A tornado is a violently rotating column of
air that is in contact with both the surface of the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
and a
cumulonimbus cloud or, in rare cases, the base of a
cumulus cloud
Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...
. It is often referred to as a twister, whirlwind or cyclone, although the word
cyclone
In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...
is used in
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...
to name a weather system with a
low-pressure area in the center around which, from an observer looking down toward the surface of the Earth, winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern. Tornadoes come in many shapes and sizes, and they are often visible in the form of a
condensation funnel
A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground or a water sur ...
originating from the base of a cumulonimbus cloud, with a cloud of rotating
debris and
dust
Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...
beneath it. Most tornadoes have wind speeds less than , are about across, and travel several kilometers (a few miles) before dissipating. The
most extreme tornadoes can attain wind speeds of more than , are more than in diameter, and stay on the ground for more than 100 km (60 miles).
[ ]
Various types of tornadoes include the
multiple vortex tornado
A multiple-vortex tornado is a tornado that contains several vortices (called subvortices or suction vortices) revolving around, ''inside'' of, and as part of the main vortex. The only times multiple vortices may be visible are when the tornado i ...
,
landspout, and
waterspout
A waterspout is an intense columnar vortex (usually appearing as a funnel cloud, funnel-shaped cloud) that occurs over a body of water. Some are connected to a cumulus congestus cloud, some to a cumuliform cloud and some to a cumulonimbus clou ...
. Waterspouts are characterized by a spiraling funnel-shaped wind current, connecting to a large cumulus or cumulonimbus cloud. They are generally classified as non-
supercellular tornadoes that develop over bodies of water, but there is disagreement over whether to classify them as true tornadoes. These spiraling columns of air frequently develop in tropical areas close to the
equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
and are less common at
high latitude
The polar regions, also called the frigid zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitudes are dominated by float ...
s. Other tornado-like phenomena that exist in nature include the
gustnado
A gustnado is a brief, shallow surface-based vortex which forms within the downburst emanating from a thunderstorm. The name is a portmanteau by elision of "gust front tornado", as gustnadoes form due to non-tornadic straight-line wind feature ...
,
dust devil
A dust devil is a strong, well-formed, and relatively short-lived whirlwind. Its size ranges from small (half a metre wide and a few metres tall) to large (more than 10 m wide and more than 1 km tall). The primary vertical motion is u ...
,
fire whirl
A fire whirl or fire devil (sometimes referred to as a fire tornado) is a whirlwind induced by a fire and often (at least partially) composed of flame or ash. These start with a whirl of wind, often made visible by smoke, and may occur when inte ...
, and
steam devil
A steam devil is a small, weak whirlwind over water (or sometimes wet land) that has drawn fog into the vortex, thus rendering it visible.
They form over large lakes and oceans during cold air outbreaks while the water is still relatively war ...
.
Tornadoes occur most frequently in North America (particularly in central and southeastern regions of the United States colloquially known as
Tornado Alley
Tornado Alley is a loosely defined area of the central United States where tornadoes are most frequent. The term was first used in 1952 as the title of a research project to study severe weather in areas of Texas, Louisiana, Oklahoma, Kansas, So ...
; the US and Canada have by far the most tornadoes of any countries in the world). Tornados also occur in
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
, much of Europe (except Spain, most of the Alps, Balkans, and northern Scandinavia), western and eastern Australia, New Zealand, Bangladesh and adjacent eastern India, Japan, Philippines, and southeastern South America (Uruguay and Argentina).
Tornadoes can be detected before or as they occur through the use of
pulse-Doppler radar
A pulse-Doppler radar is a radar system that determines the range to a target using pulse-timing techniques, and uses the Doppler effect of the returned signal to determine the target object's velocity. It combines the features of pulse radars an ...
by recognizing patterns in velocity and reflectivity data, such as
hook echo
A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms. It is found in the lower portions of a storm as air and precipitation flow into a mesocyclone, resulting in a curved feature of reflectivity. ...
es or
debris ball
A tornadic debris signature (TDS), often colloquially referred to as a debris ball, is an area of high reflectivity on weather radar caused by debris lofting into the air, usually associated with a tornado. A TDS may also be indicated by dual-polar ...
s, as well as through the efforts of
storm spotter
Storm spotting is a form of weather spotting in which observers watch for the approach of severe weather, monitor its development and progression, and actively relay their findings to local authorities.
History
Storm spotting developed in the ...
s.
Tornado rating scales
There are several scales for rating the strength of tornadoes. The
Fujita scale rates tornadoes by damage caused and has been replaced in some countries by the updated
Enhanced Fujita Scale. An F0 or EF0 tornado, the weakest category, damages trees, but not substantial structures. An
F5 or EF5 tornado, the strongest category, rips buildings off their foundations and can deform large
skyscraper
A skyscraper is a tall continuously habitable building having multiple floors. Modern sources currently define skyscrapers as being at least or in height, though there is no universally accepted definition. Skyscrapers are very tall high-ris ...
s. The similar
TORRO scale ranges from T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. Doppler
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
data,
photogrammetry
Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant ima ...
, and ground swirl patterns (
trochoidal marks) may also be analyzed to determine intensity and assign a rating.
Etymology
The word ''tornado'' comes from the
Spanish word (past participle of 'to turn', or 'to have turned', which comes from the Latin ''tonare'' 'to thunder'.
Tornadoes' opposite phenomena are the widespread, straight-line
derechoes (, from es, derecho , 'straight'). A tornado is also commonly referred to as a "twister" or the old-fashioned colloquial term ''cyclone''.
Definitions
A tornado is a violently rotating column of air, in contact with the ground, either pendant from a
cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.
For a vortex to be classified as a tornado, it must be in contact with both the ground and the cloud base. The term is not precisely defined; for example, there is disagreement as to whether separate touchdowns of the same funnel constitute separate tornadoes.
''Tornado'' refers to the
vortex
In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
of wind, not the condensation cloud.
Funnel cloud
![Tornado with no funnel](https://upload.wikimedia.org/wikipedia/commons/8/8d/Tornado_with_no_funnel.jpg)
A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (as described by
Bernoulli's principle) and rapid rotation (due to
cyclostrophic balance) usually cause
water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
in the air to condense into cloud droplets due to
adiabatic cooling. This results in the formation of a visible funnel cloud or condensation funnel.
There is some disagreement over the definition of a funnel cloud and a condensation funnel. According to the ''Glossary of Meteorology'', a funnel cloud is any rotating cloud pendant from a cumulus or cumulonimbus, and thus most tornadoes are included under this definition. Among many meteorologists, the 'funnel cloud' term is strictly defined as a rotating cloud which is not associated with strong winds at the surface, and condensation funnel is a broad term for any rotating cloud below a cumuliform cloud.
Tornadoes often begin as funnel clouds with no associated strong winds at the surface, and not all funnel clouds evolve into tornadoes. Most tornadoes produce strong winds at the surface while the visible funnel is still above the ground, so it is difficult to discern the difference between a funnel cloud and a tornado from a distance.
Outbreaks and families
Occasionally, a single storm will produce more than one tornado, either simultaneously or in succession. Multiple tornadoes produced by the same
storm cell
A storm cell is an air mass that contains up and down drafts in convective loops and that moves and reacts as a single entity, functioning as the smallest unit of a storm-producing system. An organized grouping of thunder clouds will thus be consi ...
are referred to as a "tornado family". Several tornadoes are sometimes spawned from the same large-scale storm system. If there is no break in activity, this is considered a tornado outbreak (although the term "tornado outbreak" has various definitions). A period of several successive days with tornado outbreaks in the same general area (spawned by multiple weather systems) is a tornado outbreak sequence, occasionally called an extended tornado outbreak.
Characteristics
Size and shape
![Binger Oklahoma Tornado](https://upload.wikimedia.org/wikipedia/commons/d/d1/Binger_Oklahoma_Tornado.jpg)
Most tornadoes take on the appearance of a narrow
funnel, a few hundred meters (yards) across, with a small cloud of debris near the ground. Tornadoes may be obscured completely by rain or dust. These tornadoes are especially dangerous, as even experienced meteorologists might not see them.
Small, relatively weak landspouts may be visible only as a small swirl of dust on the ground. Although the condensation funnel may not extend all the way to the ground, if associated surface winds are greater than , the circulation is considered a tornado.
A tornado with a nearly cylindrical profile and relatively low height is sometimes referred to as a "stovepipe" tornado. Large tornadoes which appear at least as wide as their cloud-to-ground height can look like large
wedges stuck into the ground, and so are known as "wedge tornadoes" or "wedges".
The "stovepipe" classification is also used for this type of tornado if it otherwise fits that profile. A wedge can be so wide that it appears to be a block of dark clouds, wider than the distance from the cloud base to the ground. Even experienced storm observers may not be able to tell the difference between a low-hanging cloud and a wedge tornado from a distance. Many, but not all major tornadoes are wedges.
![Roping tornado](https://upload.wikimedia.org/wikipedia/commons/f/f9/Roping_tornado.jpg)
Tornadoes in the dissipating stage can resemble narrow tubes or ropes, and often curl or twist into complex shapes. These tornadoes are said to be "roping out", or becoming a "rope tornado". When they rope out, the length of their funnel increases, which forces the winds within the funnel to weaken due to
conservation of angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed system ...
.
Multiple-vortex tornadoes can appear as a family of swirls circling a common center, or they may be completely obscured by condensation, dust, and debris, appearing to be a single funnel.
In the United States, tornadoes are around across on average and travel on the ground for .
However, there is a wide range of tornado sizes. Weak tornadoes, or strong yet dissipating tornadoes, can be exceedingly narrow, sometimes only a few feet or couple meters across. One tornado was reported to have a damage path only long.
On the other end of the spectrum, wedge tornadoes can have a damage path a mile (1.6 km) wide or more. A
tornado that affected Hallam, Nebraska on May 22, 2004, was up to wide at the ground, and
a tornado in El Reno, Oklahoma on May 31, 2013, was approximately wide, the widest on record.
In terms of path length, the
Tri-State Tornado
On March 18, 1925, one of the deadliest tornado outbreaks in recorded history generated at least twelve significant tornadoes and spanned a large portion of the midwestern and southern United States. In all, at least 751 people were ki ...
, which affected parts of
Missouri
Missouri is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it is bordered by eight states (tied for the most with Tennessee ...
,
Illinois
Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolita ...
, and
Indiana
Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th s ...
on March 18, 1925, was on the ground continuously for . Many tornadoes which appear to have path lengths of or longer are composed of a family of tornadoes which have formed in quick succession; however, there is no substantial evidence that this occurred in the case of the Tri-State Tornado.
In fact, modern reanalysis of the path suggests that the tornado may have begun further west than previously thought.
Appearance
Tornadoes can have a wide range of colors, depending on the environment in which they form. Those that form in dry environments can be nearly invisible, marked only by swirling debris at the base of the funnel. Condensation funnels that pick up little or no debris can be gray to white. While traveling over a body of water (as a waterspout), tornadoes can turn white or even blue. Slow-moving funnels, which ingest a considerable amount of debris and dirt, are usually darker, taking on the color of debris. Tornadoes in the
Great Plains
The Great Plains (french: Grandes Plaines), sometimes simply "the Plains", is a broad expanse of flatland in North America. It is located west of the Mississippi River and east of the Rocky Mountains, much of it covered in prairie, steppe, an ...
can turn red because of the reddish tint of the soil, and tornadoes in mountainous areas can travel over snow-covered ground, turning white.
![Waurika Oklahoma Tornado Back and Front](https://upload.wikimedia.org/wikipedia/commons/3/30/Waurika_Oklahoma_Tornado_Back_and_Front.jpg)
Lighting conditions are a major factor in the appearance of a tornado. A tornado which is "
back-lit" (viewed with the sun behind it) appears very dark. The same tornado, viewed with the sun at the observer's back, may appear gray or brilliant white. Tornadoes which occur near the time of sunset can be many different colors, appearing in hues of yellow, orange, and pink.
Dust kicked up by the winds of the parent thunderstorm, heavy rain and hail, and the darkness of night are all factors that can reduce the visibility of tornadoes. Tornadoes occurring in these conditions are especially dangerous, since only
weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly puls ...
observations, or possibly the sound of an approaching tornado, serve as any warning to those in the storm's path. Most significant tornadoes form under the storm's ''updraft base'', which is rain-free, making them visible. Also, most tornadoes occur in the late afternoon, when the bright sun can penetrate even the thickest clouds.
There is mounting evidence, including
Doppler on Wheels mobile radar images and eyewitness accounts, that most tornadoes have a clear, calm center with extremely low pressure, akin to the
eye
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...
of
tropical cyclone
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
s. Lightning is said to be the source of illumination for those who claim to have seen the interior of a tornado.
Rotation
Tornadoes normally rotate
cyclonically (when viewed from above, this is counterclockwise in the
northern hemisphere
The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
and clockwise in the
southern
Southern may refer to:
Businesses
* China Southern Airlines, airline based in Guangzhou, China
* Southern Airways, defunct US airline
* Southern Air, air cargo transportation company based in Norwalk, Connecticut, US
* Southern Airways Express, M ...
). While large-scale storms always rotate cyclonically due to the
Coriolis effect, thunderstorms and tornadoes are so small that the direct influence of the Coriolis effect is unimportant, as indicated by their large
Rossby numbers. Supercells and tornadoes rotate cyclonically in numerical simulations even when the Coriolis effect is neglected.
Low-level
mesocyclones and tornadoes owe their rotation to complex processes within the supercell and ambient environment.
Approximately 1 percent of tornadoes rotate in an anticyclonic direction in the northern hemisphere. Typically, systems as weak as landspouts and gustnadoes can rotate anticyclonically, and usually only those which form on the anticyclonic shear side of the descending
rear flank downdraft (RFD) in a cyclonic supercell.
On rare occasions,
anticyclonic tornadoes form in association with the mesoanticyclone of an anticyclonic supercell, in the same manner as the typical cyclonic tornado, or as a companion tornado either as a satellite tornado or associated with anticyclonic eddies within a supercell.
Sound and seismology
![Tornado infrasound sources](https://upload.wikimedia.org/wikipedia/commons/6/63/Tornado_infrasound_sources.png)
Tornadoes emit widely on the
acoustics
Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician ...
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
and the sounds are caused by multiple mechanisms. Various sounds of tornadoes have been reported, mostly related to familiar sounds for the witness and generally some variation of a whooshing roar. Popularly reported sounds include a freight train, rushing rapids or waterfall, a nearby jet engine, or combinations of these. Many tornadoes are not audible from much distance; the nature of and the propagation distance of the audible sound depends on atmospheric conditions and topography.
The winds of the tornado vortex and of constituent turbulent
eddies
In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid beh ...
, as well as airflow interaction with the surface and debris, contribute to the sounds. Funnel clouds also produce sounds. Funnel clouds and small tornadoes are reported as whistling, whining, humming, or the buzzing of innumerable bees or electricity, or more or less harmonic, whereas many tornadoes are reported as a continuous, deep rumbling, or an irregular sound of "noise".
Since many tornadoes are audible only when very near, sound is not to be thought of as a reliable warning signal for a tornado. Tornadoes are also not the only source of such sounds in severe thunderstorms; any strong, damaging wind, a severe hail volley, or continuous thunder in a thunderstorm may produce a roaring sound.
Tornadoes also produce identifiable inaudible
infrasonic
Infrasound, sometimes referred to as low status sound, describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz). Hearing becomes gradually less sensitive as frequency decreases, so for humans to perce ...
signatures.
Unlike audible signatures, tornadic signatures have been isolated; due to the long-distance propagation of low-frequency sound, efforts are ongoing to develop tornado prediction and detection devices with additional value in understanding tornado morphology, dynamics, and creation.
Tornadoes also produce a detectable
seismic signature, and research continues on isolating it and understanding the process.
Electromagnetic, lightning, and other effects
Tornadoes emit on the
electromagnetic spectrum
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...
, with
sferics and
E-field effects detected.
There are observed correlations between tornadoes and patterns of lightning. Tornadic storms do not contain more lightning than other storms and some tornadic cells never produce lightning at all. More often than not, overall cloud-to-ground (CG) lightning activity decreases as a tornado touches the surface and returns to the baseline level when the tornado dissipates. In many cases, intense tornadoes and thunderstorms exhibit an increased and anomalous dominance of positive polarity CG discharges.
Luminosity
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...
has been reported in the past and is probably due to misidentification of external light sources such as lightning, city lights, and
power flash
A power flash is a flash of light caused by arcing electrical discharges from damaged electrical equipment, most often severed or arcing power lines. They are often caused by strong winds, especially those from tropical cyclones and tornadoes, and ...
es from broken lines, as internal sources are now uncommonly reported and are not known to ever have been recorded. In addition to winds, tornadoes also exhibit changes in atmospheric variables such as
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
,
moisture, and
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
. For example, on June 24, 2003, near
Manchester, South Dakota
Manchester was a small unincorporated community in Kingsbury County in the east-central part of the U.S. state of South Dakota. On June 24, 2003, the town was annihilated by a large F4-rated tornado, and has since become a ghost town. In 2004, the ...
, a probe measured a pressure decrease. The pressure dropped gradually as the vortex approached then dropped extremely rapidly to in the core of the violent tornado before rising rapidly as the vortex moved away, resulting in a V-shape pressure trace. Temperature tends to decrease and moisture content to increase in the immediate vicinity of a tornado.
Life cycle
Supercell relationship
Tornadoes often develop from a class of thunderstorms known as supercells. Supercells contain
mesocyclones, an area of organized rotation a few kilometers/miles up in the atmosphere, usually across. Most intense tornadoes (EF3 to EF5 on the
Enhanced Fujita Scale) develop from supercells. In addition to tornadoes, very heavy rain, frequent lightning, strong wind gusts, and hail are common in such storms.
Most tornadoes from supercells follow a recognizable life cycle which begins when increasing rainfall drags with it an area of quickly descending air known as the
rear flank downdraft (RFD). This downdraft accelerates as it approaches the ground, and drags the supercell's rotating mesocyclone towards the ground with it.
Formation
As the mesocyclone lowers below the cloud base, it begins to take in cool, moist air from the downdraft region of the storm. The convergence of warm air in the updraft and cool air causes a rotating wall cloud to form. The RFD also focuses the mesocyclone's base, causing it to draw air from a smaller and smaller area on the ground. As the updraft intensifies, it creates an area of low pressure at the surface. This pulls the focused mesocyclone down, in the form of a visible condensation funnel. As the funnel descends, the RFD also reaches the ground, fanning outward and creating a gust front that can cause severe damage a considerable distance from the tornado. Usually, the funnel cloud begins causing damage on the ground (becoming a tornado) within a few minutes of the RFD reaching the ground.
Maturity
![Union City Oklahoma Tornado (mature)](https://upload.wikimedia.org/wikipedia/commons/6/6b/Union_City_Oklahoma_Tornado_%28mature%29.jpg)
Initially, the tornado has a good source of warm, moist air
flowing inward to power it, and it grows until it reaches the "mature stage". This can last from a few minutes to more than an hour, and during that time a tornado often causes the most damage, and in rare cases can be more than across. The low pressured atmosphere at the base of the tornado is essential to the endurance of the system. Meanwhile, the RFD, now an area of cool surface winds, begins to wrap around the tornado, cutting off the inflow of warm air which previously fed the tornado.
The flow inside the funnel of the tornado is downward, supplying water vapor from the cloud above. This is contrary to the upward flow inside hurricanes, supplying water vapor from the warm ocean below. Therefore, the energy of the tornado is supplied from the cloud above. The complicated mechanism is explained in
Dissipation
![Eastern Colorado Tornado Ropes Out](https://upload.wikimedia.org/wikipedia/commons/7/74/Eastern_Colorado_Tornado_Ropes_Out.jpg)
As the RFD completely wraps around and chokes off the tornado's air supply, the vortex begins to weaken, becoming thin and rope-like. This is the "dissipating stage", often lasting no more than a few minutes, after which the tornado ends. During this stage, the shape of the tornado becomes highly influenced by the winds of the parent storm, and can be blown into fantastic patterns.
Even though the tornado is dissipating, it is still capable of causing damage. The storm is contracting into a rope-like tube and, due to
conservation of angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed system ...
, winds can increase at this point.
Although this is a widely accepted theory for how most tornadoes form, live, and die, it does not explain the formation of smaller tornadoes, such as landspouts, long-lived tornadoes, or tornadoes with multiple vortices. These each have different mechanisms which influence their development—however, most tornadoes follow a pattern similar to this one.
Types
Multiple vortex
![1957 Dallas multi-vortex 1 edited](https://upload.wikimedia.org/wikipedia/commons/e/ea/1957_Dallas_multi-vortex_1_edited.JPG)
A ''multiple-vortex tornado'' is a type of tornado in which two or more columns of spinning air rotate about their own axes and at the same time revolve around a common center. A multi-vortex structure can occur in almost any circulation, but is very often observed in intense tornadoes. These vortices often create small areas of heavier damage along the main tornado path.
This is a phenomenon that is distinct from a
satellite tornado, which is a smaller tornado that forms very near a large, strong tornado contained within the same mesocyclone. The satellite tornado may appear to "
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
" the larger tornado (hence the name), giving the appearance of one, large multi-vortex tornado. However, a satellite tornado is a distinct circulation, and is much smaller than the main funnel.
Waterspout
![Trombe](https://upload.wikimedia.org/wikipedia/commons/e/e8/Trombe.jpg)
A ''waterspout'' is defined by the
National Weather Service
The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...
as a tornado over water. However, researchers typically distinguish "fair weather" waterspouts from tornadic (i.e. associated with a mesocyclone) waterspouts. Fair weather waterspouts are less severe but far more common, and are similar to
dust devil
A dust devil is a strong, well-formed, and relatively short-lived whirlwind. Its size ranges from small (half a metre wide and a few metres tall) to large (more than 10 m wide and more than 1 km tall). The primary vertical motion is u ...
s and
landspouts. They form at the bases of
cumulus congestus clouds over tropical and subtropical waters. They have relatively weak winds, smooth
laminar
Laminar means "flat". Laminar may refer to:
Terms in science and engineering:
* Laminar electronics or organic electronics, a branch of material sciences dealing with electrically conductive polymers and small molecules
* Laminar armour or "band ...
walls, and typically travel very slowly. They occur most commonly in the
Florida Keys
The Florida Keys are a coral cay archipelago located off the southern coast of Florida, forming the southernmost part of the continental United States. They begin at the southeastern coast of the Florida peninsula, about south of Miami, and e ...
and in the northern
Adriatic Sea
The Adriatic Sea () is a body of water separating the Italian Peninsula from the Balkan Peninsula. The Adriatic is the northernmost arm of the Mediterranean Sea, extending from the Strait of Otranto (where it connects to the Ionian Sea) to t ...
.
In contrast, tornadic waterspouts are stronger tornadoes over water. They form over water similarly to mesocyclonic tornadoes, or are stronger tornadoes which cross over water. Since they form from
severe thunderstorms and can be far more intense, faster, and longer-lived than fair weather waterspouts, they are more dangerous. In official tornado statistics, waterspouts are generally not counted unless they affect land, though some European weather agencies count waterspouts and tornadoes together.
Landspout
A ''landspout'', or ''dust-tube tornado'', is a tornado not associated with a mesocyclone. The name stems from their characterization as a "fair weather waterspout on land". Waterspouts and landspouts share many defining characteristics, including relative weakness, short lifespan, and a small, smooth condensation funnel that often does not reach the surface. Landspouts also create a distinctively laminar cloud of dust when they make contact with the ground, due to their differing mechanics from true mesoform tornadoes. Though usually weaker than classic tornadoes, they can produce strong winds which could cause serious damage.
Similar circulations
Gustnado
A ''gustnado'', or ''gust front tornado'', is a small, vertical swirl associated with a
gust front
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a ...
or
downburst. Because they are not connected with a cloud base, there is some debate as to whether or not gustnadoes are tornadoes. They are formed when fast-moving cold, dry outflow air from a
thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are someti ...
is blown through a mass of stationary, warm, moist air near the outflow boundary, resulting in a "rolling" effect (often exemplified through a
roll cloud
An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge or gust fronts o ...
). If low level
wind shear is strong enough, the rotation can be turned vertically or diagonally and make contact with the ground. The result is a gustnado.
Dust devil
![Dust devil](https://upload.wikimedia.org/wikipedia/commons/4/43/Dust_devil.jpg)
A ''dust devil'' (also known as a whirlwind) resembles a tornado in that it is a vertical swirling column of air. However, they form under clear skies and are no stronger than the weakest tornadoes. They form when a strong convective updraft is formed near the ground on a hot day. If there is enough low-level wind shear, the column of hot, rising air can develop a small cyclonic motion that can be seen near the ground. They are not considered tornadoes because they form during fair weather and are not associated with any clouds. However, they can, on occasion, result in major damage.
Fire whirls
Small-scale, tornado-like circulations can occur near any intense surface heat source. Those that occur near intense
wildfire
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire ...
s are called ''fire whirls''. They are not considered tornadoes, except in the rare case where they connect to a
pyrocumulus
A flammagenitus cloud, also known as a flammagenitus, pyrocumulus cloud, or fire cloud, is a dense cumuliform cloud associated with fire or volcanic eruptions. A flammagenitus is similar dynamically in some ways to a firestorm, and the two phen ...
or other cumuliform cloud above. Fire whirls usually are not as strong as tornadoes associated with thunderstorms. They can, however, produce significant damage.
Steam devils
A ''steam devil'' is a
rotating updraft between that involves steam or smoke. These formations do not involve high wind speeds, only completing a few rotations per minute. Steam devils are very rare. They most often form from smoke issuing from a power plant's smokestack.
Hot springs
A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by circ ...
and deserts may also be suitable locations for a tighter, faster-rotating steam devil to form. The phenomenon can occur over water, when cold arctic air passes over relatively warm water.
Intensity and damage
The Fujita scale and the Enhanced Fujita Scale rate tornadoes by damage caused. The Enhanced Fujita (EF) Scale was an update to the older Fujita scale, by
expert elicitation
In science, engineering, and research, expert elicitation is the synthesis of opinions of authorities of a subject where there is uncertainty due to insufficient data or when such data is unattainable because of physical constraints or lack of res ...
, using engineered wind estimates and better damage descriptions. The EF Scale was designed so that a tornado rated on the Fujita scale would receive the same numerical rating, and was implemented starting in the United States in 2007. An EF0 tornado will probably damage trees but not substantial structures, whereas an
EF5 tornado can rip buildings off their foundations leaving them bare and even deform large
skyscraper
A skyscraper is a tall continuously habitable building having multiple floors. Modern sources currently define skyscrapers as being at least or in height, though there is no universally accepted definition. Skyscrapers are very tall high-ris ...
s. The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes.
Doppler weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly puls ...
data,
photogrammetry
Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant ima ...
, and ground swirl patterns (
cycloidal marks) may also be analyzed to determine intensity and award a rating.
Tornadoes vary in intensity regardless of shape, size, and location, though strong tornadoes are typically larger than weak tornadoes. The association with track length and duration also varies, although longer track tornadoes tend to be stronger.
In the case of violent tornadoes, only a small portion of the path is of violent intensity, most of the higher intensity from
subvortices.
In the United States, 80% of tornadoes are EF0 and EF1 (T0 through T3) tornadoes. The rate of occurrence drops off quickly with increasing strength—less than 1% are violent tornadoes (EF4, T8 or stronger).
Current records may significantly underestimate the frequency of strong (EF2-EF3) and violent (EF4-EF5) tornadoes, as damage-based intensity estimates are limited to structures and vegetation that a tornado impacts. A tornado may be much stronger than its damage-based rating indicates if its strongest winds occur away from suitable damage indicators, such as in an open field.
Outside
Tornado Alley
Tornado Alley is a loosely defined area of the central United States where tornadoes are most frequent. The term was first used in 1952 as the title of a research project to study severe weather in areas of Texas, Louisiana, Oklahoma, Kansas, So ...
, and North America in general, violent tornadoes are extremely rare. This is apparently mostly due to the lesser number of tornadoes overall, as research shows that tornado intensity distributions are fairly similar worldwide. A few significant tornadoes occur annually in Europe, Asia, southern Africa, and southeastern South America.
Climatology
![Globdisttornado](https://upload.wikimedia.org/wikipedia/commons/1/1a/Globdisttornado.jpg)
The United States has the most tornadoes of any country, nearly four times more than estimated in all of Europe, excluding waterspouts.
This is mostly due to the unique geography of the continent. North America is a large continent that extends from the
tropics
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referred to ...
north into
arctic
The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...
areas, and has no major east–west mountain range to block air flow between these two areas. In the
middle latitudes
The middle latitudes (also called the mid-latitudes, sometimes midlatitudes, or moderate latitudes) are a spatial region on Earth located between the Tropic of Cancer (latitudes 23°26'22") to the Arctic Circle (66°33'39"), and Tropic of Caprico ...
, where most tornadoes of the world occur, the
Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...
block moisture and buckle the
atmospheric flow, forcing drier air at mid-levels of the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
due to downsloped winds, and causing
the formation of a low pressure area downwind to the east of the mountains. Increased westerly flow off the Rockies force the formation of a
dry line
A dry line (also called a dew point line, or Marfa front, after Marfa, Texas) is a line across a continent that separates moist air and dry air. One of the most prominent examples of such a separation occurs in central North America, especially T ...
when the flow aloft is strong, while the
Gulf of Mexico
The Gulf of Mexico ( es, Golfo de México) is an oceanic basin, ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of ...
fuels abundant low-level moisture in the southerly flow to its east. This unique topography allows for frequent collisions of warm and cold air, the conditions that breed strong, long-lived storms throughout the year. A large portion of these tornadoes form in an area of the
central United States known as
Tornado Alley
Tornado Alley is a loosely defined area of the central United States where tornadoes are most frequent. The term was first used in 1952 as the title of a research project to study severe weather in areas of Texas, Louisiana, Oklahoma, Kansas, So ...
.
This area extends into Canada, particularly
Ontario
Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central Ca ...
and the
Prairie Provinces, although southeast
Quebec
Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirtee ...
, the interior of
British Columbia
British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...
, and western
New Brunswick
New Brunswick (french: Nouveau-Brunswick, , locally ) is one of the thirteen provinces and territories of Canada. It is one of the three Maritime provinces and one of the four Atlantic provinces. It is the only province with both English and ...
are also tornado-prone. Tornadoes also occur across northeastern Mexico.
The United States averages about 1,200 tornadoes per year, followed by Canada, averaging 62 reported per year.
NOAA's has a higher average 100 per year in Canada.
The Netherlands has the highest average number of recorded tornadoes per area of any country (more than 20, or annually), followed by the UK (around 33, per year), although those are of lower intensity, briefer and cause minor damage.
![Tornado Alley](https://upload.wikimedia.org/wikipedia/commons/3/35/Tornado_Alley.gif)
Tornadoes kill an average of 179 people per year in Bangladesh, the most in the world.
Reasons for this include the region's high population density, poor construction quality, and lack of tornado safety knowledge.
Other areas of the world that have frequent tornadoes include South Africa, the
La Plata Basin area, portions of Europe, Australia and New Zealand, and far eastern Asia.
Tornadoes are most common in spring and least common in winter, but tornadoes can occur any time of year that favorable conditions occur.
Spring and fall experience peaks of activity as those are the seasons when stronger winds, wind shear, and atmospheric instability are present.
Tornadoes are focused in the right front quadrant of
landfalling tropical cyclones, which tend to occur in the late summer and autumn. Tornadoes can also be spawned as a result of
eyewall mesovortices, which persist until landfall.
[ ]
Tornado occurrence is highly dependent on the time of day, because of
solar heating.
Worldwide, most tornadoes occur in the late afternoon, between 15:00 (3 pm) and 19:00 (7 pm) local time, with a peak near 17:00 (5 pm). Destructive tornadoes can occur at any time of day. The
Gainesville Tornado of 1936, one of the deadliest tornadoes in history, occurred at 8:30 am local time.
The United Kingdom has the highest incidence of tornadoes per unit area of land in the world. Unsettled conditions and weather fronts transverse the British Isles at all times of the years, and are responsible for spawning the tornadoes, which consequently form at all times of the year. The United Kingdom has at least 34 tornadoes per year and possibly as many as 50. Most tornadoes in the United Kingdom are weak, but they are occasionally destructive. For example, the Birmingham tornado of 2005 and the London tornado of 2006 both registered F2 on the Fujita scale and both caused significant damage and injury.
Associations with climate and climate change
![Tornado US annual count](https://upload.wikimedia.org/wikipedia/commons/7/70/Tornado_US_annual_count.svg)
Associations with various
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
and environmental trends exist. For example, an increase in the
sea surface temperature
Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mass ...
of a source region (e.g. Gulf of Mexico and
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...
) increases atmospheric moisture content. Increased moisture can fuel an increase in
severe weather and tornado activity, particularly in the cool season.
Some evidence does suggest that the
Southern Oscillation is weakly correlated with changes in tornado activity, which vary by season and region, as well as whether the
ENSO phase is that of
El Niño or
La Niña
La Niña (; ) is an oceanic and atmospheric phenomenon that is the colder counterpart of as part of the broader El Niño–Southern Oscillation (ENSO) climate pattern. The name ''La Niña'' originates from Spanish for "the girl", by an ...
.
Research has found that fewer tornadoes and hailstorms occur in winter and spring in the U.S. central and southern plains during El Niño, and more occur during La Niña, than in years when temperatures in the
Pacific are relatively stable. Ocean conditions could be used to forecast extreme spring storm events several months in advance.
Climatic shifts may affect tornadoes via
teleconnection
Teleconnection in atmospheric science refers to climate anomalies being related to each other at large distances (typically thousands of kilometers). The most emblematic teleconnection is that linking sea-level pressure at Tahiti and Darwin, Austr ...
s in shifting the jet stream and the larger weather patterns. The climate-tornado link is confounded by the forces affecting larger patterns and by the local, nuanced nature of tornadoes. Although it is reasonable to suspect that
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
may affect trends in tornado activity,
any such effect is not yet identifiable due to the complexity, local nature of the storms, and database quality issues. Any effect would vary by region.
Detection
![Tornado1857](https://upload.wikimedia.org/wikipedia/commons/7/76/Tornado1857.jpg)
Rigorous attempts to warn of tornadoes began in the United States in the mid-20th century. Before the 1950s, the only method of detecting a tornado was by someone seeing it on the ground. Often, news of a tornado would reach a local weather office after the storm. However, with the advent of weather radar, areas near a local office could get advance warning of severe weather. The first public
tornado warnings were issued in 1950 and the first
tornado watch
A tornado watch ( SAME code: TOA) is a severe weather watch product issued by national weather forecasting agencies when meteorological conditions are favorable for the development of severe thunderstorms capable of producing tornadoes. In addit ...
es and
convective outlooks
The Storm Prediction Center (SPC) is a US government agency that is part of the National Centers for Environmental Prediction (NCEP), operating under the control of the National Weather Service (NWS), which in turn is part of the National Oceani ...
came about in 1952. In 1953, it was confirmed that
hook echo
A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms. It is found in the lower portions of a storm as air and precipitation flow into a mesocyclone, resulting in a curved feature of reflectivity. ...
es were associated with tornadoes. By recognizing these radar signatures, meteorologists could detect thunderstorms probably producing tornadoes from several miles away.
Radar
Today most developed countries have a network of weather radars, which serves as the primary method of detecting hook signatures that are likely associated with tornadoes. In the United States and a few other countries, Doppler weather radar stations are used. These devices measure the velocity and radial
direction (towards or away from the radar) of the winds within a storm, and so can spot evidence of rotation in storms from over away. When storms are distant from a radar, only areas high within the storm are observed and the important areas below are not sampled.
Data resolution also decreases with distance from the radar. Some meteorological situations leading to tornadogenesis are not readily detectable by radar and tornado development may occasionally take place more quickly than radar can complete a scan and send the batch of data. Doppler radar systems can detect
mesocyclones within a supercell thunderstorm. This allows meteorologists to predict tornado formations throughout thunderstorms.
Storm spotting
Spotters usually are trained by the NWS on behalf of their respective organizations, and report to them. The organizations activate public warning systems such as
sirens and the
Emergency Alert System
The Emergency Alert System (EAS) is a national warning system in the United States designed to allow authorized officials to broadcast emergency alerts and warning messages to the public via cable, satellite, or broadcast television, and both ...
(EAS), and they forward the report to the NWS.
There are more than 230,000 trained Skywarn weather spotters across the United States.
In Canada, a similar network of volunteer weather watchers, called
Canwarn
CANWARN, acronym for CANadian Weather Amateur Radio Network, is an organized severe weather spotting and reporting program organized and run by the Meteorological Services Division of Environment Canada. What CANWARN members do is called ground t ...
, helps spot severe weather, with more than 1,000 volunteers.
In Europe, several nations are organizing spotter networks under the auspices of
Skywarn Europe Skywarn Europe is a network of volunteer storm spotters throughout Europe based on the U.S. Skywarn program (a similar program, Canwarn operates in Canada).
Its members attend severe weather training courses and deliver real-time observations of c ...
and the Tornado and Storm Research Organisation (TORRO) has maintained a network of spotters in the United Kingdom since 1974.
Storm spotters are required because radar systems such as
NEXRAD detect signatures that suggest the presence of tornadoes, rather than tornadoes as such.
Radar may give a warning before there is any visual evidence of a tornado or an imminent one, but
ground truth
Ground truth is information that is known to be real or true, provided by direct observation and measurement (i.e. empirical evidence) as opposed to information provided by inference.
Etymology
The ''Oxford English Dictionary'' (s.v. "ground t ...
from an observer can give definitive information. The spotter's ability to see what radar cannot is especially important as distance from the radar site increases, because the radar beam becomes progressively higher in altitude further away from the radar, chiefly due to curvature of Earth, and the beam also spreads out.
Visual evidence
![Wall cloud12 - NOAA](https://upload.wikimedia.org/wikipedia/commons/1/18/Wall_cloud12_-_NOAA.jpg)
Storm spotters are trained to discern whether or not a storm seen from a distance is a supercell. They typically look to its rear, the main region of
updraft and inflow. Under that updraft is a rain-free base, and the next step of
tornadogenesis is the formation of a rotating
wall cloud
A wall cloud (murus or pedestal cloud) is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud and from which tornadoes sometimes form. It is typically beneath the r ...
. The vast majority of intense tornadoes occur with a wall cloud on the backside of a supercell.
Evidence of a supercell is based on the storm's shape and structure, and cloud tower features such as a hard and vigorous updraft tower, a persistent, large
overshooting top, a hard anvil (especially when backsheared against strong upper level
wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...
s), and a corkscrew look or
striations. Under the storm and closer to where most tornadoes are found, evidence of a supercell and the likelihood of a tornado includes inflow bands (particularly when curved) such as a "beaver tail", and other clues such as strength of inflow, warmth and moistness of inflow air, how outflow- or inflow-dominant a storm appears, and how far is the front flank precipitation core from the wall cloud. Tornadogenesis is most likely at the interface of the updraft and
rear flank downdraft, and requires a balance between the outflow and inflow.
Only wall clouds that rotate spawn tornadoes, and they usually precede the tornado between five and thirty minutes. Rotating wall clouds may be a visual manifestation of a low-level mesocyclone. Barring a low-level boundary, tornadogenesis is highly unlikely unless a rear flank downdraft occurs, which is usually visibly evidenced by evaporation of cloud adjacent to a corner of a wall cloud. A tornado often occurs as this happens or shortly afterwards; first, a funnel cloud dips and in nearly all cases by the time it reaches halfway down, a surface swirl has already developed, signifying a tornado is on the ground before condensation connects the surface circulation to the storm. Tornadoes may also develop without wall clouds, under flanking lines and on the leading edge. Spotters watch all areas of a storm, and the
cloud base and surface.
Extremes
![Super Outbreak Map](https://upload.wikimedia.org/wikipedia/commons/1/15/Super_Outbreak_Map.jpg)
The tornado which holds most records in history was the
Tri-State Tornado
On March 18, 1925, one of the deadliest tornado outbreaks in recorded history generated at least twelve significant tornadoes and spanned a large portion of the midwestern and southern United States. In all, at least 751 people were ki ...
, which roared through parts of
Missouri
Missouri is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it is bordered by eight states (tied for the most with Tennessee ...
,
Illinois
Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolita ...
, and
Indiana
Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th s ...
on March 18, 1925. It was likely an F5, though tornadoes were not ranked on any scale in that era. It holds records for longest path length (), longest duration (about 3.5 hours), and fastest forward speed for a significant tornado () anywhere on Earth. In addition, it is the deadliest single tornado in United States history (695 dead).
The tornado was also the costliest tornado in history at the time (unadjusted for inflation), but in the years since has been surpassed by several others if population changes over time are not considered. When costs are normalized for wealth and inflation, it ranks third today.
The deadliest tornado in world history was the
Daultipur-Salturia Tornado in Bangladesh on April 26, 1989, which killed approximately 1,300 people.
One of the most extensive
tornado outbreak
__NOTOC__
A tornado outbreak is the occurrence of multiple tornadoes spawned by the same synoptic scale weather system. The number of tornadoes required to qualify as an outbreak typically are at least six to ten, with at least two rotational l ...
s on record was the
1974 Super Outbreak
The 1974 Super Outbreak was the second-largest tornado outbreak on record for a single 24-hour period, just behind the 2011 Super Outbreak. It was also the most violent tornado outbreak ever recorded, with 30 F4/F5 tornadoes confirmed. From Apri ...
, which affected a large area of the central United States and extreme southern
Ontario
Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central Ca ...
on April 3 and 4, 1974. The outbreak featured 148 tornadoes in 18 hours, many of which were violent; six were of F5 intensity, and twenty-four peaked at F4 strength. Sixteen tornadoes were on the ground at the same time during its peak. More than 300 people, possibly as many as 330, were killed.
While direct measurement of the most violent tornado wind speeds is nearly impossible, since conventional
anemometers would be destroyed by the intense winds and flying debris, some tornadoes have been scanned by
mobile Doppler radar units, which can provide a good estimate of the tornado's winds. The highest wind speed ever measured in a tornado, which is also the highest wind speed ever recorded on the planet, is 301 ± 20 mph (484 ± 32 km/h) in the F5
Bridge Creek-Moore, Oklahoma, tornado which killed 36 people. The reading was taken about above the ground.
Storms that produce tornadoes can feature intense updrafts, sometimes exceeding . Debris from a tornado can be lofted into the parent storm and carried a very long distance. A tornado which affected
Great Bend, Kansas, in November 1915, was an extreme case, where a "rain of debris" occurred from the town, a sack of flour was found away, and a cancelled check from the Great Bend bank was found in a field outside of
Palmyra, Nebraska
Palmyra is a village in Otoe County, Nebraska, United States. The population was 545 at the 2010 census.
History
Palmyra was platted in 1870. Its name commemorates the ancient city of Palmyra. 1925 editionis available for download aUniversity of ...
, to the northeast.
Waterspouts and tornadoes have been advanced as an explanation for instances of
raining fish and other animals.
Safety
![Birmingham tornado 2005 damage](https://upload.wikimedia.org/wikipedia/commons/7/7c/Birmingham_tornado_2005_damage.jpg)
Though tornadoes can strike in an instant, there are precautions and preventative measures that can be taken to increase the chances of survival. Authorities such as the
Storm Prediction Center in the United States advise having a pre-determined plan should a tornado warning be issued. When a warning is issued, going to a basement or an interior first-floor room of a sturdy building greatly increases chances of survival.
In tornado-prone areas, many buildings have underground
storm cellars, which have saved thousands of lives.
Some countries have meteorological agencies which distribute tornado forecasts and increase levels of alert of a possible tornado (such as
tornado watch
A tornado watch ( SAME code: TOA) is a severe weather watch product issued by national weather forecasting agencies when meteorological conditions are favorable for the development of severe thunderstorms capable of producing tornadoes. In addit ...
es and
warnings in the United States and Canada).
Weather radio A weather radio is a specialized radio receiver that is designed to receive a public broadcast service, typically from government-owned radio stations, dedicated to broadcasting weather forecasts and reports on a continual basis, with the routine we ...
s provide an alarm when a severe weather advisory is issued for the local area, mainly available only in the United States. Unless the tornado is far away and highly visible, meteorologists advise that drivers park their vehicles far to the side of the road (so as not to block emergency traffic), and find a sturdy shelter. If no sturdy shelter is nearby, getting low in a ditch is the next best option. Highway overpasses are one of the worst places to take shelter during tornadoes, as the constricted space can be subject to increased wind speed and funneling of debris underneath the overpass.
Myths and misconceptions
Folklore often identifies a green sky with tornadoes, and though the phenomenon may be associated with severe weather, there is no evidence linking it specifically with tornadoes. It is often thought that opening windows will lessen the damage caused by the tornado. While there is a large drop in
atmospheric pressure inside a strong tornado, the pressure difference is unlikely to cause significant damage. Opening windows may instead increase the severity of the tornado's damage.
A violent tornado can destroy a house whether its windows are open or closed.
![1999 Salt Lake City tornado](https://upload.wikimedia.org/wikipedia/commons/3/3e/1999_Salt_Lake_City_tornado.jpg)
Another commonly held misconception is that highway overpasses provide adequate shelter from tornadoes. This belief is partly inspired by widely circulated video captured during the
1991 tornado outbreak near
Andover, Kansas
Andover is a city in Butler County, Kansas, United States, and a suburb of Wichita. As of the 2020 census, the population of the city was 14,892. Andover is the most populated city in Butler County.
History
19th century
When the Kansas te ...
, where a news crew and several other people took shelter under an overpass on the
Kansas Turnpike and safely rode out a tornado as it passed by.
However, a highway overpass is a dangerous place during a tornado, and the subjects of the video remained safe due to an unlikely combination of events: the storm in question was a weak tornado, the tornado did not directly strike the overpass, and the overpass itself was of a unique design.
[ Due to the Venturi effect, tornadic winds are accelerated in the confined space of an overpass. Indeed, in the ]1999 Oklahoma tornado outbreak
The 1999 Oklahoma tornado outbreak was a significant tornado outbreak that affected much of the Central and parts of the Eastern United States, with the highest record-breaking wind speeds of . During this week-long event, 154 tornadoes touched ...
of May 3, 1999, three highway overpasses were directly struck by tornadoes, and at each of the three locations there was a fatality, along with many life-threatening injuries. By comparison, during the same tornado outbreak, more than 2,000 homes were completely destroyed and another 7,000 damaged, and yet only a few dozen people died in their homes.
An old belief is that the southwest corner of a basement provides the most protection during a tornado. The safest place is the side or corner of an underground room opposite the tornado's direction of approach (usually the northeast corner), or the central-most room on the lowest floor. Taking shelter in a basement, under a staircase, or under a sturdy piece of furniture such as a workbench further increases the chances of survival.
There are areas which people believe to be protected from tornadoes, whether by being in a city, near a major river, hill, or mountain, or even protected by supernatural
Supernatural refers to phenomena or entities that are beyond the laws of nature. The term is derived from Medieval Latin , from Latin (above, beyond, or outside of) + (nature) Though the corollary term "nature", has had multiple meanings si ...
forces. Tornadoes have been known to cross major rivers, climb mountains, affect valleys, and have damaged several city centers. As a general rule, no area is safe from tornadoes, though some areas are more susceptible than others.
Ongoing research
Meteorology is a relatively young science and the study of tornadoes is newer still. Although researched for about 140 years and intensively for around 60 years, there are still aspects of tornadoes which remain a mystery. Meteorologists have a fairly good understanding of the development of thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are someti ...
s and mesocyclones, and the meteorological conditions conducive to their formation. However, the step from supercell, or other respective formative processes, to tornadogenesis and the prediction of tornadic vs. non-tornadic mesocyclones is not yet well known and is the focus of much research.
Also under study are the low-level mesocyclone and the stretching of low-level vorticity which tightens into a tornado, in particular, what are the processes and what is the relationship of the environment and the convective storm. Intense tornadoes have been observed forming simultaneously with a mesocyclone aloft (rather than succeeding mesocyclogenesis) and some intense tornadoes have occurred without a mid-level mesocyclone.
In particular, the role of downdrafts, particularly the rear-flank downdraft, and the role of baroclinic boundaries, are intense areas of study.
Reliably predicting tornado intensity and longevity remains a problem, as do details affecting characteristics of a tornado during its life cycle and tornadolysis. Other rich areas of research are tornadoes associated with mesovortices A mesovortex is a small-scale rotational feature found in a convective storm, such as a quasi-linear convective system (QLCS, i.e. squall line), a supercell, or the eyewall of a tropical cyclone. Mesovortices range in diameter from tens of miles to ...
within linear thunderstorm structures and within tropical cyclones.
Meteorologists still do not know the exact mechanisms by which most tornadoes form, and occasional tornadoes still strike without a tornado warning being issued. Analysis of observations including both stationary and mobile (surface and aerial) in-situ and remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
(passive and active) instruments generates new ideas and refines existing notions. Numerical modeling also provides new insights as observations and new discoveries are integrated into our physical understanding and then tested in computer simulation
Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be dete ...
s which validate new notions as well as produce entirely new theoretical findings, many of which are otherwise unattainable. Importantly, development of new observation technologies and installation of finer spatial and temporal resolution observation networks have aided increased understanding and better predictions.
Research programs, including field projects such as the VORTEX projects (Verification of the Origins of Rotation in Tornadoes Experiment), deployment of TOTO (the TOtable Tornado Observatory), Doppler on Wheels (DOW), and dozens of other programs, hope to solve many questions that still plague meteorologists. Universities, government agencies such as the National Severe Storms Laboratory, private-sector meteorologists, and the National Center for Atmospheric Research are some of the organizations very active in research; with various sources of funding, both private and public, a chief entity being the National Science Foundation
The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National I ...
. The pace of research is partly constrained by the number of observations that can be taken; gaps in information about the wind, pressure, and moisture content throughout the local atmosphere; and the computing power available for simulation.
Solar storms similar to tornadoes have been recorded, but it is unknown how closely related they are to their terrestrial counterparts.
Gallery
File:Tornado time lapse.webm, Time-lapse of a tornado's life cycle near Prospect Valley, Colorado
Prospect Valley is a populated place in Weld County, Colorado
Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northea ...
on June 19, 2018.
File:Seymour Texas Tornado.jpg, A tornado that occurred at Seymour, Texas in April 1979.
File:2013ElRenoOKtornado.jpg, The record-breaking wide El Reno, Oklahoma tornado.
File:Roanoke tornado.jpg, F4 tornado in Roanoke, Illinois on July 13, 2004.
File:Tornadic classic supercell radar.gif, A radar reflectivity image of a classic tornadic supercell near Oklahoma City, Oklahoma
Oklahoma City (), officially the City of Oklahoma City, and often shortened to OKC, is the capital and largest city of the U.S. state of Oklahoma. The county seat of Oklahoma County, it ranks 20th among United States cities in population, and ...
on May 3, 1999.
File:Tornado near Punkin Center, CO.jpg, A wall cloud with tornado South of Limon, Colorado.
File:April 14, 2012 Marquette, Kansas EF4 tornado.JPG, EF4 tornado near Marquette, Kansas
Marquette is a city in McPherson County, Kansas, United States. As of the 2020 census, the population of the city was 599. It is located between K-4 and the Smoky Hill River.
History
For millennia, the land now known as Kansas was inhabited ...
on April 14, 2012.
File:NorthSeaTornadoEF0.jpg, F0 tornado in its final stages over the North Sea near Vrångö, Sweden on July 17, 2011.
See also
* Cultural significance of tornadoes
Tornado damage to human-made structures is a result of the high wind velocity and windblown debris. Tornadic winds have been measured in excess of 300 mph (480 km/h). Tornadoes are a serious hazard to life and limb. As such, people i ...
* Cyclone
In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...
* Derecho
* List of tornadoes and tornado outbreaks
A ''list'' is any set of items in a row. List or lists may also refer to:
People
* List (surname)
Organizations
* List College, an undergraduate division of the Jewish Theological Seminary of America
* SC Germania List, German rugby unio ...
**List of F5 and EF5 tornadoes
This is a list of tornadoes which have been officially or unofficially labeled as F5, EF5, T10-T11, IF5, or an equivalent rating, the highest possible ratings on the various tornado intensity scales. These scales – the Fujita scale, the Enhance ...
**List of F4 and EF4 tornadoes
This is a list of tornadoes which have been officially or unofficially labeled as F4, EF4, IF4, or an equivalent rating. These scales – the Fujita scale, the Enhanced Fujita scale, the International Fujita scale, and the TORRO tornado inten ...
*** List of F4 and EF4 tornadoes (2020–present)
* List of tropical cyclone-spawned tornadoes
Intense tropical cyclones usually produce tornadoes, the majority of those weak, especially upon landfall.
List of tornadoes
These are the tropical cyclones that are known to have spawned tornadoes. The list is most complete for the U.S., but ...
* List of tornadoes with confirmed satellite tornadoes
* Secondary flow
* Skipping tornado
A skipping tornado is a process tornado (or a series of tornadoes) which has a discontinuous damage path.
There are several possible causes for this phenomenon:
# The tornado actually lifting from the surface (which technically may make the two da ...
* Space tornado
A space tornado is a solar windstorm and is exceptionally larger and more powerful than conventional tornadoes on Earth. They are also thought to produce the ''aurora borealis'' phenomenon.
Tornadoes on Earth are formed within the atmosphere by ...
* Tornado preparedness
The term "tornado preparedness" refers to safety precautions made before the arrival of and during a tornado. Historically, the steps taken have varied greatly, depending on location, or time remaining before a tornado was expected. For example, i ...
* Tornadoes of
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
* Tropical cyclone
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
* Hypercane
A hypercane is a hypothetical class of extreme tropical cyclone that could form if sea surface temperatures reached approximately , which is warmer than the warmest ocean temperature ever recorded. Such an increase could be caused by a large ast ...
* Typhoon
A typhoon is a mature tropical cyclone that develops between 180° and 100°E in the Northern Hemisphere. This region is referred to as the Northwestern Pacific Basin, and is the most active tropical cyclone basin on Earth, accounting for a ...
* Whirlwind
References
Further reading
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* Heavily illustrated.
External links
NOAA Storm Events Database 1950–present
European Severe Weather Database
Electronic Journal of Severe Storms Meteorology
Tornado History Project – Maps and statistics from 1950 to present
Physics Today What we know and don't know about Tornadoes September 2014
U.S. Billion-dollar Weather and Climate Disasters
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Weather hazards
Severe weather and convection
Types of cyclone
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