Thunderstorms

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Thunderstorms
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Stages of Development of Thunderstorms

• Most thunderstorms typically go through three
  stages of development
• cumulus stage
• mature stage
• dissipating stage

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Cumulus Stage

• (1) Cumulus stage warm, humid air rises, cools,
  becomes saturated and water vapor condenses to
  form a cumulus cloud. If the air is unstable,
  then the cloud continues to grow vertically.
  During this stage most of the air is rising, but
  there is no lightning or thunder.

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Cumulus Stage (Cont.)
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Mature Thunderstorm Stage

• As the cloud grows the formation of ice crystals
  brings about the development of precipitation.
• When the precipitation begins falling toward the
  surface, frictional drag between the drops and
  ice crystals starts pulling air downwards
  creating downdrafts.

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Mature Thunderstorm Stage (Cont.)

• Regions of updrafts and downdrafts are found in
  the mature thunderstorm.
• The existence of both updrafts and downdrafts can
  create significant turbulence in a mature
  thunderstorm.

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Mature Thunderstorm Stage (Cont.)

• The downdrafts may draw some drier, environmental
  air into the cloud in a process called
  entrainment.
• Entrainment leads to the evaporation of water,
  which causes the air to cool and enhances the
  downdrafts.

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The Gust Front

• When the downdrafts reach the surface, the air
  flows out horizontally beneath the thunderstorm.
• This produces an area of strong winds called the
  gust front that spreads out ahead of the
  thunderstorm.

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The Gust Front (Cont.)

• As the gust front passes a location, the wind
  speed increases, the temperature decreases, and
  the pressure increases slightly.

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The Roll Cloud

• Along the gust front, warm, moist air may be
  forced upwards and produce a roll cloud.

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The Mature Stage (Cont.)

• Lightning and thunder occur during the mature
  stage.
• The heaviest rain occurs during the mature stage
  and hail may occur.

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Mature Stage (Cont.)
anvil
Drier environmental air is entrained into the
thunderstorm enhancing downdrafts
Roll Cloud
Gust Front
Heavy Rain
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The Dissipating Stage

• Eventually the downdrafts spread throughout the
  thunderstorm.
• In the case of a slow-moving thunderstorm the
  sinking air cuts off the supply of warm, moist
  air and the thunderstorm begins to dissipate.

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The Dissipating Stage (Cont.)

• The dissipating stage is characterized by weak
  downdrafts throughout the thunderstorm, and light
  rain
• Lightning and thunder may continue to occur
  during the dissipating stage of a thunderstorm.

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The Dissipating Stage (Cont.)
Weak downdrafts occur throughout the dissipating
thunderstorm
Light Rain
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Types of Thunderstorms

• There are three basic types of thunderstorms
• air mass
• multicell
• supercell

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Air Mass Thunderstorms

• Air mass thunderstorms develop individually in
  warm, moist maritime tropical air masses.
• During a sunny day absorption of solar radiation
  increases the surface temperature to the
  Convective Temperature and the air parcels become
  unstable.

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Air Mass Thunderstorms (Cont.)

• The unstable air starts to rise and the
  thunderstorms progress from the cumulus stage,
  through the mature stage to the dissipating stage
  within an hour or so.
• Air mass thunderstorms form when the winds are
  light and move little. Eventually the downdrafts
  cut off the supply of warm moist air and the
  thunderstorm dissipates.

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Multicell Thunderstorms

• At times the gust front (also sometimes called an
  outflow boundary) from one thunderstorm will
  generate enough rising motion to produce a new
  thunderstorm cell right next to the original
  thunderstorm.

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Multicell Thunderstorms (Cont.)

• Thus, it is possible to have multiple
  thunderstorms at different stages of development
  next to each other.
• The identifiable group of thunderstorms is called
  a multicell thunderstorm.

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Multicell Thunderstorm (Cont.)
New Thunderstorm Cell at the Cumulus Stage
Gust Front
Mature Thunderstorm Cell
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Squall Lines

• At times, depending on the larger scale wind
  conditions, the gust fronts (or outflow
  boundaries) merge and produce lines of
  thunderstorms called squall lines.

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Squall Lines (Cont.)
N
Gust Fronts Merge
Separate Multicell Thunderstorms
E
Creating a linear squall line with embedded cores
of heavier precipitation.
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Supercell Thunderstorms

• Supercell thunderstorms are very large
  thunderstorms whose updrafts and downdrafts
  maintain the storm for hours.
• Supercell thunderstorms may also contain a
  rotating updraft.

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Supercell Thunderstorms (Cont.)

• Their structure, large size and long duration
  mean that supercell thunderstorms are capable of
  producing flooding rainfall, damaging surface
  winds, large hail and large tornadoes.

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Supercell Thunderstorms (Cont.)

• Supercell thunderstorms are sometimes classified
  according to how much precipitation they produce.
• (1) High Precipitation (HP) Supercell or
• (2) Low Precipitation (LP) Supercell

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The Dry Line

• The dry line is the name given to the boundary
  that separates hot, dry continental tropical air
  from warm, moist maritime tropical air.
• Since the hot dry air is more dense than the warm
  moist air the dry line is sometimes capable of
  producing enough lift to generate supercell
  thunderstorms.

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Mesoscale Convective Complexes (MCCs)

• Separate multicell and/or supercell thunderstorms
  sometimes produce gust fronts or that produce
  large squall lines.
• The upper level outflows (i.e. the anvils) at the
  tops of the thunderstorms can merge on satellite
  images making it look like one mass of
  thunderstorms covering over 100,000 square
  kilometers.

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Mesoscale Convective Complexes (MCCs) (Cont.)

• These larger features are called Mesoscale
  Convective Complexes (MCCs).
• MCCs can last for 12-18 hours, may produce very
  heavy rainfall over large areas and can generate
  a number of tornadoes.

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Lightning and Thunder

• Lightning is produced by a discharge of
  electricity that can heat the air to a
  temperature of 30,000C.
• The nearly instantaneous increase of temperature
  by 30,000C causes the air to expand very rapidly
  and creates a shock wave that we hear as thunder.

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Distance of Lightning Strokes

• The visible radiation we see as a lightning
  stroke travels at the speed of light and we see
  it almost immediately.
• Thunder travels at the speed of sound (about 330
  m s-1).

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Distance of Lightning Strokes (Cont.)

• If you count the number of seconds between the
  lightning stroke and the arrival of the thunder,
  you can determine how far away the lightning
  stroke occurred.

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Distance of Lightning Strokes (Cont.)

• For example, suppose 10 seconds elapse between
  the time you see a flash of lightning and hear
  the thunder.
• 10 s x 330 m s-1 3300 m
• The lightning stroke was approximately
• 3300 m (about 2 miles) away.

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Generation of Lightning

• Clouds tend to develop a positive charge in the
  upper portion of the cloud.
• Clouds tend to develop a negative charge in the
  lower portion of the cloud.
• The surface of the Earth and objects on the
  surface tend to be positively charged.

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Positive Charge in the Upper Portion of the Cloud


- - - - - - - - - -
Negative Charge in the Lower Portion of the Cloud




Positive Charge at the Surface and on Objects on
the Surface
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Generation of Lightning (Cont.)

• When the electrical field grows strong enough a
  current begins to flow from the cloud toward the
  surface.
• The current flows in steps of 50-100 meters.
• This current is called a stepped leader and it is
  nearly invisible to the human eye.

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Positive Charge in the Upper Portion of the Cloud


- - - - - - - - - -
Negative Charge in the Lower Portion of the Cloud
A stepped leader starts to flow down from the
cloud.




Positive Charge at the Surface and on Objects on
the Surface
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Generation of Lightning (Cont.)

• As the end of the stepped leader approaches the
  ground, a current of positively charged particles
  starts upward from the surface.

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Positive Charge in the Upper Portion of the Cloud


- - - - - - - - - -
Negative Charge in the Lower Portion of the Cloud
A stepped leader starts to flow down from the
cloud.
A current of positively charged particles starts
upward




Positive Charge at the Surface and on Objects on
the Surface
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Generation of Lightning (Cont.)

• When the two currents meet, a large number of
  electrons flow to the ground and a much larger
  visible return stroke moves upward into the cloud.

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Positive Charge in the Upper Portion of the Cloud


- - - - - - - - - -
Negative Charge in the Lower Portion of the Cloud
A large number of electrons flow to the ground
A current of positively charged particles starts
upward




Positive Charge at the Surface and on Objects on
the Surface
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Positive Charge in the Upper Portion of the Cloud


- - - - - - - - - -
Negative Charge in the Lower Portion of the Cloud
A large number of electrons flow to the ground
A visible return stroke moves upward into the
cloud




Positive Charge at the Surface and on Objects on
the Surface
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Generation of Lightning (Cont.)

• Typically this process repeats itself several
  times to create the lightning flash we see.
• The subsequent leaders coming back down from the
  cloud are called dart leaders.

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Positive Charge in the Upper Portion of the Cloud


- - - - - - - - - -
Negative Charge in the Lower Portion of the Cloud
The subsequent leaders coming down are called
dart leaders




Positive Charge at the Surface and on Objects on
the Surface
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Thunderstorm Days in the U.S.

• Thunderstorms are most common over Florida and
  the region around the Gulf of Mexico.
• Central Ohio averages about 40 days with
  thunderstorms each year.

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Hail

• Hail forms in the strongest updrafts of
  thunderstorms.
• Drops of liquid water collide with the original
  ice pellet (also called a hail embryo).
• The liquid water freezes on the ice pellet and
  the hailstone grows by accretion.

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Hail (Cont.)

• When the hailstone grows too large for the
  updraft to support it against the pull of the
  gravitational force, then it falls to the
  surface.

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Hail Occurrence in the U.S.

• Hail is most common over eastern Wyoming, eastern
  Colorado, western Nebraska and western Kansas
  which experience 6-10 days a year with hail.
• On average over central Ohio we see about 1-3
  days per year with hail.

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Tornado

• A tornado is a strong, small rapidly rotating low
  pressure system that extends beneath a
  thunderstorm to the ground.
• The exact processes that lead to the formation of
  a tornado are still not fully understood.

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Tornado (Cont.)

• Some process thought to be related to the change
  of wind direction with height (i.e. directional
  shear) causes a part of the thunderstorm to begin
  to rotate.
• This initial rotation is called a mesocyclone.

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Tornado (Cont.)

• If the rotation extends beneath the thunderstorm,
  condensation may occur due to the expansion and
  cooling of the air and a visible funnel cloud may
  be observed.
• If the bottom of the funnel cloud touches the
  surface of the Earth, then it is called a tornado.

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Mesocyclones
Funnel Cloud
Tornado
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Tornado Frequency

• You are most likely to see tornadoes in parts of
• Oklahoma
• Texas
• Kansas
• Indiana
• Nebraska
• Florida

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Tornado Frequency (Cont.)

• Tornadoes occur more frequently in western Ohio.
• On average 12-13 tornadoes occur over Ohio during
  a year.

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Enhance Fujita (EF) Scale for Tornado Damage

• Theodore Fujita developed a scale for assessing
  the damage caused by tornadoes.
• An updated and Enhanced Fujita (EF) scale was
  implemented by the U.S. National Weather Service
  on February 1, 2007.

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Operational EF Scale

• EF number 3 second gust (mph)
• 0 65-85
• 1 86-110
• 2 111-135
• 3 136-165
• 4 166-200
• 5 over 200

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Straight Line Winds

• Sometimes it is difficult to determine if damage
  was caused by a tornado or by straight line
  winds.
• Thunderstorms can sometimes produce very strong
  downdrafts called downbursts or microbursts that
  generate powerful straight line winds when they
  reach the surface.

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Severe Thunderstorm

• The U.S. National Weather Service defines a
  Severe Thunderstorm as having three-quarter inch
  hail or large and/or wind gusts of at least 50
  knots.

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Severe Thunderstorm Watch and Warning

• A Severe Thunderstorm Watch indicates that
  conditions over a region are such that it is
  possible for severe thunderstorms to form during
  the next few hours.
• A Severe Thunderstorm Warning indicates that a
  severe thunderstorm has been identified visually
  by spotters or by radar.

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Tornado Watch and Warning

• A Tornado Watch indicates that conditions over a
  region are such that it is possible for tornadoes
  to form during the next few hours.
• A Tornado Warning indicates that a tornado had
  been identified visually by spotters or by radar.