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Thunderstorms and Tornadoes


Understanding the Weather EAS-107 Chapter 14 Thunderstorms and Tornadoes Chapter 14 Overview Ordinary Cell Thunderstorms Severe Thunderstorms Mesoscale Convective ... – PowerPoint PPT presentation

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Title: Thunderstorms and Tornadoes

Understanding the WeatherEAS-107
  • Chapter 14
  • Thunderstorms and Tornadoes

Understanding the WeatherEAS-107
  • Chapter 14 Overview
  • Ordinary Cell Thunderstorms
  • Severe Thunderstorms
  • Mesoscale Convective
  • Lightning
  • Tornadoes

Understanding the WeatherEAS-107
  • Thunderstorms
  • A thunderstorm contains thunder and lightning.
  • Come in many different shapes and sizes.
  • Warm, moist air rises in a conditionally unstable
  • As long as parcel is warmer than environment then
    it will continue to rise, it is buoyant.
  • Greater the temp difference, the faster the air
    will rise.
  • Rising air must be triggered/forcing mechanism.
  • Unequal heating, terrain, lifting of air along
    shallow boundaries of converging winds.
  • Frontal lifting.
  • Large scale divergence aloft.

Understanding the WeatherEAS-107
  • Ordinary Cell Thunderstorms
  • Afternoon storms develop away from fronts.
  • Form in a region with limited wind shear wind
    speed and direction do not change with height.
  • Form along shallow convergence zones.
  • Topography, sea-breezes, cold outflow from a
    prior thunderstorm (differences in temp) can be
    locations for development.

Understanding the WeatherEAS-107
  • Ordinary Cell Thunderstorms (Cumulus/Growth
  • Warm and humid parcel of air rises, cools, and
    condenses into one cloud or a cluster of clouds.
  • Gradually moistens the dry air aloft.
  • Latent heat release keeps the parcels warm.
  • Must have continuous source of rising air, the
    cloud needs to be constantly fed rising air from
  • Updrafts suspend the liquid until it grows large
    enough to fall.
  • This usually does not happen until ice is

Understanding the WeatherEAS-107
  • Ordinary Cell Thunderstorms (Mature Stage)
  • The entrainment of drier air at the edges of the
    storm evaporates some of the liquid which cools
    the surrounding air. Evaporative cooling.
  • The air, now heavier and cooler than the air
    around it, begins to descend a downdraft. This
    can be enhanced by falling precip.
  • When this occurs the storm is mature.
  • The updraft and downdraft now constitute the

Understanding the WeatherEAS-107
  • Ordinary Cell Thunderstorms (Mature Stage)
  • The most intense stage. Lightning, thunder, rain,
    and occasionally small hail.
  • Top of cloud reaches a stable region (tropopause)
    and spreads out into anvil, up to 40,000 ft (12
  • Some updrafts are so strong they penetrate the
    stable air, a condition know as overshooting.
  • The cold downdraft forms a gust front.
  • The gust front can act to force more warm, humid
    air into the storm.

Understanding the WeatherEAS-107
  • Ordinary Cell Thunderstorms (Dissipating Stage)
  • Once the storm enters the mature stage, it begins
    to dissipate after 15 30 mins.
  • Downdrafts dominate through much of the cloud and
    cut off the supply of warm, moist air.
  • Deprived of the rich supply of warm, humid air,
    cloud droplets no longer form.
  • As the storm dies, the lower-level cloud
    particles evaporate rapidly.

Understanding the WeatherEAS-107
  • Severe Thunderstorms
  • Criteria A thunderstorm that produces hail at
    least ¾ diameter (penny/dime), and/or surface
    wind gusts of 50 kts, and/or produces a tornado.
  • The longer a storm survives, the more likely it
    will become severe.
  • Forms in same way as an ordinary storm, only the
    environment features vertical wind shear.
  • The lack of wind shear in an ordinary
    thunderstorm causes the precip to fall into the
  • Increased winds aloft push the precip away from
  • The downdraft can still undercut the updraft
    without the precip falling into it. This produces
    a long-lasting multicell storm.

Understanding the WeatherEAS-107
  • Severe Thunderstorms (Multicell Storm)
  • Most ordinary thunderstorms are multicell storms
    (storms with a cluster of cells at various stages
    of their life cycles), however many multicell
    storms are severe.
  • The gust front initiates new storms (cells).
  • This process may repeat over and over, producing
    a long lasting storm.
  • Each cell in a different state of development.

Understanding the WeatherEAS-107
  • Severe Thunderstorms (Supercell)
  • When the upper level winds are even stronger and
    also change direction with height
  • The storm can move in such a way that the
    downdraft never undercuts the updraft.
  • Can create horizontal spin which may be tilted
    into a rotating updraft.
  • The result is a supercell.
  • A supercell modifies its own environment.
  • The vertical wind shear creates a storm structure
    that allows the storm to continually move towards
    the area of warm moist air.
  • Supercells move to the right of the mean flow.

Understanding the WeatherEAS-107
  • Severe Thunderstorms (Supercell)
  • The supercell thunderstorm is typically a large
    thunderstorm that consists primarily of a single
    rotating updraft.
  • The internal structure is organized in a way so
    that the storm may maintain itself as a single
    entity for hours on end.
  • Storms of this magnitude can produce updrafts
    that exceed 90 kts, hail the size of grapefruit,
    damaging surface winds, and large, long lasting

Understanding the WeatherEAS-107
  • Mesoscale Convective Complex
  • A large, convectively driven system that is made
    up of many individual thunderstorms. Many times,
    as much as 1000 times larger than an individual
    thunderstorm cell.
  • Within the MCCs, the individual thunderstorms
    work together to generate a long-lasting weather
    system that moves slowly and quite often for
    periods exceeding 12 hrs.
  • Typically about the size of Ohio.
  • Often forms during summer nights.
  • Major source of rainfall for Plains states in the

Understanding the WeatherEAS-107
  • Lightning
  • Lightning is simply a discharge of electricity
    (giant spark) which usually occurs in mature
  • Cloud to cloud, cloud to ground, cloud to air,
    within cloud.
  • Can heat the air to 54,000F!! (5 times hotter
    than sun)
  • The extreme heating causes the air to rapidly
    expand, initiating a shock wave that becomes a
    sound wave, called thunder.
  • Sound travels 330 m/sec vs. 300 million m/sec for
  • Sound travels faster in warm air than in cold
  • It takes sound about 5 seconds to travel 1 mile.
  • If you see lightning and hear thunder 15 seconds
    later, the lightning stroke occurred 3 miles

Understanding the WeatherEAS-107
  • Lightning (Electric Charge)
  • In normal fair weather, the electric field of the
    atmosphere is characterized by a negatively
    charged surface and a positively charged upper
  • For lightning to occur, separate regions
    containing opposite electrical charges must
    exists within the cloud.
  • Separation of charge is brought about by
    precipitation processes.
  • There is a net transfer of positive ions (charge)
    from a warmer object to a colder object.
  • Ice crystals are colder than hailstones
  • Hailstones become negatively charged, ice
    crystals positively.
  • Charge is then distributed by weight.
  • Ice and a strong updraft are necessary for
    lightning to occur.

Understanding the WeatherEAS-107
  • Lightning (Exchange of Charge)
  • When the negative charge near the bottom of the
    cloud is large enough to overcome the airs
    resistance, a flow of electrons (stepped leader)
    rushes towards the earth.
  • As the electrons approach the ground, a region of
    positive charge moves up into the air through any
    conducting objects (trees, buildings, even
  • When the downward flow of electrons meets the
    upward surge of positive charge, a strong
    electric current (bright return stroke) carries
    the positive charge up into the cloud.
  • Takes 3 or 4 return strokes to fully exchange the

Understanding the WeatherEAS-107
  • Tornadoes
  • A tornado is a rapidly rotating column of air
    that blows around a small intense area of low
  • Circulation must reach the ground as either a
    cloud or area of swirling debris.
  • Funnel cloud is a tornado that does not reach the
  • The majority of tornadoes rotate
  • Diameter 300 2000 ft, some gt 1 mi.
  • Usually move NE at 20 40 kts, up to 70 kts.
  • Most tornadoes usually last only a few minutes
    and have an average path length of 4 miles.
  • But some can last many hours and travel hundreds
    of miles.
  • Most are found in the Plains (tornado alley).

Understanding the WeatherEAS-107
  • Tornadoes (Occurrence)
  • Top number Number of tornadoes in 25 years.
  • Bottom number Number of tornadoes per 10,000
    square miles.
  • Tornado alley is from north Texas through
  • Can occur in any state.

Understanding the WeatherEAS-107
  • Tornadoes (Watch vs Warning)
  • Tornado watch. Tornadoes are likely to form
    within the next 4 to 6 hours somewhere in the
    region (issued by Storm Prediction Center).
  • Tornado warning. A tornado has been spotted or
    indicated on Doppler radar (issued by the local
    weather service office).
  • Take shelter in the basement in an interior room.
    In a structure without a basement, the safest
    place is in an interior room on the lowest floor.
  • In a dorm or apartment building, move to an
    interior hallway and lie flat with your head

Understanding the WeatherEAS-107
  • Tornadoes (Life Cycle)
  • 1. Dust Whirl Stage
  • 2. Organizing Stage
  • 3. Mature Stage
  • 4. Shrinking Stage
  • 5. Decay Stage

Understanding the WeatherEAS-107
  • Tornadic Thunderstorms
  • Not everything is known about the formation of a
  • It is known that tornadoes tend to form with
    intense thunderstorms and that a conditionally
    unstable atmosphere is essential for their
  • Tornadoes form with both supercell thunderstorms
    and non-supercell thunderstorms.
  • The most intense tornadoes form with supercell
    thunderstorms. Thunderstorms with a strong,
    single rotating updraft that develop in a region
    of strong vertical wind shear.

Understanding the WeatherEAS-107
  • Tornadic Thunderstorms (Mesocyclones)
  • The rotating updraft of a supercell thunderstorm
    is called a mesocyclone.
  • Change of wind speed and direction with height is
    responsible for the rotating updraft.
  • Creates horizontally rotating vortex tubes.
  • The strong updraft will tilt the tube and draw it
    into the storm.
  • This creates a mesocyclone 5-10 km wide.

Understanding the WeatherEAS-107
  • Tornadic Thunderstorms (Bounded Weak Echo Region)
  • The updraft is so strong in a supercell that
    precipitation cannot fall through it.
    Southwesterly winds aloft usually help to blow
    the precipitation northeastward.
  • Large hailstones that have remained in the cloud
    for some time, usually fall just north of the
  • If a mesocyclone is strong and persistent, the
    precip can be wrapped around the updraft.
  • This swirling area of precip shows up on the
    radar, where the area inside the mesocyclone does
  • This is called the bounded weak echo region, an
    area that is bounded by precip. On radar, it can
    appear as a hook on the southern side of the

Understanding the WeatherEAS-107
  • Tornadic Thunderstorms (Tornado Formation)
  • The mesocyclone can be compared to a small low
    pressure system, like those that we have studied.
  • At this point, the updraft, counterclockwise
    swirling precip, and the surrounding air interact
    to form the rear flank downdraft (southwestern
    side of supercell).
  • When the downdraft strikes the ground, it may
    interact with the region of inflow to help
    initiate the tornado.

Understanding the WeatherEAS-107
  • Tornadic Thunderstorms (Tornado Formation)
  • As air rushes into the low-level core of the
    mesocyclone, the air expands, cools, and if
    moist, condenses into a cloud (funnel cloud).
  • As the air beneath the funnel cloud is drawn into
    its core, the air cools rapidly and condenses,
    and the funnel descends to the surface.
  • As the funnel reaches the ground, it usually
    picks up dirt and debris, making it appear dark.
  • While the air along the outside of the funnel is
    spiraling upward, in the most violent tornadoes,
    the air is descending towards the extreme low
    pressure at the ground (sometimes 100 mb lower
    than surrounding air).

Understanding the WeatherEAS-107
  • Tornadic Thunderstorms (Final Thoughts)
  • Not all supercells produce tornadoes in fact,
    only around 15 do.
  • However, studies reveal that supercells are more
    likely to produce tornadoes when they interact
    with a pre-existing boundary.
  • Outflow boundary from previous convection.
  • Warm front
  • When supercells interact with these boundaries,
    low-level wind shear is enhanced. This can lead
    to a stronger and deeper mesocyclone.

Understanding the WeatherEAS-107
  • Fujita Scale
  • Classifies wind speed based on damage. Tornado
    winds are estimated based on the damage caused by
    the storm.
  • Studies indicate that the majority of tornadoes
    are F0 and F1, and only a few are above F3.
  • On average, only 1 or 2 F5s occur a year.

Understanding the WeatherEAS-107
  • Fujita Scale (F0 and F1)

Understanding the WeatherEAS-107
  • Fujita Scale (F2 and F3)

Understanding the WeatherEAS-107
  • Fujita Scale (F4 and F5)

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