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Thunderstorm

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather;
Storms

   A shelf cloud associated with a heavy or severe thunderstorm over
   Enschede, The Netherlands.
   A shelf cloud associated with a heavy or severe thunderstorm over
   Enschede, The Netherlands.
   The setting sun illuminates the top of a classic anvil-shaped
   thunderstorm cloud, eastern Nebraska, United States.
   The setting sun illuminates the top of a classic anvil-shaped
   thunderstorm cloud, eastern Nebraska, United States.

   A thunderstorm, also called an electrical storm, is a form of weather
   characterized by the presence of lightning and its attendant thunder
   produced from a cumulonimbus cloud. Thunderstorms are usually
   accompanied by heavy rainfall and they can also be accompanied by
   strong winds, hail and tornadoes. In the winter months, snowfall can
   occasionally take place in a thunderstorm. Such is often termed
   thundersnow.

   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 conditions: sufficient
   moisture accumulated in the lower atmosphere, reflected by high
   temperatures; a significant fall in air temperature with increasing
   height, known as a steep adiabatic lapse rate; 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) orographic lifting due to
   topographic obstruction of air flow, and (3) dynamic lifting because of
   the presence of a frontal zone.

   Thunderstorms have had a lasting and powerful influence on early
   civilizations. Romans thought them to be battles waged by Jupiter, who
   hurled lightning bolts forged by Vulcan. Thunderstorms were associated
   with the Thunderbirds, held by Native Americans to be a servant of the
   Great Spirit.

   According to Encyclopedia Britannica, 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 an average
   thunderstorm, the energy released amounts to about 10,000,000
   kilowatt-hours ( 3.6 x 10^13 joule), which is equivalent to a
   20-kiloton nuclear warhead. A large, severe thunderstorm might be 10 to
   100 times more energetic.

Classification

   A single cell thunderstorm.
   A single cell thunderstorm.
   A lightning strike during a thunderstorm in Denver.
   A lightning strike during a thunderstorm in Denver.

   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"):
     * Single cell storms form when the atmosphere is not strong enought,
       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.

     * 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.
       The storm itself will have different portions sequentially going
       through the various thunderstorm stages. In many cases the immature
       cells develop along a flanking line, resulting in what is known as
       a line multicell.

     * Squall line or multicell line storms are formed as an organized
       line or lines of multicell storms frequently with a gust front.
       This kind of storm is also known as "Wind of the Stony Lake"
       (Traditional Chinese:石湖風, Simplified Chinese: 石湖风) in southern
       China. They often arise from convective updrafts in or near
       mountain ranges and linear weather boundaries, usually strong cold
       fronts or troughs of low pressure. Occasionally, squall lines are
       also formed near the outer rain band of the tropical cyclones. The
       squall line is propelled by its own outflow, which reinforces
       continuous development of updrafts along the leading edge. Squall
       lines tend to be hundreds of miles long, sometimes stretching
       across the Midwestern United States, covering five states at a
       time. These lines can move swiftly and in some parts of the line,
       bow echoes can form, bringing with it high winds, dangerous
       lightning, and possibly tornadoes. Heavy rain, hail and very strong
       winds over a large area such as derechos can occur in a squall
       line.

     * 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 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 (130
       km/h), and flash floods. In fact, most tornadoes occur from this
       kind of thunderstorm.

   Multicell or squall line systems may form within a meteorologically
   important feature known as mesoscale convective system (MCS) stretching
   for hundreds of kilometres. The mesoscale convective complex is a
   closely related phenomenon. They are large enough to have a pronounced
   effect on the upper-level and surface weather pattern, and may
   influence forecasts over a large area. MCS systems are common in the
   Midwest region of the United States and the Canadian Prairies during
   the summer months and produce much of the region's important
   agricultural rainfall. Prior to the discovery of the MCS phenomenon,
   the individual thunderstorms were thought of as independent entities,
   each being effectively impossible to predict. The MCS is amenable to
   forecasting, and a meteorlogist can now predict with high accuracy the
   percentage of the MCS that will be affected by thunderstorms. However,
   the meteorlologist still cannot predict exactly where each thunderstorm
   will occur within the MCS.

Severe thunderstorm

   A severe thunderstorm is a thunderstorm with winds 92.5 kilometers/hour
   (57.5 mph) or greater, 1.9 centimeter (¾ in) or larger hail, funnel
   clouds or tornadoes. These storms may contain frequent 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.

Where thunderstorms occur

   Thunderstorms occur throughout the world, even in the polar regions,
   with the greatest frequency in tropical rainforest areas, where they
   may occur nearly daily. Kampala and Tororo in Uganda have each been
   mentioned as the most thunderous places on Earth, an accolade which has
   also been bestowed upon Bogor on Java, Indonesia. In temperate regions,
   they are most frequent in spring and summer, although they can occur in
   cold fronts at any time of year. Thunderstorms are rare in polar
   regions due to the cold climate and stable air masses that are
   generally in place, but they do occur from time to time, mainly in the
   summer months.

   In more contemporary times, thunderstorms now have taken on the role of
   a curiosity. Every spring, storm chasers head to the Great Plains of
   the United States and the Canadian Prairies in summer to explore the
   visual and scientific aspects of storms and tornadoes.

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 due to the process of convection (hence
       the term convective precipitation). This creates a low-pressure
       zone beneath the forming thunderstorm. In a typical thunderstorm,
       some 5×10^8 kg of water vapour 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, the warmed air continues to rise until it
       reaches existing air which is itself 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 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 manifests as 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
   Anvil shaped thundercloud

Lightning

   Cloud to ground lightning
   Cloud to ground lightning

   Lightning is an electrical discharge that occurs in a thunderstorm. It
   can be seen in the form of 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.
     * In-cloud lightning is the most common. It is lightning within a
       cloud. Sometimes called cloud to cloud or sheet lightning.
     * 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.

   Retrieved from " http://en.wikipedia.org/wiki/Thunderstorm"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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