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| A typical
thunderstorm over a field. |
Thunderstorms
A thunderstorm, also known as an electrical storm or a
lightning storm, is a storm characterized by the
presence of lightning and its acoustic effect on the
Earth's atmosphere, known as thunder. Relatively weak
thunderstorms are sometimes called thundershowers.
Thunderstorms occur in a type of cloud known as a
cumulonimbus. They are usually accompanied by strong
winds, and often produce heavy rain and sometimes snow,
sleet, or hail, but some thunderstorms produce little
precipitation or no precipitation at all. Thunderstorms
may line up in a series or become a rainband, known as a
squall line. Strong or severe thunderstorms include some
of the most dangerous weather phenomena, including large
hail, strong winds, and tornadoes. Some of the most
persistent severe thunderstorms, known as supercells,
rotate as do cyclones. While most thunderstorms move
with the mean wind flow through the layer of the
troposphere that they occupy, vertical wind shear
sometimes causes a deviation in their course at a right
angle to the wind shear direction.
Thunderstorms result from the rapid upward movement of
warm, moist air, sometimes along a front. As the warm,
moist air moves upward, it cools, condenses, and forms a
cumulonimbus cloud that can reach heights of over 20
kilometres (12 mi). As the rising air reaches its dew
point temperature, water vapor condenses into water
droplets or ice, reducing pressure locally within the
thunderstorm cell. Any precipitation falls the long
distance through the clouds towards the Earth's surface.
As the droplets fall, they collide with other droplets
and become larger. The falling droplets create a
downdraft as it pulls cold air with it, and this cold
air spreads out at the Earth's surface, occasionally
causing strong winds that are commonly associated with
thunderstorms.
Thunderstorms can form and develop in any geographic
location but most frequently within the mid-latitude,
where warm, moist air from tropical latitudes collides
with cooler air from polar latitudes. Thunderstorms are
responsible for the development and formation of many
severe weather phenomena. Thunderstorms, and the
phenomena that occur along with them, pose great
hazards. Damage that results from thunderstorms is
mainly inflicted by downburst winds, large hailstones,
and flash flooding caused by heavy precipitation.
Stronger thunderstorm cells are capable of producing
tornadoes and waterspouts.
There are four types of thunderstorms: single-cell,
multi-cell cluster, multi-cell lines and supercells.
Supercell thunderstorms are the strongest and most
severe. Mesoscale convective systems formed by favorable
vertical wind shear within the tropics and subtropics
can be responsible for the development of hurricanes.
Dry thunderstorms, with no precipitation, can cause the
outbreak of wildfires from the heat generated from the
cloud-to-ground lightning that accompanies them. Several
means are used to study thunderstorms: weather radar,
weather stations, and video photography. Past
civilizations held various myths concerning
thunderstorms and their development as late as the 18th
century. Beyond the Earth's atmosphere, thunderstorms
have also been observed on the planets of Jupiter,
Saturn, Neptune, and, probably, Venus. |
|
 |
| Stages of a
thunderstorm's life. |
Life cycle
Warm air has a lower density than cool air, so warmer
air rises upwards and cooler air will settle at the
bottom (this effect can be seen with a hot air balloon).
Clouds form as relatively warmer air, carrying moisture,
rises within cooler air. The moist air rises, and, as it
does so, it cools and some of the water vapor in that
rising air condenses. When the moisture condenses, it
releases energy known as latent heat of condensation,
which allows the rising packet of air to cool less than
the cooler surrounding air continuing the cloud's
ascension. If enough instability is present in the
atmosphere, this process will continue long enough for
cumulonimbus clouds to form and produce lightning and
thunder. Meteorological indices such as convective
available potential energy (CAPE) and the lifted index
can be used to assist in determining potential upward
vertical development of clouds. Generally, thunderstorms
require three conditions to form: |
- Moisture
- An unstable air mass
- A lifting force (heat)
|
All thunderstorms, regardless of type, go through three
stages: the developing stage, the mature stage, and the
dissipation stage. The average thunderstorm has a 24 km
(15 mi) diameter. Depending on the conditions present in
the atmosphere, each of these three stages take an
average of 30 minutes.
Developing stage
The first stage of a thunderstorm is the cumulus stage
or developing stage. During this stage, masses of
moisture are lifted upwards into the atmosphere. The
trigger for this lift can be solar illumination, where
the heating of the ground produces thermals, or where
two winds converge forcing air upwards, or where winds
blow over terrain of increasing elevation. The moisture
carried upward cools into liquid drops of water due to
lower temperatures at high altitude, which appear as
cumulus clouds. As the water vapor condenses into
liquid, latent heat is released, which warms the air,
causing it to become less dense than the surrounding,
drier air. The air tends to rise in an updraft through
the process of convection (hence the term convective
precipitation). This process creates a low-pressure zone
within and beneath the forming thunderstorm. In a
typical thunderstorm, approximately 500 million
kilograms of water vapor are lifted into the Earth's
atmosphere. |
 |
| Anvil-shaped
thundercloud in the mature stage. |
Mature stage
In the mature stage of a thunderstorm, the warmed air
continues to rise until it reaches an area of warmer air
and can rise no farther. Often this 'cap' is the
tropopause. The air is instead forced to spread out,
giving the storm a characteristic anvil shape. The
resulting cloud is called cumulonimbus incus. The water
droplets coalesce into larger and heavier droplets and
freeze to become ice particles. As these fall, they melt
to become rain. If the updraft is strong enough, the
droplets are held aloft long enough to become so large
that they do not melt completely but fall as hail. While
updrafts are still present, the falling rain drags the
surrounding air with it, creating downdrafts as well.
The simultaneous presence of both an updraft and a
downdraft marks the mature stage of the storm and
produces cumulonimbus clouds. During this stage,
considerable internal turbulence can occur, which
manifests as strong winds, severe lightning, and even
tornadoes.
Typically, if there is little wind shear, the storm will
rapidly enter the dissipating stage and 'rain itself
out', but, if there is sufficient change in wind speed
or direction, the downdraft will be separated from the
updraft, and the storm may become a supercell, where the
mature stage can sustain itself for several hours. |
 |
| A thunderstorm in an
environment with no winds to shear the storm or
blow the anvil in any one direction. |
Dissipating
stage
In the dissipation stage, the thunderstorm is dominated
by the downdraft. If atmospheric conditions do not
support super cellular development, this stage occurs
rather quickly, approximately 20–30 minutes into the
life of the thunderstorm. The downdraft will push down
out of the thunderstorm, hit the ground and spread out.
This phenomenon is known as a downburst. The cool air
carried to the ground by the downdraft cuts off the
inflow of the thunderstorm, the updraft disappears and
the thunderstorm will dissipate. Thunderstorms in an
atmosphere with virtually no vertical wind shear weaken
as soon as they send out an outflow boundary in all
directions, which then quickly cuts off its inflow of
relatively warm, moist air, and kills the thunderstorm's
further growth. The downdraft hitting the ground creates
an outflow boundary. This can cause downbursts, a
potential hazardous condition for aircraft to fly
through, as a substantial change in wind speed and
direction occurs, resulting in a decrease of airspeed
and the subsequent reduction in lift for the aircraft.
The stronger the outflow boundary is, the stronger the
resultant vertical wind shear becomes. |
|
 |
| A return stroke,
cloud-to-ground lightning strike during a
thunderstorm. |
Hazards
Each year, many people are killed or seriously injured
by severe thunderstorms despite the advance warning.
While severe thunderstorms are most common in the spring
and summer, they can occur at just about any time of the
year.
Cloud-to-ground lightning
Cloud-to-ground lightning frequently occurs within the
phenomena of thunderstorms and have numerous hazards
towards landscapes and populations. One of the more
significant hazards lightning can pose is the wildfires
they are capable of igniting. Under a regime of low
precipitation (LP) thunderstorms, where little
precipitation is present, rainfall cannot prevent fires
from starting when vegetation is dry as lightning
produces a concentrated amount of extreme heat. Direct
damage caused by lightning strikes occurs on occasion.
In areas with a high frequency for cloud-to-ground
lightning, like Florida, lightning causes several
fatalities per year, most commonly to people working
outside.
Acid rain is also a frequent risk produced by lightning.
Distilled water has a neutral pH of 7. “Clean” or
unpolluted rain has a slightly acidic pH of about 5.2,
because carbon dioxide and water in the air react
together to form carbonic acid, a weak acid (pH 5.6 in
distilled water), but unpolluted rain also contains
other chemicals.[58] Nitric oxide present during
thunderstorm phenomena, caused by the oxidation of
atmospheric nitrogen, can result in the production of
acid rain, if nitric oxide forms compounds with the
water molecules in precipitation, thus creating acid
rain. Acid rain can damage infrastructures containing
calcite or certain other solid chemical compounds. In
ecosystems, acid rain can dissolve plant tissues of
vegetations and increase acidification process in bodies
of water and in soil, resulting in deaths of marine and
terrestrial organisms. |
 |
| A supercell
thunderstorm over Chaparral, New Mexico. |
Hail
Any thunderstorm that produces hail that reaches the
ground is known as a hailstorm. Thunderclouds that are
capable of producing hailstones are often seen obtaining
green coloration. Hail is more common along mountain
ranges because mountains force horizontal winds upwards
(known as orographic lifting), thereby intensifying the
updrafts within thunderstorms and making hail more
likely. One of the more common regions for large hail is
across mountainous northern India, which reported one of
the highest hail-related death tolls on record in 1888.
China also experiences significant hailstorms. Across
Europe, Croatia experiences frequent occurrences of
hail.
In North America, hail is most common in the area where
Colorado, Nebraska, and Wyoming meet, known as "Hail
Alley". Hail in this region occurs between the months of
March and October during the afternoon and evening
hours, with the bulk of the occurrences from May through
September. Cheyenne, Wyoming is North America's most
hail-prone city with an average of nine to ten
hailstorms per season. In South America, areas prone to
hail are cities like Bogotá, Colombia.
Hail can cause serious damage, notably to automobiles,
aircraft, skylights, glass-roofed structures, livestock,
and most commonly, farmers' crops. Hail is one of the
most significant thunderstorm hazards to aircraft. When
hail stones exceed 13 millimetres (0.5 in) in diameter,
planes can be seriously damaged within seconds. The
hailstones accumulating on the ground can also be
hazardous to landing aircraft. Wheat, corn, soybeans,
and tobacco are the most sensitive crops to hail damage.
Hail is one of Canada's most costly hazards. Hailstorms
have been the cause of costly and deadly events
throughout history. One of the earliest recorded
incidents occurred around the 9th century in Roopkund,
Uttarakhand, India. The largest hailstone in terms of
maximum circumference and length ever recorded in the
United States fell in 2003 in Aurora, Nebraska, United
States. |
 |
| In June 2007, the
town of Elie, Manitoba was struck by an F5
tornado. |
Tornadoes and
waterspouts
A tornado is a violent, rotating column of air in
contact with both the surface of the earth and a
cumulonimbus cloud (otherwise known as a thundercloud)
or, in rare cases, the base of a cumulus cloud.
Tornadoes come in many 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 and 110 mph (64 and 177 km/h), are approximately 75
metres (246 ft) across, and travel several kilometers (a
few miles) before dissipating. Some attain wind speeds
of more than 300 mph (480 km/h), stretch more than 1,600
metres (1 mi) across, and stay on the ground for more
than 100 kilometres (dozens of miles).
The Fujita scale and the Enhanced Fujita Scale rate
tornadoes by damage caused. An EF0 tornado, the weakest
category, damages trees but does not cause significant
damage to 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.
Waterspouts have similar characteristics as tornadoes,
characterized by a spiraling funnel-shaped wind current
that form over bodies of water, connecting to large
cumulonimbus clouds. Waterspouts are generally
classified as forms of tornadoes, or more specifically,
non-supercelled tornadoes that develop over large bodies
of water. These spiralling columns of air frequently
develop within tropical areas close to the equator, but
are less common within areas of high latitude. |
 |
| A flash flood caused
by a severe thunderstorm. |
Flash flood
Flash flooding is the process where a landscape, most
notably an urban environment, is subjected to rapid
floods. These rapid floods occur more quickly and are
more localized than seasonal river flooding or areal
flooding and are frequently (though not always)
associated with intense rainfall. Flash flooding can
frequently occur in slow-moving thunderstorms and is
usually caused by the heavy liquid precipitation that
accompanies it. Flash floods are most common in densely
populated urban environments, where few plants and
bodies of water are present to absorb and contain the
extra water. Flash flooding can be hazardous to small
infrastructure, such as bridges, and weakly constructed
buildings. Plants and crops in agricultural areas can be
destroyed and devastated by the force of raging water.
Automobiles parked within affected areas can also be
displaced. Soil erosion can occur as well, exposing
risks of landslide phenomena. |
 |
| Trees uprooted or
displaced by the force of a downburst wind in
northwest Monroe County, Wisconsin. |
Downburst
Downburst winds can produce numerous hazards to
landscapes experiencing thunderstorms. Downburst winds
are generally very powerful, and are often mistaken for
wind speeds produced by tornadoes, due to the
concentrated amount of force exerted by their
straight-horizontal characteristic. Downburst winds can
be hazardous to unstable, incomplete, or weakly
constructed infrastructures and buildings. Agricultural
crops, and other plants in nearby environments can be
uprooted and damaged. Aircraft engaged in takeoff or
landing can crash. Automobiles can be displaced by the
force exerted by downburst winds. Downburst winds are
usually formed in areas when high pressure air systems
of downdrafts begin to sink and displace the air masses
below it, due to their higher density. When these
downdrafts reach the surface, they spread out and turn
into the destructive straight-horizontal winds.
Thunderstorm asthma
Thunderstorm asthma is the triggering of an asthma
attack by environmental conditions directly caused by a
local thunderstorm. During a thunderstorm, pollen grains
can absorb moisture and then burst into much smaller
fragments with these fragments being easily dispersed by
wind. While larger pollen grains are usually filtered by
hairs in the nose, the smaller pollen fragments are able
to pass through and enter the lungs, triggering the
asthma attack. |
|
 |
| A mild thunderstorm
over Niagara Falls, Ontario. |
Safety
precautions
Most thunderstorms come and go fairly uneventfully;
however, any thunderstorm can become severe, and all
thunderstorms, by definition, present the danger of
lightning. Thunderstorm preparedness and safety refers
to taking steps before, during, and after a thunderstorm
to minimize injury and damage.
Preparedness
Preparedness refers to precautions that should be taken
before a thunderstorm. Some preparedness takes the form
of general readiness (as a thunderstorm can occur at any
time of the day or year). Preparing a family emergency
plan, for example, can save valuable time if a storm
arises quickly and unexpectedly. Preparing the home by
removing dead or rotting limbs and trees, which can be
blown over in high winds, can also significantly reduce
the risk of property damage and personal injury.
The National Weather Service (NWS) in the United States
recommends several precautions that people should take
if thunderstorms are likely to occur: |
- Know the names of local
counties, cities, and towns, as these are how
warnings are described.
- Monitor forecasts and weather
conditions and know whether thunderstorms are likely
in the area.
- Be alert for natural signs of an
approaching storm.
- Cancel or reschedule outdoor
events (to avoid being caught outdoors when a storm
hits).
- Take action early so you have
time to get to a safe place.
- Get inside a substantial
building or hard-topped metal vehicle before
threatening weather arrives.
- If you hear thunder, get to the
safe place immediately.
- Avoid open areas like hilltops,
fields, and beaches, and don't be or be near the
tallest objects in an area when thunderstorms are
occurring.
- Don't shelter under tall or
isolated trees during thunderstorms.
- If in the woods, put as much
distance as possible between you and any trees
during thunderstorms.
- If in a group, spread out to
increase the chances of survivors who could come to
the aid of any victims from a lightning strike.
|
Safety
While safety and preparedness often overlap,
“thunderstorm safety” generally refers to what people
should do during and after a storm. The American Red
Cross recommends that people follow these precautions if
a storm is imminent or in progress: |
- Take action immediately upon
hearing thunder. Anyone close enough to the storm to
hear thunder can be struck by lightning.
- Avoid electrical appliances,
including corded telephones. Cordless and wireless
telephones are safe to use during a thunderstorm.
- Close and stay away from windows
and doors, as glass can become a serious hazard in
high wind.
- Do not bathe or shower, as
plumbing conducts electricity.
- If driving, safely exit the
roadway, turn on hazard lights, and park. Remain in
the vehicle and avoid touching metal.
|
|
The NWS stopped recommending the "lightning crouch" in
2008 as it doesn't provide a significant level of
protection and will not significantly lower the risk of
being killed or injured from a nearby lightning strike. |
 |
| Formation of
numerous waterspouts in the Great Lakes region.
(North America). |
Frequent
occurrences
Thunderstorms occur throughout the world, even in the
polar regions, with the greatest frequency in tropical
rainforest areas, where they may occur nearly daily. At
any given time approximately 2,000 thunderstorms are
occurring on Earth. Kampala and Tororo in Uganda have
each been mentioned as the most thunderous places on
Earth, a claim also made for Singapore and Bogor on the
Indonesian island of Java. Other cities known for
frequent storm activity include Darwin, Caracas, Manila
and Mumbai. Thunderstorms are associated with the
various monsoon seasons around the globe, and they
populate the rainbands of tropical cyclones. In
temperate regions, they are most frequent in spring and
summer, although they can occur along or ahead of cold
fronts at any time of year. They may also occur within a
cooler air mass following the passage of a cold front
over a relatively warmer body of water. Thunderstorms
are rare in polar regions because of cold surface
temperatures.
Some of the most powerful thunderstorms over the United
States occur in the Midwest and the Southern states.
These storms can produce large hail and powerful
tornadoes. Thunderstorms are relatively uncommon along
much of the West Coast of the United States,[99] but
they occur with greater frequency in the inland areas,
particularly the Sacramento and San Joaquin Valleys of
California. In spring and summer, they occur nearly
daily in certain areas of the Rocky Mountains as part of
the North American Monsoon regime. In the Northeast,
storms take on similar characteristics and patterns as
the Midwest, but with less frequency and severity.
During the summer, air-mass thunderstorms are an almost
daily occurrence over central and southern parts of
Florida.
Energy
If the quantity of water that is condensed in and
subsequently precipitated from a cloud is known, then
the total energy of a thunderstorm can be calculated. In
a typical thunderstorm, approximately 5×108 kg of water
vapor are lifted, and the amount of energy released when
this condenses is 1015 joules. This is on the same order
of magnitude of energy released within a tropical
cyclone, and more energy than that released during the
atomic bomb blast at Hiroshima, Japan in 1945.
The Fermi Gamma-ray Burst Monitor results show that
gamma rays and antimatter particles (positrons) can be
generated in powerful thunderstorms. It is suggested
that the antimatter positrons are formed in terrestrial
gamma-ray flashes (TGF). TGFs are brief bursts occurring
inside thunderstorms and associated with lightning. The
streams of positrons and electrons collide higher in the
atmosphere to generate more gamma rays. About 500 TGFs
may occur every day worldwide, but mostly go undetected.
Studies
In more contemporary times, thunderstorms have taken on
the role of a scientific curiosity. Every spring, storm
chasers head to the Great Plains of the United States
and the Canadian Prairies to explore the scientific
aspects of storms and tornadoes through use of
videotaping. Radio pulses produced by cosmic rays are
being used to study how electric charges develop within
thunderstorms. More organized meteorological projects
such as VORTEX2 use an array of sensors, such as the
Doppler on Wheels, vehicles with mounted automated
weather stations, weather balloons, and unmanned
aircraft to investigate thunderstorms expected to
produce severe weather. Lightning is detected remotely
using sensors that detect cloud-to-ground lightning
strokes with 95 percent accuracy in detection and within
250 metres (820 ft) of their point of origin.
Mythology and religion
Thunderstorms strongly influenced many early
civilizations. Greeks believed that they were battles
waged by Zeus, who hurled lightning bolts forged by
Hephaestus. Some American Indian tribes associated
thunderstorms with the Thunderbird, who they believed
was a servant of the Great Spirit. The Norse considered
thunderstorms to occur when Thor went to fight Jötnar,
with the thunder and lightning being the effect of his
strikes with the hammer Mjölnir. Hinduism recognizes
Indra as the god of rain and thunderstorms. Christian
doctrine accepts that fierce storms are the work of God.
These ideas were still within the mainstream as late as
the 18th century.
Martin Luther was out walking when a thunderstorm began,
causing him to pray to God for being saved and promising
to become a monk.
Outside of Earth
Thunderstorms, evidenced by flashes of lightning, on
Jupiter have been detected and are associated with
clouds where water may exist as both a liquid and ice,
suggesting a mechanism similar to that on Earth. (Water
is a polar molecule that can carry a charge, so it is
capable of creating the charge separation needed to
produce lightning). These electrical discharges can be
up to a thousand times more powerful than lightning on
the Earth. The water clouds can form thunderstorms
driven by the heat rising from the interior. The clouds
of Venus may also be capable of producing lightning;
some observations suggest that the lightning rate is at
least half of that on Earth. |
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