Thunderstorm
A thunderstorm, also called an electrical storm, is a form of weather that has lightning and
thunder produced from a cumulonimbus cloud. Thunderstorms usually come with by heavy
rainfall and they can also make strong winds, hail and tornadoes.
Thunderstorms form when significant condensation (resulting in the production of a wide
range of water droplets and ice crystals) occurs in an atmosphere that is unstable and supports
deep, rapid upward motion. This often occurs in the presence of three things: 1) sufficient
moisture accumulated in the lower atmosphere, 2) reflected by high temperatures; 3) a
significant fall in air temperature and a force such as mechanical convergence along a cold front
to focus the lift.[The process to initiate vertical lifting can be caused by (1) unequal warming of
the surface of the Earth, (2) lifting due to topographic obstruction of air flow, and (3) dynamic
lifting because of the presence of a frontal zone.
Classification
A single cell thunderstorm.
A lightning strike.
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.
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Single cell storms form when the atmosphere is unstable, but there is little or no wind
shear, meaning precipitation falls back down through the updraft that led to it, cooling it
and eventually killing it. These storms are short lived, and last for less than an hour after
becoming strong enough to produce lightning. Days with suitable weather conditions
often see the repeated forming and dissipation of such storms, leading them to be known
as "pulse" storms.
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Multicell storms are groups of cells in different stages of development which have
merged into a larger system. The cloud becomes divided into updraft and downdraft
regions separated by a gust front. The gust front may extend for several miles ahead of
the storm, bringing with it increases in wind speed and atmospheric pressure, decreases in
temperature, and shifts in wind direction.
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Supercell storms are large, severe quasi-steady-state storms which form when the wind
speed and direction vary with height separates downdrafts from updrafts (i.e.,
precipitation is not falling through the updraft) and contain a strong, rotating updraft.
These storms normally have such powerful updrafts that the top of the cloud (or anvil)
can reach miles into the air and can be 15 miles wide. These storms produce destructive
tornadoes, sometimes F3 or higher, extremely large hailstones (4 inch—10 cm—
diameter), straight-line winds in excess of 80 mph, and flash floods. In fact, most
tornadoes occur from this kind of thunderstorms
Severe thunderstorm
A severe thunderstorm is a thunderstorm with winds 57.5 mph or greater, ¾ in or larger hail,
funnel clouds or tornadoes. These storms may have a lot cloud-to-ground lightning and heavy
downpours which can lead to localized flooding. This is a general definition which varies by
country and is somewhat contentious An otherwise weak thunderstorm which produces a wind
gust of the required strength would be defined as 'severe' whereas a very violent thunderstorm
with continuous lightning and very heavy rain (but without the required wind gusts, hail or
tornado/funnel cloud) would not. Many of the violent local thunderstorms which affect Florida
so frequently during the summer months would not be defined as severe.
Severe thunderstorms may occur as supercell thunderstorms, although multicell and squall lines
are the most common forms.
Life cycle
An airflow diagram of the towering
cumulus stage
An airflow diagram of the
mature stage
An airflow diagram of the
dissipation stage
Diagram showing formation of a thunderstorm cloud
A given cell of a thunderstorm goes through three stages: the cumulus stage, the mature stage,
and the dissipation stage.
In the cumulus stage of a thunderstorm cell, masses of moisture are pushed upwards. The
trigger for this can be solar insolation heating the ground producing thermals, areas where two
winds converge forcing air upwards, or where winds blow over areas of high ground. The
moisture rapidly cools into liquid drops of water, which appears as cumulus clouds. As the
water vapour condenses into liquid, latent heat is released which warms the air, causing it to
become less dense than the surrounding dry air, and so the air will tend to rise in an updraft
because of convection . This creates a low-pressure zone beneath the forming thunderstorm.
In a typical thunderstorm, In the mature stage, the warmed air keeps rising until it reaches
existing air which is itself warmer, and the air can rise no further. 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 combine 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 up long enough to be so large that they do not melt completely as they fall and fall as
hail. While updrafts are still present, the falling rain creates downdrafts as well. The presence of
both updrafts and downdrafts during this stage can cause considerable internal turbulence in the
storm system, which sometimes becomes strong winds, severe lightning, and even tornadoes. If
there is little wind shear, the storm will rapidly '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.
Finally, in the dissipation stage, updraft conditions no longer exist, and the storm is
characterized largely by weak downdrafts. Because most of the moisture has precipitated out,
there is not sufficient moisture in the lower air to sustain the cycle and the thunderstorm
dissipates.
Anvil shaped thundercloud
Lightning
Lightning is an electrical discharge that occurs in a thunderstorm. It can
be seen as a bright streak (or bolt) from the sky. Lightning occurs when
a charge is built up within a cloud. When a large enough charge is built
up, a large discharge will occur and can be seen as lightning. The
temperature of a lightning bolt can be hotter than the surface of the sun.
Although the lightning is extremely hot, the short duration makes it not
necessarily fatal. Contrary to the popular idea that lightning doesn’t
strike twice in the same spot, some people have been struck by
lightning over three times and skyscrapers like the Empire State
Building have been struck numerous times in the same storm.
There are several kinds of lightning.
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In-Cloud Lightning is the most common. It is lightning within a cloud.
Cloud to Ground Lightning is when a bolt of lightning from a cloud strikes the ground.
This form poses the greatest threat to life and property.
Ground to Cloud Lightning is when a lightning bolt is induced from the ground to the
cloud.
Cloud to Cloud Lightning is rarely seen and is when a bolt of lightning arches from one
cloud to another.
Ball Lightning is extremely rare and has no known scientific explanation. It is seen in
the form of a 20 to 200 centimeter ball.
Cloud to Air Lightning is when lightning from a cloud hits air of a different charge.