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A thunderstorm, also known as an
electrical storm or a lightning storm, is a form of
weather characterized by the presence of lightning
and its effect: thunder. It is usually
accompanied by heavy rain and sometimes snow, hail,
or no precipitation at all. Thunderstorms may line
up in a series, and strong or severe thunderstorms
may rotate.
  
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Warm air has a lower
density than cool air, so warm air rises within
cooler air, similar to hot air balloons. Clouds form
as warm air carrying moisture rises within cooler
air. As the warm air rises, it cools. The moist
water vapor begins to condense. When the moisture
condenses, this releases energy that keeps the air
warmer than its surroundings, so that it continues
to rise. If enough instability is present in the
atmosphere, this process will continue long enough
for cumulonimbus clouds to form, which support
lightning and thunder.
All thunderstorms, regardless of type, go through
three stages: the cumulus stage, the mature stage,
and the dissipation stage. Depending on the
conditions present in the atmosphere, these three
stages can take anywhere from 20 minutes to several
hours to occur.
The first stage of a
thunderstorm is the cumulus stage, or developing
stage. In this stage, masses of moisture are lifted
upwards into the atmosphere. The trigger for this
lift can be insulation heating the ground producing
thermals, areas where two winds converge forcing air
upwards, or where winds blow over terrain of
increasing elevation. The moisture rapidly cools
into liquid drops of water, which appears 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 dry
air. The air tends to rise in an updraft through the
process of convection (hence the term convective
precipitation). This creates a low-pressure zone
beneath the forming thunderstorm. In a typical
thunderstorm, approximately 5×108 kg of water vapor
are lifted, and the amount of energy released when
this condenses is about equal to the energy used by
a city (US-2002) of 100,000 during a month.
In the mature stage
of a thunderstorm, the warmed air continues to rise
until it reaches existing air which is warmer, and
the air can rise no further. 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 heavy 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
be so large that they do not melt completely and
fall as hail. While updrafts are still present, the
falling rain creates downdrafts as well. The
simultaneous presence of both an updraft and
downdrafts marks the mature stage of the storm, and
during this stage considerable internal turbulence
can occur in the storm system, which sometimes
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 and/or direction the downdraft will be
separated from the updraft, and the storm may become
a supercell, and the mature stage can sustain itself
for several hours.
In certain cases however, even with little wind
shear, if there is enough atmospheric support and
instability in place for the thunderstorm to feed
on, it may even maintain its mature stage a bit
longer than most storms.
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. 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.
There are four main
types of thunderstorms: single cell, multicell,
squall line (also called multicell line) and
supercell. Which type forms depends on the
instability and relative wind conditions at
different layers of the atmosphere ("wind shear").
[see below for details of
each]
This term technically applies to a
single thunderstorm with one main updraft. Within a
cluster of thunderstorms, the term "cell" refers to
each separate principal updraft.
Thunderstorm cells can and do form in isolation to
other cells. Such storms are rarely severe and are a
result of local atmospheric instability; hence the
term "air mass thunderstorm". These are the typical
summer thunderstorm in many temperate locales. They
also occur in the cool unstable air which often
follows the passage of a cold front from the sea
during winter.
While most single cell thunderstorms move, there are
some unusual circumstances where they remain
stationary. When this happens, catastrophic flooding
is possible. In Rapid City, South Dakota, in 1972,
an unusual alignment of winds at various levels of
the atmosphere combined to produce a continuous,
stationary cell which dropped an enormous quantity
of rain, resulting in devastating flash flooding.
A similar event occurred in Boscastle, England, on
16 August 2004.
Multicell storms form as clusters
of storms but may then evolve into one or more
squall lines. They often arise from convective
updrafts in or near mountain ranges and linear
weather boundaries, usually strong cold fronts or
troughs of low pressure.
Multicell line storms, commonly
referred to as "squall lines", occur when
multi-cellular storms form in a line rather than
clusters. They can be hundreds of miles long, move
swiftly, and be preceded by a gust front. Heavy
rain, hail, lightning, very strong winds and even
isolated tornadoes can occur over a large area in a
squall line. Bow echoes can form within squall
lines, bringing with them even higher winds.
An unusually powerful type of squall line called a "derecho"
occurs when an intense squall line travels for
several hundred miles, often leaving widespread
damage over thousands of square miles.
Occasionally, squall lines also form near the outer
rain band of tropical cyclones. The squall line is
propelled by its own outflow, which reinforces
continuous development of updrafts along the leading
edge. This kind of storm is also known as
"Wind of the Stony Lake" (石湖風 or 石湖风) in southern
China.
Supercell storms are large, severe
quasi-steady-state storms which feature wind speed
and direction that vary with height ("wind shear"),
separate downdrafts and updrafts (i.e.,
precipitation is not falling through the updraft)
and a strong, rotating updraft (a "mesocyclone").
These storms normally have such powerful updrafts
that the top of the cloud (or anvil) can break
through the troposphere and reach into the lower
levels of the stratosphere and can be 15 miles (24
km) wide. These storms can produce destructive
tornadoes, sometimes F3 or higher, extremely large
hailstones (4 inch or 10 cm diameter), straight-line
winds in excess of 80 mph (130 km/h), and flash
floods. In fact, most tornadoes occur from this type
of thunderstorm.
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