From Wikipedia, the free encyclopedia
A
tornado is a rapidly rotating column of
air that is in contact with both the surface of the
Earth and a
cumulonimbus cloud or, in rare cases, the base of a
cumulus cloud. The windstorm is often referred to as a
twister,
whirlwind or
cyclone, although the word
cyclone is used in meteorology to name a weather system with a
low-pressure
area in the center around which, from an observer looking down toward
the surface of the earth, winds blow counterclockwise in the Northern
Hemisphere and clockwise in the Southern. Tornadoes come in many shapes and sizes, and they are often visible in the form of a
condensation funnel originating from the base of a cumulonimbus cloud, with a cloud of rotating
debris and
dust
beneath it. Most tornadoes have wind speeds less than 110 miles per
hour (180 km/h), are about 250 feet (80 m) across, and travel a few
miles (several kilometers) before dissipating. The
most extreme
tornadoes can attain wind speeds of more than 300 miles per hour
(480 km/h), are more than two miles (3 km) in diameter, and stay on the
ground for dozens of miles (more than 100 km).
Various types of tornadoes include the
multiple vortex tornado,
landspout, and
waterspout.
Waterspouts are characterized by a spiraling funnel-shaped wind
current, connecting to a large cumulus or cumulonimbus cloud. They are
generally classified as non-
supercellular
tornadoes that develop over bodies of water, but there is disagreement
over whether to classify them as true tornadoes. These spiraling columns
of air frequently develop in tropical areas close to the
equator and are less common at
high latitudes. Other tornado-like phenomena that exist in nature include the
gustnado,
dust devil,
fire whirl, and
steam devil.
Tornadoes occur most frequently in North America, particularly in central and southeastern regions of the
United States colloquially known as
tornado alley, as well as in
Southern Africa,
northwestern and southeast Europe, western and southeastern Australia,
New Zealand, Bangladesh and adjacent eastern India, and southeastern
South America. Tornadoes can be detected before or as they occur through the use of
Pulse-Doppler radar by recognizing patterns in velocity and reflectivity data, such as
hook echoes or
debris balls, as well as through the efforts of
storm spotters.
There are several scales for rating the strength of tornadoes. The
Fujita scale rates tornadoes by damage caused and has been replaced in some countries by the updated
Enhanced Fujita Scale. An F0 or EF0 tornado, the weakest category, damages trees, but not substantial structures. An
F5 or EF5 tornado, the strongest category, rips buildings off their foundations and can deform large
skyscrapers. The similar
TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. Doppler
radar data,
photogrammetry, and ground swirl patterns (
trochoidal marks) may also be analyzed to determine intensity and assign a rating.
A tornado near Anadarko, Oklahoma, 1999. The funnel is the thin tube reaching from the cloud to the ground. The lower part of this tornado is surrounded by a translucent
dust cloud, kicked up by the tornado's strong winds at the surface. The
wind of the tornado has a much wider radius than the funnel itself.
Etymology
The word
tornado comes from the Spanish word tornado (past participle of to turn, or to have torn). Tornadoes' opposite phenomena are the
derechoes (
, from
Spanish:
derecho [deˈɾetʃo],
"straight"). A tornado is also commonly referred to as a "twister", and
is also sometimes referred to by the old-fashioned colloquial term
cyclone. The term "cyclone" is used as a synonym for "tornado" in the often-aired 1939 film
The Wizard of Oz. 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 of wind, not the condensation cloud.
Funnel cloud
This
tornado has no funnel cloud; however, the rotating dust cloud indicates
that strong winds are occurring at the surface, and thus it is a true
tornado.
A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (as described by
Bernoulli's principle) and rapid rotation (due to
cyclostrophic balance) usually cause
water vapor in the air to condense into cloud droplets due to
adiabatic cooling. This results in the formation of a visible funnel cloud or condensation funnel.
There is some disagreement over the definition of a funnel cloud and a condensation funnel. According to the Glossary of Meteorology,
a funnel cloud is any rotating cloud pendant from a cumulus or
cumulonimbus, and thus most tornadoes are included under this
definition.
Among many meteorologists, the 'funnel cloud' term is strictly defined
as a rotating cloud which is not associated with strong winds at the
surface, and condensation funnel is a broad term for any rotating cloud
below a cumuliform cloud.
Tornadoes often begin as funnel clouds with no associated strong
winds at the surface, and not all funnel clouds evolve into tornadoes.
Most tornadoes produce strong winds at the surface while the visible
funnel is still above the ground, so it is difficult to discern the
difference between a funnel cloud and a tornado from a distance.
Outbreaks and families
Occasionally, a single storm will produce more than one tornado,
either simultaneously or in succession. Multiple tornadoes produced by
the same
storm cell are referred to as a "tornado family".
Several tornadoes are sometimes spawned from the same large-scale storm
system. If there is no break in activity, this is considered a tornado
outbreak (although the term "tornado outbreak" has various definitions).
A period of several successive days with tornado outbreaks in the same
general area (spawned by multiple weather systems) is a tornado outbreak
sequence, occasionally called an extended tornado outbreak.
Characteristics
Size and shape
Most tornadoes take on the appearance of a narrow
funnel,
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 they 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 travel on the ground for 5 miles (8.0 km).
However, there is a wide range of tornado sizes. Weak tornadoes, or
strong yet dissipating tornadoes, can be exceedingly narrow, sometimes
only a few feet or couple meters across. One tornado was reported to
have a damage path only 7 feet (2.1 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 (4.0 km) wide at the ground, and
a tornado in El Reno, Oklahoma on May 31, 2013 was approximately 2.6 miles (4.2 km) wide, the widest on record.
In terms of path length, the
Tri-State Tornado, which affected parts of
Missouri,
Illinois, and
Indiana
on March 18, 1925, was on the ground continuously for 219 miles
(352 km). Many tornadoes which appear to have path lengths of 100 miles
(160 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.
In fact, modern reanalysis of the path suggests that the tornado may
have begun 15 miles (24 km) further west than previously thought.
Appearance
Tornadoes can have a wide range of colors, depending on the
environment in which they form. Those that form in dry environments can
be nearly invisible, marked only by swirling debris at the base of the
funnel. Condensation funnels that pick up little or no debris can be
gray to white. While traveling over a body of water (as a waterspout),
tornadoes can turn white or even blue. Slow-moving funnels, which ingest
a considerable amount of debris and dirt, are usually darker, taking on
the color of debris. Tornadoes in the
Great Plains
can turn red because of the reddish tint of the soil, and tornadoes in
mountainous areas can travel over snow-covered ground, turning white.
Photographs of the Waurika, Oklahoma
tornado of May 30, 1976, taken at nearly the same time by two
photographers. In the top picture, the tornado is lit with the sunlight
focused from behind the camera,
thus the funnel appears bluish. In the lower image, where the camera is
facing the opposite direction, the sun is behind the tornado, giving it
a dark appearance.
Lighting conditions are a major factor in the appearance of a tornado. A tornado which is "
back-lit"
(viewed with the sun behind it) appears very dark. The same tornado,
viewed with the sun at the observer's back, may appear gray or brilliant
white. Tornadoes which occur near the time of sunset can be many
different colors, appearing in hues of yellow, orange, and pink.
Dust kicked up by the winds of the parent thunderstorm, heavy
rain and hail, and the darkness of night are all factors which can
reduce the visibility of tornadoes. Tornadoes occurring in these
conditions are especially dangerous, since only
weather radar
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
mobile radar images and eyewitness accounts, that most tornadoes have a
clear, calm center with extremely low pressure, akin to the
eye of
tropical cyclones. Lightning is said to be the source of illumination for those who claim to have seen the interior of a tornado.
Rotation
Tornadoes normally rotate
cyclonically (when viewed from above, this is counterclockwise in the
northern hemisphere and clockwise in the
southern). While large-scale storms always rotate cyclonically due to the
Coriolis effect,
thunderstorms and tornadoes are so small that the direct influence of
the Coriolis effect is unimportant, as indicated by their large
Rossby numbers. Supercells and tornadoes rotate cyclonically in numerical simulations even when the Coriolis effect is neglected. Low-level
mesocyclones and tornadoes owe their rotation to complex processes within the supercell and ambient environment.
Approximately 1 percent of tornadoes rotate in an anticyclonic
direction in the northern hemisphere. Typically, systems as weak as
landspouts and gustnadoes can rotate anticyclonically, and usually only
those which form on the anticyclonic shear side of the descending
rear flank downdraft (RFD) in a cyclonic supercell. On rare occasions,
anticyclonic tornadoes
form in association with the mesoanticyclone of an anticyclonic
supercell, in the same manner as the typical cyclonic tornado, or as a
companion tornado either as a satellite tornado or associated with
anticyclonic eddies within a supercell.
Sound and seismology
Tornadoes emit widely on the
acoustics spectrum
and the sounds are caused by multiple mechanisms. Various sounds of
tornadoes have been reported, mostly related to familiar sounds for the
witness and generally some variation of a whooshing roar. Popularly
reported sounds include a freight train, rushing rapids or waterfall, a
nearby jet engine, or combinations of these. Many tornadoes are not
audible from much distance; the nature of and the propagation distance
of the audible sound depends on atmospheric conditions and topography.
The winds of the tornado vortex and of constituent turbulent
eddies,
as well as airflow interaction with the surface and debris, contribute
to the sounds. Funnel clouds also produce sounds. Funnel clouds and
small tornadoes are reported as whistling, whining, humming, or the
buzzing of innumerable bees or electricity, or more or less harmonic,
whereas many tornadoes are reported as a continuous, deep rumbling, or
an irregular sound of "noise".
Since many tornadoes are audible only when very near, sound is
not to be thought of as a reliable warning signal for a tornado.
Tornadoes are also not the only source of such sounds in severe
thunderstorms; any strong, damaging wind, a severe hail volley, or
continuous thunder in a thunderstorm may produce a roaring sound.
Tornadoes also produce identifiable inaudible
infrasonic 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, with
sferics and
E-field effects detected.
[45][47][48]
There are observed correlations between tornadoes and patterns of
lightning. Tornadic storms do not contain more lightning than other
storms and some tornadic cells never produce lightning at all. More
often than not, overall cloud-to-ground (CG) lightning activity
decreases as a tornado touches the surface and returns to the baseline
level when the tornado dissipates. In many cases, intense tornadoes and
thunderstorms exhibit an increased and anomalous dominance of positive
polarity CG discharges.
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
has been reported in the past and is probably due to misidentification
of external light sources such as lightning, city lights, and
power flashes
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,
moisture, and
pressure. For example, on June 24, 2003 near
Manchester, South Dakota, a probe measured a 100
mbar (
hPa) (2.95
inHg) pressure decrease. The pressure dropped gradually as the vortex approached then dropped extremely rapidly to 850
mbar (
hPa) (25.10
inHg)
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
A
sequence of images showing the birth of a tornado. First, the rotating
cloud base lowers. This lowering becomes a funnel, which continues
descending while winds build near the surface, kicking up dust and
debris and causing damage. As the pressure continues to drop, the
visible funnel extends to the ground. This tornado, near Dimmitt, Texas, was one of the best-observed violent tornadoes in history.
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 (1.6–9.7 kilometres) across. Most intense tornadoes (EF3 to
EF5 on the
Enhanced Fujita Scale)
develop from supercells. In addition to tornadoes, very heavy rain,
frequent lightning, strong wind gusts, and hail are common in such
storms.
Most tornadoes from supercells follow a recognizable life cycle.
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.
Composite of eight images shot in sequence as a tornado formed in Kansas in 2016
Formation
As the mesocyclone lowers below the cloud base, it begins to take in
cool, moist air from the downdraft region of the storm. The convergence
of warm air in the updraft and cool air causes a rotating wall cloud to
form. The RFD also focuses the mesocyclone's base, causing it to draw
air from a smaller and smaller area on the ground. As the updraft
intensifies, it creates an area of low pressure at the surface. This
pulls the focused mesocyclone down, in the form of a visible
condensation funnel. As the funnel descends, the RFD also reaches the
ground, fanning outward and creating a gust front that can cause severe
damage a considerable distance from the tornado. Usually, the funnel
cloud begins causing damage on the ground (becoming a tornado) within a
few minutes of the RFD reaching the ground.
Maturity
Initially, the tornado has a good source of warm, moist air
flowing inward
to power it, and it grows until it reaches the "mature stage". This can
last 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. The low pressured atmosphere at the
base of the tornado is essential to the endurance of the system.
Meanwhile, the RFD, now an area of cool surface winds, begins to wrap
around the tornado, cutting off the inflow of warm air which previously
fed the tornado.
Dissipation
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 ends. During this stage the shape of the tornado
becomes highly influenced by the winds of the parent storm, and can be
blown into fantastic patterns.
Even though the tornado is dissipating, it is still capable of causing
damage. The storm is contracting into a rope-like tube and, due to
conservation of angular momentum, 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. Sometimes, in 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 and possibly feed a new mesocyclone. 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.
Types
Multiple vortex
A
multiple-vortex tornado is a type of tornado in which two or
more columns of spinning air rotate about their own axis and at the
same time around a common center. A multi-vortex structure can occur in
almost any circulation, but is very often observed in intense tornadoes.
These vortices often create small areas of heavier damage along the
main tornado path. This is a phenomenon that is distinct from a
satellite tornado,
which is a smaller tornado which forms very near a large, strong
tornado contained within the same mesocyclone. The satellite tornado may
appear to "
orbit"
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
as a tornado over water. However, researchers typically distinguish
"fair weather" waterspouts from tornadic (i.e. associated with a
mesocyclone) waterspouts. Fair weather waterspouts are less severe but
far more common, and are similar to
dust devils and
landspouts. They form at the bases of
cumulus congestus clouds over tropical and subtropical waters. They have relatively weak winds, smooth
laminar walls, and typically travel very slowly. They occur most commonly in the
Florida Keys and in the northern
Adriatic Sea.
In contrast, tornadic waterspouts are stronger tornadoes over water.
They form over water similarly to mesocyclonic tornadoes, or are
stronger tornadoes which cross over water. Since they form from
severe thunderstorms and can be far more intense, faster, and longer-lived than fair weather waterspouts, they are more dangerous.
In official tornado statistics, waterspouts are generally not counted
unless they affect land, though some European weather agencies count
waterspouts and tornadoes together.
Landspout
A landspout, or dust-tube tornado, is a tornado not
associated with a mesocyclone. The name stems from their
characterization as a "fair weather waterspout on land". Waterspouts and
landspouts share many defining characteristics, including relative
weakness, short lifespan, and a small, smooth condensation funnel 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.
Similar circulations
Gustnado
A
gustnado, or
gust front tornado, is a small, vertical swirl associated with a
gust front or
downburst.
Because they are not connected with a cloud base, there is some debate
as to whether or not gustnadoes are tornadoes. They are formed when fast
moving cold, dry outflow air from a
thunderstorm
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
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 (also known as a whirlwind) resembles a tornado
in that it is a vertical swirling column of air. However, they form
under clear skies and are no stronger than the weakest tornadoes. They
form when a strong convective updraft is formed near the ground on a hot
day. If there is enough low level wind shear, the column of hot, rising
air can develop a small cyclonic motion that can be seen near the
ground. They are not considered tornadoes because they form during fair
weather and are not associated with any clouds. However, they can, on
occasion, result in major damage.
Fire whirls
Small-scale, tornado-like circulations can occur near any intense surface heat source. Those that occur near intense
wildfires 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.
Steam devils
A
steam devil is a
rotating updraft
between 50 and 200 meters wide that involves steam or smoke. These
formations do not involve high wind speeds, only completing a few
rotations per minute. Steam devils are very rare. They most often form
from smoke issuing from a power plant's smokestack.
Hot springs
and deserts may also be suitable locations for a tighter,
faster-rotating steam devil to form. The phenomenon can occur over
water, when cold arctic air passes over relatively warm water.
Intensity and damage
The Fujita scale and the Enhanced Fujita Scale rate tornadoes by
damage caused. The Enhanced Fujita (EF) Scale was an update to the older
Fujita scale, by
expert elicitation,
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
skyscrapers. The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes.
Doppler weather radar data,
photogrammetry, and ground swirl patterns (
cycloidal marks) may also be analyzed to determine intensity and award a rating.
A house displaying EF1 damage. The roof and garage door have been damaged, but walls and supporting structures are still intact.
Tornadoes vary in intensity regardless of shape, size, and location,
though strong tornadoes are typically larger than weak tornadoes. The
association with track length and duration also varies, although longer
track tornadoes tend to be stronger. In the case of violent tornadoes, only a small portion of the path is of violent intensity, most of the higher intensity from
subvortices.
In the United States, 80% of tornadoes are EF0 and EF1 (T0
through T3) tornadoes. The rate of occurrence drops off quickly with
increasing strength—less than 1% are violent tornadoes (EF4, T8 or
stronger). 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
Areas worldwide where tornadoes are most likely, indicated by orange shading
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 north into
arctic areas, and has no major east-west mountain range to block air flow between these two areas. In the
middle latitudes, where most tornadoes of the world occur, the
Rocky Mountains block moisture and buckle the
atmospheric flow, forcing drier air at mid-levels of the
troposphere 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 when the flow aloft is strong, while the
Gulf of Mexico
fuels abundant low-level moisture in the southerly flow to its east.
This unique topography allows for frequent collisions of warm and cold
air, the conditions that breed strong, long-lived storms throughout the
year. A large portion of these tornadoes form in an area of the
central United States known as
Tornado Alley. This area extends into
Canada, particularly
Ontario and the
Prairie Provinces, although southeast
Quebec, the interior of
British Columbia, and western
New Brunswick are also tornado-prone. Tornadoes also occur across northeastern Mexico.
The United States averages about 1,200 tornadoes per year, followed by Canada, averaging 62 reported per year. NOAA's has a higher average 100 per year in Canada.
The Netherlands has the highest average number of recorded tornadoes
per area of any country (more than 20, or 0.0013 per sq mi (0.00048 per
km
2), annually), followed by the UK (around 33, or 0.00035 per sq mi (0.00013 per km
2), per year), although those are of lower intensity, briefer
] and cause minor damage.
Intense tornado activity in the United States. The darker-colored areas denote the area commonly referred to as Tornado Alley.
Tornadoes kill an average of 179 people per year in
Bangladesh, the most in the world. Reasons for this include the region's high population density, poor construction quality, and lack of tornado safety knowledge. Other areas of the world that have frequent tornadoes include South Africa, the
La Plata Basin area, portions of Europe, Australia and New Zealand, and far eastern Asia.
Tornadoes are most common in spring and least common in winter,
but tornadoes can occur any time of year that favorable conditions
occur.
Spring and fall experience peaks of activity as those are the seasons
when stronger winds, wind shear, and atmospheric instability are
present. Tornadoes are focused in the right front quadrant of
landfalling tropical cyclones, which tend to occur in the late summer and autumn. Tornadoes can also be spawned as a result of
eyewall mesovortices, which persist until landfall.
Tornado occurrence is highly dependent on the time of day, because of
solar heating. Worldwide, most tornadoes occur in the late afternoon, between 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.
The United Kingdom has the highest incidence of tornadoes, measured by unit area of land, than any other country in the world.
Unsettled conditions and weather fronts transverse the Islands at all
times of the years and are responsible for spawning the tornadoes, which
consequently form at all times of the year. The United Kingdom has at
least 34 tornadoes per year and possibly as many as 50,
more than any other country in the world relative to its land area.
Most tornadoes in the United Kingdom are weak, but they are occasionally
destructive. For example, the Birmingham tornado of 2005 and the London
tornado of 2006 both registered F2 on the Fujita scale and both caused
significant damage and injury.
Associations with climate and climate change
U. S. Annual January–December Tornado Count 1976–2011 from NOAA National Climatic Data Center
Associations with various
climate and environmental trends exist. For example, an increase in the
sea surface temperature of a source region (e.g. Gulf of Mexico and
Mediterranean Sea) increases atmospheric moisture content. Increased moisture can fuel an increase in
severe weather and tornado activity, particularly in the cool season.
Some evidence does suggest that the
Southern Oscillation is weakly correlated with changes in tornado activity, which vary by season and region, as well as whether the
ENSO phase is that of
El Niño or
La Niña.
Research has found that fewer tornadoes and hailstorms occur in winter
and spring in the U.S. central and southern plains during
El Niño, and more occur during
La Niña, than in years when temperatures in the
Pacific are relatively stable. Ocean conditions could be used to forecast extreme spring storm events several months in advance.
Climatic shifts may affect tornadoes via
teleconnections
in shifting the jet stream and the larger weather patterns. The
climate-tornado link is confounded by the forces affecting larger
patterns and by the local, nuanced nature of tornadoes. Although it is
reasonable to suspect that
global warming 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
Path of a tornado across Wisconsin on August 21, 1857
Rigorous attempts to warn of tornadoes began in the United States in
the mid-20th century. Before the 1950s, the only method of detecting a
tornado was by someone seeing it on the ground. Often, news of a tornado
would reach a local weather office after the storm. However, with the
advent of weather radar, areas near a local office could get advance
warning of severe weather. The first public
tornado warnings were issued in 1950 and the first
tornado watches and
convective outlooks came about in 1952. In 1953, it was confirmed that
hook echoes were associated with tornadoes.
By recognizing these radar signatures, meteorologists could detect
thunderstorms probably producing tornadoes from several miles away.
Radar
Today, most developed countries have a network of weather radars,
which serves as the primary method of detecting hook signatures that are
likely associated with tornadoes. In the United States and a few other
countries, Doppler weather radar stations are used. These devices
measure the velocity and radial
direction
(towards or away from the radar) of the winds within a storm, and so
can spot evidence of rotation in storms from over one 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 tornado development may occasionally take place
more quickly than radar can complete a scan and send the batch of data.
Doppler radar systems can detect
mesocyclones within a supercell thunderstorm. This allows meteorologists to predict tornado formations throughout thunderstorms.
Storm spotting
In the mid-1970s, the U.S. National Weather Service (NWS) increased its efforts to train
storm spotters
so they could spot key features of storms that indicate severe hail,
damaging winds, and tornadoes, as well as storm damage and
flash flooding. The program was called
Skywarn, and the spotters were local sheriff's deputies, state troopers, firefighters, ambulance drivers,
amateur radio operators,
civil defense (now
emergency management) spotters,
storm chasers,
and ordinary citizens. When severe weather is anticipated, local
weather service offices request these spotters to look out for severe
weather and report any tornadoes immediately, so that the office can
warn of the hazard.
Spotters usually are trained by the
NWS on behalf of their respective organizations, and report to them. The organizations activate public warning systems such as
sirens and the
Emergency Alert System (EAS), and they forward the report to the NWS.
There are more than 230,000 trained Skywarn weather spotters across the United States.
In Canada, a similar network of volunteer weather watchers, called
Canwarn, helps spot severe weather, with more than 1,000 volunteers. In Europe, several nations are organizing spotter networks under the auspices of
Skywarn Europe and the Tornado and Storm Research Organisation (TORRO) has maintained a network of spotters in the United Kingdom since 1974.
Storm spotters are required because radar systems such as
NEXRAD do not really detect tornadoes; merely signatures which hint at the presence of tornadoes. Radar may give a warning before there is any visual evidence of a tornado or an imminent one, but
ground truth from an observer can either verify the threat or determine that a tornado is not imminent.
The spotter's ability to see what radar can't 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 or not a storm seen
from a distance is a supercell. They typically look to its rear, the
main region of
updraft and inflow. Under that updraft is a rain-free base, and the next step of
tornadogenesis is the formation of a rotating
wall cloud. The vast majority of intense tornadoes occur with a wall cloud on the backside of a supercell.
Evidence of a supercell is based on the storm's shape and
structure, and cloud tower features such as a hard and vigorous updraft
tower, a persistent, large
overshooting top, a hard anvil (especially when backsheared against strong upper level
winds), and a corkscrew look or
striations.
Under the storm and closer to where most tornadoes are found, evidence
of a supercell and the likelihood of a tornado includes inflow bands
(particularly when curved) such as a "beaver tail", and other clues such
as strength of inflow, warmth and moistness of inflow air, how outflow-
or inflow-dominant a storm appears, and how far is the front flank
precipitation core from the wall cloud. Tornadogenesis is most likely at
the interface of the updraft and
rear flank downdraft, and requires a balance between the outflow and inflow.
Only wall clouds that rotate spawn tornadoes, and they usually
precede the tornado between five and thirty minutes. Rotating wall
clouds may be a visual manifestation of a low-level mesocyclone. Barring
a low-level boundary, tornadogenesis is highly unlikely unless a rear
flank downdraft occurs, which is usually visibly evidenced by
evaporation of
cloud
adjacent to a corner of a wall cloud. A tornado often occurs as this
happens or shortly afterwards; first, a funnel cloud dips and in nearly
all cases by the time it reaches halfway down, a surface swirl has
already developed, signifying a tornado is on the ground before
condensation connects the surface circulation to the storm. Tornadoes
may also develop without wall clouds, under flanking lines and on the
leading edge. Spotters watch all areas of a storm, and the
cloud base and surface.
Extremes
A map of the tornado paths in the Super Outbreak (April 3–4, 1974)
The most record-breaking tornado in recorded history was the
Tri-State Tornado, which roared through parts of
Missouri,
Illinois, and
Indiana
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 costliest tornado in history at the time
(unadjusted for inflation), but in the years since has been surpassed by
several others if population changes over time are not considered. When
costs are normalized for wealth and inflation, it ranks third today.
The most extensive
tornado outbreak on record was the
2011 Super Outbreak,
which spawned 360 confirmed tornadoes over the southeastern United
States – 216 of them within a single 24-hour period. The previous record
was the
1974 Super Outbreak which spawned 148 tornadoes.
While direct measurement of the most violent tornado wind speeds is nearly impossible, since conventional
anemometers would be destroyed by the intense winds and flying debris, some tornadoes have been scanned by
mobile Doppler radar units,
which can provide a good estimate of the tornado's winds. The highest
wind speed ever measured in a tornado, which is also the highest wind
speed ever recorded on the planet, is 301 ± 20 mph (484 ± 32 km/h) in
the F5
Bridge Creek-Moore, Oklahoma, tornado which killed 36 people.
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 mph (240 km/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,
in November 1915, was an extreme case, where a "rain of debris"
occurred 80 miles (130 km) from the town, a sack of flour was found 110
miles (180 km) away, and a cancelled check from the Great Bend bank was
found in a field outside of
Palmyra, Nebraska, 305 miles (491 km) to the northeast. Waterspouts and tornadoes have been advanced as an explanation for instances of
raining fish and other animals.
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
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 cellars 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 watches and
warnings 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 the constricted space can be subject to increased wind
speed and funneling of debris underneath the overpass.
Myths and misconceptions
Folklore often identifies a green sky with tornadoes, and though the
phenomenon may be associated with severe weather, there is no evidence
linking it specifically with tornadoes. It is often thought that opening windows will lessen the damage caused by the tornado. While there is a large drop in
atmospheric pressure
inside a strong tornado, it is unlikely that the pressure drop would be
enough to cause the house to explode. 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 misconception is that highway overpasses
provide adequate shelter from tornadoes. This belief is partly inspired
by widely circulated video captured during the
1991 tornado outbreak near
Andover, Kansas, where a news crew and several other people take shelter under an overpass on the
Kansas Turnpike and safely ride out a tornado as it passes by.
However, a highway overpass is a dangerous place during a tornado, and
the subjects of the video remained safe due to an unlikely combination
of events: the storm in question was a weak tornado, the tornado did not
directly strike the overpass, and the overpass itself was of a unique
design. Due to the
Venturi effect, tornadic winds are accelerated in the confined space of an overpass. Indeed, in the
1999 Oklahoma tornado outbreak
of May 3, 1999, three highway overpasses were directly struck by
tornadoes, and at each of the three locations there was a fatality,
along with many life-threatening injuries.
By comparison, during the same tornado outbreak, more than 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.
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 forces. Tornadoes have been known to cross major rivers, climb mountains, affect valleys, and have damaged
several city centers. As a general rule, no area is safe from tornadoes, though some areas are more susceptible than others.
Ongoing research
Meteorology is a relatively young science and the study of tornadoes
is newer still. Although researched for about 140 years and intensively
for around 60 years, there are still aspects of tornadoes which remain a
mystery. Scientists have a fairly good understanding of the development of
thunderstorms and mesocyclones, and the meteorological conditions conducive to their formation. However, the step from
supercell, or other respective formative processes, to
tornadogenesis and the prediction of tornadic vs. non-tornadic mesocyclones is not yet well known and is the focus of much research.
Also under study are the low-level mesocyclone and the
stretching of low-level
vorticity which tightens into a tornado,
in particular, what are the processes and what is the relationship of
the environment and the convective storm. Intense tornadoes have been
observed forming simultaneously with a mesocyclone aloft (rather than
succeeding mesocyclogenesis) and some intense tornadoes have occurred
without a mid-level mesocyclone.
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 (passive and active) instruments generates new ideas and refines existing notions.
Numerical modeling
also provides new insights as observations and new discoveries are
integrated into our physical understanding and then tested in
computer simulations
which validate new notions as well as produce entirely new theoretical
findings, many of which are otherwise unattainable. Importantly,
development of new observation technologies and installation of finer
spatial and temporal resolution observation networks have aided
increased understanding and better predictions.
Research programs, including field projects such as the
VORTEX projects (Verification of the Origins of Rotation in Tornadoes Experiment), deployment of
TOTO (the TOtable Tornado Observatory),
Doppler on Wheels (DOW), and dozens of other programs, hope to solve many questions that still plague meteorologists. Universities, government agencies such as the
National Severe Storms Laboratory, private-sector meteorologists, and the
National Center for Atmospheric Research
are some of the organizations very active in research; with various
sources of funding, both private and public, a chief entity being the
National Science Foundation.
The pace of research is partly constrained by the number of
observations that can be taken; gaps in information about the wind,
pressure, and moisture content throughout the local atmosphere; and the
computing power available for simulation.
Solar storms similar to tornadoes have been recorded, but it is
unknown how closely related they are to their terrestrial counterparts.
