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Tornado

Tornado

Overview

A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud
Cumulonimbus cloud
Cumulonimbus is a towering vertical cloud that is very tall, dense, and involved in thunderstorms and other inclement weather. Cumulonimbus originates from Latin: Cumulus "Heap" and nimbus "rain". It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold...

 or, in rare cases, the base of a cumulus cloud
Cumulus cloud
Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...

. They are often referred to as a twister or a cyclone, although the word cyclone
Cyclone
In meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale...

 is used in meteorology in a wider sense, to name any closed low pressure
Low pressure area
A low-pressure area, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low-pressure systems form under areas of wind divergence which occur in upper levels of the troposphere. The formation process of a low-pressure area is known as...

 circulation. Tornadoes come in many shapes and sizes, but are typically in the form of a visible condensation funnel
Funnel cloud
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 but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud...

, whose narrow end touches the earth and is often encircled by a cloud of debris
Debris
Debris is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, in geology, large rock fragments left by a melting glacier etc. The singular form of debris is debris...

 and dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

.
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Timeline

1680   The first confirmed tornado in America kills a servant at Cambridge, Massachusetts.

1840   The Great Natchez Tornado strikes Natchez, Mississippi killing 317 people. It is the second deadliest tornado in United States history.

1880   An F4 tornado strikes Marshfield, Missouri, killing 99 people and injuring 100.

1893   The small town of Pomeroy, Iowa, is nearly destroyed by a tornado that kills 71 people and injures 200.

1899   New Richmond Tornado: the eighth deadliest tornado in U.S. history kills 117 people and injures around 200.

1926   The deadliest November tornado outbreak in U.S. history strikes on Thanksgiving day. 27 twisters of great strength are reported in the Midwest, including the strongest November tornado, an estimated F4, that devastates Heber Springs, Arkansas. There are 51 deaths in Arkansas alone, 76 deaths and over 400 injuries in all.

1948   The first successful tornado forecast predicts that a tornado will strike Tinker Air Force Base, Oklahoma.

1953   In Warner Robins, Georgia, an F4 tornado kills 18 people.

1953   Flint-Worcester tornado outbreak sequence: A tornado hits Flint, Michigan, and kills 115. This is the last tornado to claim more than 100 lives.

1955   In the United States, a night time F5 tornado strikes the small city of Udall, Kansas, killing 80 and injuring 273. It is the deadliest tornado to ever occur in the state and the 23rd deadliest in the U.S.

 
Quotations

Scenes of suffering and horror marked the storm and fire. Throughout the night relief workers and ambulances endeavored to make their way through the streets strewn with wreckage, fallen telegraph poles and wires and burning embers. The only light afforded was that of the burning area…

St. Louis Post-Dispatch, March 20, 1925
Encyclopedia

A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud
Cumulonimbus cloud
Cumulonimbus is a towering vertical cloud that is very tall, dense, and involved in thunderstorms and other inclement weather. Cumulonimbus originates from Latin: Cumulus "Heap" and nimbus "rain". It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold...

 or, in rare cases, the base of a cumulus cloud
Cumulus cloud
Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...

. They are often referred to as a twister or a cyclone, although the word cyclone
Cyclone
In meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale...

 is used in meteorology in a wider sense, to name any closed low pressure
Low pressure area
A low-pressure area, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low-pressure systems form under areas of wind divergence which occur in upper levels of the troposphere. The formation process of a low-pressure area is known as...

 circulation. Tornadoes come in many shapes and sizes, but are typically in the form of a visible condensation funnel
Funnel cloud
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 but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud...

, whose narrow end touches the earth and is often encircled by a cloud of debris
Debris
Debris is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, in geology, large rock fragments left by a melting glacier etc. The singular form of debris is debris...

 and dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

. Most tornadoes have wind speeds less than 110 miles per hour (177 km/h), are approximately 250 feet (76.2 m) across, and travel a few miles (several kilometers) before dissipating. The most extreme
1999 Bridge Creek – Moore tornado
The 1999 Bridge Creek – Moore tornado was an extremely powerful F5 tornado which devastated towns just outside of Oklahoma City on May 3, 1999. Throughout its one hour and 25 minute existence, the tornado covered , destroying thousands of homes, killing 41 people and leaving...

 tornadoes can attain wind speeds of more than 300 mph (480 km/h), stretch more than two miles (3 km) across, and stay on the ground for dozens of miles (more than 100 km).

Various types of tornadoes include the landspout
Landspout
A lololol is a slang term coined by meteorologist Howard B. Bluestein in 1985 for a kind of tornado not associated with the mesocyclone of a thunderstorm...

, multiple vortex tornado
Multiple vortex tornado
A multiple-vortex tornado is a tornado that contains several vortices rotating around, inside of, and as part of the main vortex. These multiple vortices are somewhat similar to eyewall mesovortices found in intense tropical cyclones...

, and waterspout
Waterspout
A waterspout is an intense columnar vortex that occurs over a body of water and is connected to a cumuliform cloud. In the common form, it is a non-supercell tornado over water. While it is often weaker than most of its land counterparts, stronger versions spawned by mesocyclones do occur...

. Waterspouts are characterized by a spiraling funnel-shaped wind current, connecting to a large cumulus or cumulonimbus cloud. They are generally classified as non-supercell
Supercell
A supercell is a thunderstorm that is characterized by the presence of a mesocyclone: a deep, continuously-rotating updraft. For this reason, these storms are sometimes referred to as rotating thunderstorms...

ular tornadoes that develop over bodies of water. These spiraling columns of air frequently develop in tropical areas close to the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

, and are less common at high latitudes. Other tornado-like phenomena that exist in nature include the gustnado
Gustnado
A gustnado is a specific type of short-lived, low-level rotating cloud that can form in a severe thunderstorm. The name is a portmanteau of "gust front tornado", as gustnadoes form due to non-tornadic cyclonic features in the downdraft from the gust front of a strong thunderstorm, especially one...

, dust devil
Dust devil
A dust devil is a strong, well-formed, and relatively long-lived whirlwind, ranging from small to large . The primary vertical motion is upward...

, fire whirl
Fire whirl
A fire whirl, colloquially fire devil or fire tornado, is a phenomenon—rarely captured on camera—in which a fire, under certain conditions , acquires a vertical vorticity and forms a whirl, or a tornado-like vertically oriented rotating column of air...

s, and steam devil
Steam devil
A steam devil is a small, weak whirlwind over water 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 warm, and can be an important mechanism in vertically transporting...

.

Tornadoes have been observed on every continent except Antarctica. However, the vast majority of tornadoes in the world occur in the Tornado Alley
Tornado Alley
Tornado Alley is a colloquial and popular media term that most often refers to the area of the United States where tornadoes are most frequent. Although an official location is not defined, the area between the Rocky Mountains and Appalachian Mountains is usually associated with it.The areas...

 region of the United States, although they can occur nearly anywhere in North America. They also occasionally occur in south-central and eastern Asia, the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...

, south east Asia, like Malaysia, northern and east-central South America, Southern Africa
Southern Africa
Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...

, northwestern and southeast Europe, western and southeastern Australia, and New Zealand. Tornadoes can be detected before or as they occur through the use of Pulse-Doppler radar
Pulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...

 by recognizing patterns in velocity and reflectivity data, such as hook echo
Hook echo
The hook echo is one of the classical hallmarks of tornado-producing supercell thunderstorms as seen on a weather radar. The echo is produced by rain, hail, or even debris being wrapped around the supercell...

es, as well as by the efforts of storm spotter
Storm spotter
A storm spotter is a specific type of weather spotter who actively maintains a visual watch of the development and progression of specific weather events while actively relaying important information to the local weather agency in a timely manner.- History :...

s.

There are several different scales for rating the strength of tornadoes. The Fujita scale
Fujita scale
The Fujita scale , or Fujita-Pearson scale, is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation...

 rates tornadoes by damage caused, and has been replaced in some countries by the updated Enhanced Fujita Scale
Enhanced Fujita Scale
The Enhanced Fujita Scale rates the strength of tornadoes in the United States based on the damage they cause.Implemented in place of the Fujita scale introduced in 1971 by Ted Fujita, it began operational use on February 1, 2007. The scale has the same basic design as the original 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
Skyscraper
A skyscraper is a tall, continuously habitable building of many stories, often designed for office and commercial use. There is no official definition or height above which a building may be classified as a skyscraper...

s. The similar TORRO scale
TORRO scale
The TORRO tornado intensity scale is a scale measuring tornado intensity between T0 and T11. It was developed by Terence Meaden of the Tornado and Storm Research Organisation , a meteorological organisation in the United Kingdom, as an extension of the Beaufort scale.- History and derivation from...

 ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. Doppler radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...

 data, photogrammetry
Photogrammetry
Photogrammetry is the practice of determining the geometric properties of objects from photographic images. Photogrammetry is as old as modern photography and can be dated to the mid-nineteenth century....

, and ground swirl patterns (cycloid
Cycloid
A cycloid is the curve traced by a point on the rim of a circular wheel as the wheel rolls along a straight line.It is an example of a roulette, a curve generated by a curve rolling on another curve....

al marks) may also be analyzed to determine intensity and assign a rating.

Etymology


The word tornado is an altered form of the Spanish word tronada, which means "thunderstorm". This in turn was taken from the Latin tonare, meaning "to thunder". It most likely reached its present form through a combination of the Spanish tronada and tornar ("to turn"); however, this may be a folk etymology. A tornado is also commonly referred to as a "twister", and is also sometimes referred to by the old-fashioned colloquial term cyclone
Cyclone
In meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale...

. The term "cyclone" is used as a synonym for "tornado" in the often-aired 1939 film, The Wizard of Oz
The Wizard of Oz (1939 film)
The Wizard of Oz is a 1939 American musical fantasy film produced by Metro-Goldwyn-Mayer. It was directed primarily by Victor Fleming. Noel Langley, Florence Ryerson and Edgar Allan Woolf received credit for the screenplay, but there were uncredited contributions by others. The lyrics for the songs...

. The term "twister" is also used in that film, along with being the title of the 1996 tornado-related film Twister.

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. Scientists have not yet created a complete definition of the word; for example, there is disagreement as to whether separate touchdowns of the same funnel constitute separate tornadoes. Tornado refers to the vortex
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...

 of wind, not the condensation cloud.

Funnel cloud




A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (as described by Bernoulli's principle
Bernoulli's principle
In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy...

) and rapid rotation (due to cyclostrophic balance) usually causes water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 in the air to become visible as a funnel cloud or condensation funnel.

There is some disagreement over the definition of funnel cloud and 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, although not all evolve into a tornado. However, many tornadoes are preceded by a funnel cloud. 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
Storm cell
A storm cell is an air mass that contains up and down drafts in convective loops, moves and reacts as a single entity, and functions as the smallest unit of a storm-producing system....

 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 there are 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.

Size and shape



Most tornadoes take on the appearance of a narrow funnel
Funnel
A funnel is a pipe with a wide, often conical mouth and a narrow stem. It is used to channel liquid or fine-grained substances into containers with a small opening. Without a funnel, spillage would occur....

, a few hundred yards (meters) 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. Tornadoes can appear in many shapes and sizes.

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 40 mph (64 km/h), the circulation is considered a tornado. A tornado with a nearly cylindrical profile and relative low height is sometimes referred to as a "stovepipe" tornado. Large single-vortex tornadoes 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.

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. Multiple-vortex tornadoes can appear as a family of swirls circling a common center, or may be completely obscured by condensation, dust, and debris, appearing to be a single funnel.

In the United States, tornadoes are around 500 feet (150 m) across on average and stay on the ground for 5 miles (8 km). Yet, 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 7 feet (2 m) 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 2.5 miles (4 km) wide at the ground.

In terms of path length, the Tri-State Tornado
Tri-State Tornado
The Tri-State Tornado of Wednesday, March 18, 1925, was the deadliest tornado in U.S. history. With 695 confirmed fatalities, the tornado killed more than twice as many as the second deadliest, the 1840 Great Natchez Tornado...

, which affected parts of Missouri
Missouri
Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...

, Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

, and Indiana
Indiana
Indiana is a US state, admitted to the United States as the 19th on December 11, 1816. It is located in the Midwestern United States and Great Lakes Region. With 6,483,802 residents, the state is ranked 15th in population and 16th in population density. Indiana is ranked 38th in land area and is...

 on March 18, 1925, was on the ground continuously for 219 miles (352.4 km). Many tornadoes which appear to have path lengths of 100 miles (160.9 km) 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. Modern reanalysis of the path suggests that the tornado may have begun 15 miles (24.1 km) further west than previously thought, lengthening its track.

Appearance


Tornadoes can have a wide range of colors, depending on the environment in which they form. Those which form in a dry environment can be nearly invisible, marked only by swirling debris at the base of the funnel. Condensation funnels which pick up little or no debris can be gray to white. While traveling over a body of water as a waterspout, they can turn very white or even blue. Funnels which move slowly, ingesting a lot of debris and dirt, are usually darker, taking on the color of debris. Tornadoes in the Great Plains
Great Plains
The Great Plains are a broad expanse of flat land, much of it covered in prairie, steppe and grassland, which lies west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S...

 can turn red because of the reddish tint of the soil, and tornadoes in mountainous areas can travel over snow-covered ground, turning white.

Lighting conditions are a major factor in the appearance of a tornado. A tornado which is "back-lit
Backlighting (lighting design)
Backlighting refers to the process of illuminating the subject from the back. In other words, the lighting instrument and the viewer are facing towards each other, with the subject in between. This causes the edges of the subject to glow, while the other areas remain darker. The backlight can be a...

" (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 which can reduce the visibility of tornadoes. Tornadoes occurring in these conditions are especially dangerous, since only weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...

 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. Night-time tornadoes are often illuminated by frequent lightning.

There is mounting evidence, including Doppler On Wheels
Doppler On Wheels
Doppler On Wheels is a fleet of radar trucks maintained by the Center for Severe Weather Research led by Joshua Wurman, with the funding mainly provided by the National Science Foundation...

 mobile radar images and eyewitness accounts, that most tornadoes have a clear, calm center with extremely low pressure, akin to the eye
Eye (cyclone)
The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the second most severe weather of a cyclone...

 of tropical cyclone
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...

s. This area would be clear (possibly full of dust), have relatively light winds, and be very dark, since the light would be blocked by swirling debris on the outside of the tornado. 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
Cyclone
In meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale...

 in direction (counterclockwise in the northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

, clockwise in the southern
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

). While large-scale storms always rotate cyclonically due to the Coriolis effect
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...

, thunderstorms and tornadoes are so small that the direct influence of the Coriolis effect is unimportant, as indicated by their large Rossby number
Rossby number
The Rossby number, named for Carl-Gustav Arvid Rossby, is a dimensionless number used in describing fluid flow. The Rossby number is the ratio of inertial to Coriolis force, terms v\cdot\nabla v\sim U^2 / L and \Omega\times v\sim U\Omega in the Navier–Stokes equations, respectively...

s. Supercells and tornadoes rotate cyclonically in numerical simulations even when the Coriolis effect is neglected. Low-level mesocyclone
Mesocyclone
A mesocyclone is a vortex of air, approximately 2 to 10 miles in diameter , within a convective storm....

s 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
Rear flank downdraft
The rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm. These areas of descending air are thought to be essential in the production of many supercellular tornadoes...

 in a cyclonic supercell. On rare occasions, anticyclonic tornado
Anticyclonic tornado
An anticyclonic tornado is a tornado which rotates in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. It is simply a naming convention denoting the anomaly from normal rotation which is cyclonic in upwards of 98 percent of tornadoes...

es 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


Tornadoes emit widely on the acoustics
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...

 spectrum
Audio frequency
An audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...

 and the sounds are caused by multiple mechanisms. Various sounds of tornadoes have been reported throughout time, 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 and propagation distance of the audible sound depends on atmospheric conditions and topography.

The winds of the tornado vortex and of constituent turbulent eddies
Eddy (fluid dynamics)
In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object...

, 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 reliable warning of a tornado. And, any strong, damaging wind, even a severe hail volley or continuous thunder in a thunderstorm may produce a roaring sound.


Tornadoes also produce identifiable inaudible infrasonic 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
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

, 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. More often than not, overall cloud-to-ground (CG) lightning activity decreases as a tornado reaches the surface and returns to the baseline level when the tornado lifts. In many cases, intense tornadoes and thunderstorms exhibit an increased and anomalous dominance of positive polarity CG discharges. Electromagnetics and lightning have little or nothing to do directly with what drives tornadoes (tornadoes are basically a thermodynamic phenomenon), although there are likely connections with the storm and environment affecting both phenomena.

Luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

 has been reported in the past and is probably due to misidentification of external light sources such as lightning, city lights, and power flash
Power flash
A power flash is a flash of light caused by arcing electricity from damaged electrical equipment. They are often caused by tornadoes, and thus are used by storm spotters to spot tornadoes which would otherwise be invisible due to rain or darkness....

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
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, moisture
Moisture
Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

, and pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

. For example, on June 24, 2003 near Manchester, South Dakota
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 completely annihilated by a large F4-rated tornado, and has since become a ghost town.-History:...

, a probe measured a 100 mbar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 (hPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

) (2.95 inHg
Inch of mercury
Inches of mercury, ' is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports, refrigeration and aviation in the United States, but is seldom used elsewhere....

) pressure decrease. The pressure dropped gradually as the vortex approached then dropped extremely rapidly to 850 mbar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 (hPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

) (25.10 inHg
Inch of mercury
Inches of mercury, ' is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports, refrigeration and aviation in the United States, but is seldom used elsewhere....

) 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 miles up in the atmosphere, usually 1–6 miles (2–10 km) across. Most intense tornadoes (EF3 to EF5 on the Enhanced Fujita Scale
Enhanced Fujita Scale
The Enhanced Fujita Scale rates the strength of tornadoes in the United States based on the damage they cause.Implemented in place of the Fujita scale introduced in 1971 by Ted Fujita, it began operational use on February 1, 2007. The scale has the same basic design as the original 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. That 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 approaches the ground, a visible condensation funnel appears to descend from the base of the storm, often from a rotating wall cloud
Wall cloud
A wall cloud is a large, lowering, and rotating base of a cumulonimbus cloud that potentially forms tornadoes. It is typically beneath the rain-free base portion of a deep cumulus cloud , and indicates the area of primary and strongest updraft which condenses into cloud at altitudes lower than...

. As the funnel descends, the RFD also reaches the ground, creating a gust front that can cause damage a good distance from the tornado. Usually, the funnel cloud becomes a tornado within minutes of the RFD reaching the ground.

Maturity


Initially, the tornado has a good source of warm, moist inflow
Inflow (meteorology)
Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and moisture from air within the Earth's atmosphere into storm systems. Extratropical cyclones are fed by inflow focused along their cold front and warm fronts...

 to power it, so it grows until it reaches the "mature stage". This can last anywhere 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 one mile (1.6 km) across. Meanwhile, the RFD, now an area of cool surface winds, begins to wrap around the tornado, cutting off the inflow of warm air which feeds the tornado.

Demise


As the RFD completely wraps around and chokes off the tornado's air supply, the vortex begins to weaken, and become thin and rope-like. This is the "dissipating stage"; often lasting no more than a few minutes, after which the tornado fizzles. 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, like the ice skater who pulls her arms in to spin faster, winds can increase at this point.

As the tornado enters the dissipating stage, its associated mesocyclone often weakens as well, as the rear flank downdraft cuts off the inflow powering it. In particular, intense supercells tornadoes can develop cyclically. As the first mesocyclone and associated tornado dissipate, the storm's inflow may be concentrated into a new area closer to the center of the storm. If a new mesocyclone develops, the cycle may start again, producing one or more new tornadoes. Occasionally, the old (occluded) mesocyclone and the new mesocyclone produce a tornado at the same time.

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.

Multiple vortex


A multiple-vortex tornado is a type of tornado in which two or more columns of spinning air rotate around a common center. Multivortex 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 distinct phenomenon from a satellite tornado, which is a weaker tornado which forms very near a large, strong tornado contained within the same mesocyclone. The satellite tornado may appear to "orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

" 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


A waterspout is defined by the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...

 as a tornado over water. However, researchers typically distinguish "fair weather" waterspouts from tornadic waterspouts. Fair weather waterspouts are less severe but far more common, and are similar to dust devils and landspouts. They form at the bases of cumulus congestus clouds over tropical and subtropical waters. They have relatively weak winds, smooth laminar
Laminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...

 walls, and typically travel very slowly. They occur most commonly in the Florida Keys
Florida Keys
The Florida Keys are a coral archipelago in southeast United States. They begin at the southeastern tip of the Florida peninsula, about south of Miami, and extend in a gentle arc south-southwest and then westward to Key West, the westernmost of the inhabited islands, and on to the uninhabited Dry...

 and in the northern Adriatic Sea
Adriatic Sea
The Adriatic Sea is a body of water separating the Italian Peninsula from the Balkan peninsula, and the system of the Apennine Mountains from that of the Dinaric Alps and adjacent ranges...

. 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.

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 which 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.

Gustnado


A gustnado, or gust front tornado, is a small, vertical swirl associated with a gust front
Outflow boundary
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump...

 or downburst
Downburst
A downburst is created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. Unlike winds in a tornado, winds in a downburst are directed outwards from the point where it hits land or water...

. 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
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...

 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). If low level wind shear
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...

 is strong enough, the rotation can be turned vertically or diagonally and make contact with the ground. The result is a gustnado. They usually cause small areas of heavier rotational wind damage among areas of straight-line wind damage.

Dust devil


A dust devil 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 in arid
Arid
A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...

 areas.

Fire whirls and steam devils


Small-scale, tornado-like circulations can occur near any intense surface heat source. Those that occur near intense wildfire
Wildfire
A wildfire is any uncontrolled fire in combustible vegetation that occurs in the countryside or a wilderness area. Other names such as brush fire, bushfire, forest fire, desert fire, grass fire, hill fire, squirrel fire, vegetation fire, veldfire, and wilkjjofire may be used to describe the same...

s are called fire whirls. They are not considered tornadoes, except in the rare case where they connect to a pyrocumulus or other cumuliform cloud above. Fire whirls usually are not as strong as tornadoes associated with thunderstorms. They can, however, produce significant damage. A steam devil is a rotating updraft that involves steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

 or smoke
Smoke
Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...

. Steam devils are very rare. They most often form from smoke issuing from a power plant smokestack. Hot springs
Hot Springs
Hot Springs may refer to:* Hot Springs, Arkansas** Hot Springs National Park, Arkansas*Hot Springs, California**Hot Springs, Lassen County, California**Hot Springs, Modoc County, California**Hot Springs, Placer County, California...

 and deserts
Déserts
Déserts is a piece by Edgard Varèse for brass , percussion , piano, and tape. Percussion instruments are exploited for their resonant potential, rather than used solely as accompaniment...

 may also be suitable locations for a 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 upgrade to the older Fujita scale, by expert elicitation
Expert elicitation
In science, engineering, and research, expert elicitation is the synthesis of opinions of experts of a subject where there is uncertainty due to insufficient data or when such data is unattainable because of physical constraints or lack of resources. Expert elicitation is essentially a scientific...

, 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
Skyscraper
A skyscraper is a tall, continuously habitable building of many stories, often designed for office and commercial use. There is no official definition or height above which a building may be classified as a skyscraper...

s. The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. Doppler
Pulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...

 weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...

 data, photogrammetry
Photogrammetry
Photogrammetry is the practice of determining the geometric properties of objects from photographic images. Photogrammetry is as old as modern photography and can be dated to the mid-nineteenth century....

, 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
Multiple vortex tornado
A multiple-vortex tornado is a tornado that contains several vortices rotating around, inside of, and as part of the main vortex. These multiple vortices are somewhat similar to eyewall mesovortices found in intense tropical cyclones...

.

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). Outside Tornado Alley, 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, respectively.

Climatology



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
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

 north into arctic
Arctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...

 areas, and has no major east-west mountain range to block air flow between these two areas. In the middle latitudes
Middle latitudes
The middle latitudes are between 23°26'22" North and 66°33'39" North, and between 23°26'22" South and 66°33'39" South latitude, or, the Earth's temperate zones between the tropics and the Arctic and Antarctic. The prevailing winds in the middle latitudes are often very strong...

, where most tornadoes of the world occur, the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

 block moisture and buckle the atmospheric flow
Prevailing winds
Prevailing winds are winds that blow predominantly from a single general direction over a particular point on Earth's surface. The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface. A region's prevailing and dominant winds...

, forcing drier air at mid-levels of the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 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
Dry line
A dry line separates moist air from the Gulf of Mexico and dry desert air from the south-western states . The dry line is an important factor in severe weather frequency in the Great Plains of North America...

 when the flow aloft is strong, while the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

 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
Central United States
The Central United States is sometimes conceived as between the Eastern United States and Western United States as part of a three-region model, roughly coincident with the Midwestern United States plus the western and central portions of the Southern United States; the term is also sometimes used...

 known as Tornado Alley
Tornado Alley
Tornado Alley is a colloquial and popular media term that most often refers to the area of the United States where tornadoes are most frequent. Although an official location is not defined, the area between the Rocky Mountains and Appalachian Mountains is usually associated with it.The areas...

. This area extends into Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, particularly Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....

 and the Prairie Provinces, although southeast Quebec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....

, the interior of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...

, and western New Brunswick
New Brunswick
New Brunswick is one of Canada's three Maritime provinces and is the only province in the federation that is constitutionally bilingual . The provincial capital is Fredericton and Saint John is the most populous city. Greater Moncton is the largest Census Metropolitan Area...

 are also tornado-prone. Tornadoes also occur across northeastern Mexico.

The United States averages about 1,200 tornadoes per year. The Netherlands has the highest average number of recorded tornadoes per area of any country (more than 20, or 0.0013 per sq mi (0.00048 per km2), annually), followed by the UK (around 33, or 0.00035 per sq mi (0.00013 per km2), per year), but most are small and cause minor damage. In absolute number of events, ignoring area, the UK experiences more tornadoes than any other European country, excluding waterspouts.


Tornadoes kill an average of 179 people per year in Bangladesh
Bangladesh
Bangladesh , officially the People's Republic of Bangladesh is a sovereign state located in South Asia. It is bordered by India on all sides except for a small border with Burma to the far southeast and by the Bay of Bengal to the south...

, the most in the world. This is due to high population density, poor quality of construction and lack of tornado safety knowledge, as well as other factors. Other areas of the world that have frequent tornadoes include South Africa, parts of Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

, Paraguay
Paraguay
Paraguay , officially the Republic of Paraguay , is a landlocked country in South America. It is bordered by Argentina to the south and southwest, Brazil to the east and northeast, and Bolivia to the northwest. Paraguay lies on both banks of the Paraguay River, which runs through the center of the...

, and southern Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

, as well as portions of Europe, Australia and New Zealand, and far eastern Asia.

Tornadoes are most common in spring and least common in winter. 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
Landfall (meteorology)
Landfall is the event of a tropical cyclone or a waterspout coming onto land after being over water. When a waterspout makes landfall it is reclassified as a tornado, which can then cause damage inland...

 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. Favorable conditions can occur any time of the year.

Tornado occurrence is highly dependent on the time of day, because of solar heating. Worldwide, most tornadoes occur in the late afternoon, between 3 pm and 7 pm local time, with a peak near 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.

Associations with climate and climate change


Associations to various climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

 and environmental trends exist. For example, an increase in the sea surface temperature
Sea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...

 of a source region (e.g. Gulf of Mexico and Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...

) increases atmospheric moisture content. Increased moisture can fuel an increase in severe weather
Severe weather
Severe weather phenomena are weather conditions that are hazardous to human life and property.- Examples Include :Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, and , instability in the...

 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
Enso
Ensō is a Japanese word meaning "circle" and a concept strongly associated with Zen. Ensō is one of the most common subjects of Japanese calligraphy even though it is a symbol and not a character. It symbolizes the Absolute enlightenment, strength, elegance, the Universe, and the void; it can...

 phase is that of El Niño or La Niña
La Niña
La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3–5 °C...

.

Climatic shifts may affect tornadoes via teleconnection
Teleconnection
Teleconnection in atmospheric science refers to climate anomalies being related to each other at large distances...

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 that global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

 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



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 warning
Tornado warning
A tornado warning is an alert issued by government weather services to warn that severe thunderstorms with tornadoes may be imminent. It can be issued after a tornado or funnel cloud has been spotted by eye, or more commonly if there are radar indications of tornado formation...

s were issued in 1950 and the first tornado watch
Tornado watch
A tornado watch is issued when conditions are right for a tornado to form. Since any thunderstorm capable of producing a tornado is defined as severe, a tornado watch is also automatically a severe thunderstorm watch...

es and convective outlooks in 1952. In 1953 it was confirmed that hook echo
Hook echo
The hook echo is one of the classical hallmarks of tornado-producing supercell thunderstorms as seen on a weather radar. The echo is produced by rain, hail, or even debris being wrapped around the supercell...

es are associated with tornadoes. By recognizing these radar signatures, meteorologists could detect thunderstorms probably producing tornadoes from dozens of miles away.

Radar



Today, most developed countries have a network of weather radars, which remains the main method of detecting signatures probably 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
Direction (geometry, geography)
Direction is the information contained in the relative position of one point with respect to another point without the distance information. Directions may be either relative to some indicated reference , or absolute according to some previously agreed upon frame of reference Direction is the...

 (towards or away from the radar) of the winds in a storm, and so can spot evidence of rotation in storms from more than a hundred miles (160 km) 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 on occasion tornado development may occur more quickly than radar can complete a scan and send the batch of data. Also, most populated areas on Earth are now visible from the Geostationary Operational Environmental Satellite
Geostationary Operational Environmental Satellite
The Geostationary Satellite system, operated by the United States National Environmental Satellite, Data, and Information Service , supports weather forecasting, severe storm tracking, and meteorology research. Spacecraft and ground-based elements of the system work together to provide a continuous...

s (GOES), which aid in the nowcasting of tornadic storms.

Storm spotting


In the mid-1970s, the U.S. National Weather Service (NWS) increased its efforts to train storm spotter
Storm spotter
A storm spotter is a specific type of weather spotter who actively maintains a visual watch of the development and progression of specific weather events while actively relaying important information to the local weather agency in a timely manner.- History :...

s to spot key features of storms which indicate severe hail, damaging winds, and tornadoes, as well as damage itself and flash flood
Flash flood
A flash flood is a rapid flooding of geomorphic low-lying areas—washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a storm, hurricane, or tropical storm or meltwater from ice or snow flowing over ice sheets or snowfields...

ing. The program was called Skywarn
Skywarn
SKYWARN is a program of the United States' National Weather Service . Its mission is to collect reports of localized severe weather. These reports are used to aid forecasters in issuing and verifying severe weather watches and warnings and to improve the forecasting and warning processes and the...

, and the spotters were local sheriff's deputies, state troopers, firefighters, ambulance drivers, amateur radio operator
Amateur radio operator
An amateur radio operator is an individual who typically uses equipment at an amateur radio station to engage in two-way personal communications with other similar individuals on radio frequencies assigned to the amateur radio service. Amateur radio operators have been granted an amateur radio...

s, civil defense
Civil defense
Civil defense, civil defence or civil protection is an effort to protect the citizens of a state from military attack. It uses the principles of emergency operations: prevention, mitigation, preparation, response, or emergency evacuation, and recovery...

 (now emergency management
Emergency management
Emergency management is the generic name of an interdisciplinary field dealing with the strategic organizational management processes used to protect critical assets of an organization from hazard risks that can cause events like disasters or catastrophes and to ensure the continuance of the...

) spotters, storm chasers
Storm chasing
Storm chasing is broadly defined as the pursuit of any severe weather condition, regardless of motive, which can be curiosity, adventure, scientific exploration or for news professions/media coverage....

, and ordinary citizens. When severe weather is anticipated, local weather service offices request that these spotters look out for severe weather, and report any tornadoes immediately, so that the office can warn of the hazard.

Usually spotters are trained by the NWS on behalf of their respective organizations, and report to them. The organizations activate public warning systems such as sirens
Civil defense siren
A civil defense siren is a mechanical or electronic device for generating sound to...

 and the Emergency Alert System
Emergency Alert System
The Emergency Alert System is a national warning system in the United States put into place on January 1, 1997, when it superseded the Emergency Broadcast System , which itself had superseded the CONELRAD System...

, and 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
CANWARN is an acronym for CANadian Weather Amateur Radio NetworkThe CANWARN program is an organized severe weather spotting and reporting program organized and run by the Meteorological Services Division of Environment Canada...

, 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
Skywarn Europe is a network of volunteer storm spotters throughout Europe based on the U.S. Skywarn program.Its members attend severe weather training courses and deliver real-time assessments of current weather conditions that may be used to warn the public...

 and the Tornado and Storm Research Organisation (TORRO) has maintained a network of spotters in the United Kingdom since 1974.

Storm spotters are needed because radar systems such as NEXRAD
NEXRAD
NEXRAD or Nexrad is a network of 159 high-resolution Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration within the United States Department of Commerce...

 do not detect a tornado; merely signatures which hint at the presence of tornadoes. Radar may give a warning before there is any visual evidence of a tornado or imminent tornado, but ground truth
Ground truth
Ground truth is a term used in cartography, meteorology, analysis of aerial photographs, satellite imagery and a range of other remote sensing techniques in which data are gathered at a distance. Ground truth refers to information that is collected "on location." In remote sensing, this is...

 from an observer can either verify the threat or determine that a tornado is not imminent. 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



Storm spotters are trained to discern whether a storm seen from a distance is a supercell. They typically look to its rear, the main region of updraft and inflow. Under the updraft is a rain-free base, and the next step of tornadogenesis
Tornadogenesis
Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes, and each type of tornado can have several different methods of formation. Scientific study is ongoing, as some aspects of tornado formation remain a mystery....

 is the formation of a rotating wall cloud
Wall cloud
A wall cloud is a large, lowering, and rotating base of a cumulonimbus cloud that potentially forms tornadoes. It is typically beneath the rain-free base portion of a deep cumulus cloud , and indicates the area of primary and strongest updraft which condenses into cloud at altitudes lower than...

. The vast majority of intense tornadoes occur with a wall cloud on the backside of a supercell.

Evidence of a supercell comes from the storm's shape and structure, and cloud tower features such as a hard and vigorous updraft tower, a persistent, large overshooting top
Overshooting top
An overshooting top is a dome-like protrusion that shoots out of the top of the anvil of a thunderstorm. When an overshooting top is present for 10 minutes or longer, it is a strong indication that the thunderstorm may be severe....

, a hard anvil (especially when backsheared against strong upper level wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

s), and a corkscrew look or striations. Under the storm and closer to where most tornadoes are found, evidence of a supercell and 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
Rear flank downdraft
The rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm. These areas of descending air are thought to be essential in the production of many supercellular tornadoes...

, and requires a balance between the outflow and inflow.

Only wall clouds that rotate spawn tornadoes, and usually precede the tornado by five to thirty minutes. Rotating wall clouds are the visual manifestation of a 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
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 adjacent to a corner of a wall cloud. A tornado often occurs as this happens or shortly after; 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 occur without wall clouds, under flanking lines, and on the leading edge. Spotters watch all areas of a storm, and the cloud base
Cloud base
This article refers to meteorology. For the airborne base of the TV series Captain Scarlet and the Mysterons, see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...

 and surface.

Extremes




The most extreme tornado in recorded history was the Tri-State Tornado
Tri-State Tornado
The Tri-State Tornado of Wednesday, March 18, 1925, was the deadliest tornado in U.S. history. With 695 confirmed fatalities, the tornado killed more than twice as many as the second deadliest, the 1840 Great Natchez Tornado...

, which roared through parts of Missouri
Missouri
Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...

, Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

, and Indiana
Indiana
Indiana is a US state, admitted to the United States as the 19th on December 11, 1816. It is located in the Midwestern United States and Great Lakes Region. With 6,483,802 residents, the state is ranked 15th in population and 16th in population density. Indiana is ranked 38th in land area and is...

 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 (219 miles, 352 km), longest duration (about 3.5 hours), and fastest forward speed for a significant tornado (73 mph, 117 km/h) anywhere on Earth. In addition, it is the deadliest single tornado in United States history (695 dead). The tornado was also the second costliest tornado in history at the time, 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
Daultipur-Salturia Tornado
The Daulatpur-Saturia, Bangladesh Tornado was an extremely destructive tornado that occurred in the Manikganj District, Bangladesh on April 26, 1989. It was the costliest and deadliest tornado in Bangladesh's history. There is great uncertainty about the death toll, but estimates indicate that it...

 in Bangladesh
Bangladesh
Bangladesh , officially the People's Republic of Bangladesh is a sovereign state located in South Asia. It is bordered by India on all sides except for a small border with Burma to the far southeast and by the Bay of Bengal to the south...

 on April 26, 1989, which killed approximately 1300 people. Bangladesh has had at least 19 tornadoes in its history kill more than 100 people, almost half of the total in the rest of the world.

The most extensive tornado outbreak
Tornado outbreak
While there is no single agreed upon definition, generally at least 6-10 tornadoes produced by the same synoptic scale weather system is considered a tornado outbreak. The tornadoes usually occur within the same day, or continue into the early morning hours of the succeeding day, and within the...

 on record was the Super Outbreak
Super Outbreak
The Super Outbreak is the second largest tornado outbreak on record for a single 24-hour period, just behind the tornado outbreak of April 25–28, 2011...

, which affected a large area of the central United States and extreme southern Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....

 in Canada on April 3 and 4, 1974. This outbreak, which saw 148 tornadoes develop in 18 hours, included six of F5 intensity and twenty-four that 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 by tornadoes during this outbreak.

While direct measurement of the most violent tornado wind speeds is nearly impossible, since conventional anemometer
Anemometer
An anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics...

s would be destroyed by the intense winds, some tornadoes have been scanned by mobile Doppler radar units
Doppler On Wheels
Doppler On Wheels is a fleet of radar trucks maintained by the Center for Severe Weather Research led by Joshua Wurman, with the funding mainly provided by the National Science Foundation...

, 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. Though the reading was taken about 100 feet (30 m) above the ground, this is a testament to the power of the strongest tornadoes.

Storms that produce tornadoes can feature intense updrafts, sometimes exceeding 150 mi/h. Debris from a tornado can be lofted into the parent storm and carried a very long distance. A tornado which affected Great Bend, Kansas
Great Bend, Kansas
Great Bend, named for its location at the historic big bend of the Arkansas River, is the most populous city in and the county seat of Barton County, Kansas, United States. As of the 2010 census, the city population was 15,995.-History:...

, in November 1915, was an extreme case, where a "rain of debris" occurred 80 miles (128.7 km) from the town, a sack of flour was found 110 miles (177 km) away, and a cancelled check from the Great Bend bank was found in a field outside of Palmyra, Nebraska
Palmyra, Nebraska
Palmyra is a village in Otoe County, Nebraska, United States. The population was 546 at the 2000 census.-Geography:Palmyra is located at ....

, 305 miles (490.8 km) to the northeast. Waterspouts and tornadoes have been advanced as an explanation for instances of raining fish and other animals
Raining animals
Raining animals is a rare meteorological phenomenon in which flightless animals "rain" from the sky. Such occurrences have been reported from many countries throughout history. One hypothesis offered to explain this phenomenon is that strong winds traveling over water sometimes pick up creatures...

.

Safety


Though tornadoes can strike in an instant, there are precautions and preventative measures that people can take to increase the chances of surviving a tornado. Authorities such as the Storm Prediction Center
Storm Prediction Center
The Storm Prediction Center , located in Norman, Oklahoma, is tasked with forecasting the risk of severe thunderstorms and tornadoes in the contiguous United States. The agency issues convective outlooks, mesoscale discussions, and watches as a part of this process...

 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 storm cellar
Storm cellar
A storm shelter or storm cellar is a type of underground bunker designed to protect the occupants from violent severe weather, particularly tornadoes...

s on the property. These underground refuges 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
Tornado watch
A tornado watch is issued when conditions are right for a tornado to form. Since any thunderstorm capable of producing a tornado is defined as severe, a tornado watch is also automatically a severe thunderstorm watch...

es and warnings
Tornado warning
A tornado warning is an alert issued by government weather services to warn that severe thunderstorms with tornadoes may be imminent. It can be issued after a tornado or funnel cloud has been spotted by eye, or more commonly if there are radar indications of tornado formation...

 in the United States and Canada). Weather radios provide an alarm when a severe weather advisory is issued for the local area, though these are 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 they are believed to create a Venturi effect
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...

, increasing the danger from the tornado by increasing the wind speed and funneling 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
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

 inside a strong tornado, it is unlikely that the pressure drop would be enough to cause the house to explode. Some research indicates that opening windows may actually increase the severity of the tornado's damage. A violent tornado can destroy a house whether its windows are open or closed.

Another commonly held belief is that highway overpasses provide adequate shelter from tornadoes. On the contrary, a highway overpass is a dangerous place during a tornado. In the 1999 Oklahoma tornado outbreak of May 3, 1999, three highway overpasses were directly struck by tornadoes, and at all three locations there was a fatality, along with many life-threatening injuries. The small area under the overpasses is believed to cause a Venturi effect
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...

. By comparison, during the same tornado outbreak, more than 2000 homes were completely destroyed, with another 7000 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 chances of survival.

Finally, 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
The supernatural or is that which is not subject to the laws of nature, or more figuratively, that which is said to exist above and beyond nature...

 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.

One long-held myth was that no city whose name begins with the letter X
X
X is the twenty-fourth letter in the basic modern Latin alphabet.-Uses:In mathematics, x is commonly used as the name for an independent variable or unknown value. The usage of x to represent an independent or unknown variable can be traced back to the Arabic word šay شيء = “thing,” used in Arabic...

 would ever be hit by a tornado, the letter X as an initial letter of a place-name in the United States being extremely rare—until Xenia, Ohio
Xenia, Ohio
Xenia is a city in and the county seat of Greene County, Ohio, United States. The municipality is located in southwestern Ohio 21 miles from Dayton and is part of the Dayton Metropolitan Statistical Area...

 experienced an F-5 tornado that killed 34 people in 1974.

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. Scientists have a fairly good understanding of the development of thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...

s and mesocyclones, and the meteorological conditions conducive to their formation. However, the step from supercell
Supercell
A supercell is a thunderstorm that is characterized by the presence of a mesocyclone: a deep, continuously-rotating updraft. For this reason, these storms are sometimes referred to as rotating thunderstorms...

 (or other respective formative processes) to tornadogenesis
Tornadogenesis
Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes, and each type of tornado can have several different methods of formation. Scientific study is ongoing, as some aspects of tornado formation remain a mystery....

 and predicting 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
Vortex stretching
In fluid dynamics, vortex stretching is the lengthening of vortices in three-dimensional fluid flow, associated with a corresponding increase of the component of vorticity in the stretching direction—due to the conservation of angular momentum....

 of low-level vorticity which tightens into a tornado, namely, 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 within linear thunderstorm structures and within tropical cyclones.

Scientists 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
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...

 (passive and active) instruments generates new ideas and refines existing notions. Numerical modeling
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...

 also provides new insights as observations and new discoveries are integrated into our physical understanding and then tested in computer simulation
Computer simulation
A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...

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
VORTEX projects
The Verification of the Origins of Rotation in Tornadoes Experiment or VORTEX, field projects study tornadoes. VORTEX1 was the first time scientists completely researched the entire evolution of a tornado, enabling a greater understanding of the processes involved with tornadogenesis...

 (Verification of the Origins of Rotation in Tornadoes Experiment), deployment of TOTO
TOtable Tornado Observatory
The TOtable Tornado Observatory is a large, instrumented barrel-shaped device invented in 1979 by Dr. Al Bedard and Carl Ramzy of the National Oceanic and Atmospheric Administration Environmental Technology Laboratory...

 (the TOtable Tornado Observatory), Doppler On Wheels
Doppler On Wheels
Doppler On Wheels is a fleet of radar trucks maintained by the Center for Severe Weather Research led by Joshua Wurman, with the funding mainly provided by the National Science Foundation...

 (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
National Severe Storms Laboratory
The National Severe Storms Laboratory is a National Oceanic and Atmospheric Administration weather research laboratory located at the National Weather Center in Norman, Oklahoma....

, private-sector meteorologists, and the National Center for Atmospheric Research
National Center for Atmospheric Research
The National Center for Atmospheric Research has multiple facilities, including the I. M. Pei-designed Mesa Laboratory headquarters in Boulder, Colorado. NCAR is managed by the nonprofit University Corporation for Atmospheric Research and sponsored by the National Science Foundation...

 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
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...

.

See also


  • Cultural significance of tornadoes
    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 . Tornadoes are a serious hazard to life and limb...

  • Derecho
    Derecho
    A derecho is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo...

  • History of tropical cyclone-spawned tornadoes
    History of tropical cyclone-spawned tornadoes
    Intense tropical cyclones usually produce tornadoes, the majority of them weak, especially upon landfall.- List of Atlantic tropical cyclones which spawned tornadoes :These are the Atlantic tropical cyclones that are known to have spawned tornadoes in the U.S....

  • List of tornadoes and tornado outbreaks
  • Secondary flow
    Secondary flow
    In fluid dynamics, a secondary flow is a relatively minor flow superimposed on the primary flow, where the primary flow usually matches very closely the flow pattern predicted using simple analytical techniques and assuming the fluid is inviscid...

  • Skipping tornado
    Skipping tornado
    A skipping tornado is a vaguely defined term which refers to a tornado which has a discontinuous damage path. This may be due to several factors:#The tornado actually lifting from the surface...

  • Tornado drill
    Tornado drill
    A tornado drill is a method of practicing to take cover in a specified location in the event that a tornado strikes an area. Generally, a signal is given, such as a series of tones , or a voice announcement...

  • Tornadoes of
  • Whirlwind (atmospheric phenomenon)
    Whirlwind (atmospheric phenomenon)
    A whirlwind is a weather phenomenon in which a vortex of wind forms due to instabilities and turbulence created by heating and flow gradients. Whirlwinds occur all over the world and in any season....


External links