: the central, low-pressure axis of any rotating fluid, as occurs in whirlpools and tornadoes

waterspout: a tornado that exists over water; less violent and smaller waterspouts form in fair weather just as dust devils do over dry land

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