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Thunderstorms
  A storm
containing lightning and
thunder; convective storms
  Severe thunderstorms: At least one:
Chapter 14
  large hail
  wind gusts greater than or equal to 50 kt
  Tornado
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Ordinary Cell Thunderstorms
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Ordinary Cell Thunderstorms
  AKA Air-mass
thunderstorms: form with
limited wind sheer, vertically stacked
 
Mature Stage
  Droplets grow large enough to fall.
  Drier air is drawn into cloud from sides (entrainment). This
  Stages: cumulus, mature, dissipating
causes evaporates some drops, which cools air.
  Cooling air leads to downdrafts, also enhanced by falling
  Cumulus Stage (Growth Stage)
precipitation.
○  Warm air rises. Cloud droplets evaporate at
  Anvil-Shaped cumulonimbus is formed, cloud top can be
top and make air more humid, allowing higher
growth.
○  Condensation releases heat, which causes
more rising.
○  No precipitation (held aloft by updrafts), no
lightning.
Concordia University Geog/Sci-381 Chapter 14
Concordia University Geog/Sci-381 Chapter 14
40000’ high.
  Updrafts collide with downdrafts- turbulence.
  Cool downdrafts spread sideways at the ground along a
gust front.
○  Turbulence along gust front.
○  Warm air rises over gust front.
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Ordinary Cell Thunderstorms
  Dissipating
Stage
  Storm generally dissipates after 15-30
minutes.
  Gust front moves away from the storm and
no longer enhances updrafts.
  Downdrafts dominate, no more updrafts to
fuel the storm.
  Whole process can last only one hour.
  Thunderstorms bring summer rain and
welcome temperature relief, though brief.
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Thunderstorms
 
Multi-cell Thunderstorms
  Thunderstorms that contain a number of convection cells, each
in a different stage of development
  Caused by moderate to strong wind shear, which produces tilt
  Sometimes there is an over shooting top
  Mammatus clouds can form below the anvil
 
Gust Front: leading edge of the cold air out-flowing air
  Can form shelf clouds or roll clouds.
  Combined edge of gust fronts is called the outflow boundary.
 
Downbursts: localized downdraft that hits the ground and
spreads horizontally in a radial burst of wind
  Microbusts: 4km spread or less.
  Cause wind shear, dangerous to planes.
  Plane crash in Dallas, 1985: 100 deaths.
  Virga may form.
  Warm downbursts are called heat bursts.
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Fig. 14-4, p. 11
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Fig. 14-4, p. 10
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Multi-cell Thunderstorms
 
Squall-line thunderstorms: a line of multi-cell thunderstorms
  Pre-frontal squall-line
  Bow Echo: a bow-shaped squall line
  Derecho: High winds along several hundred kilometers of squall
line
 
The Rear Inflow Jet
 
Meso-scale Convective Complex (MCC): a number of
individual multi-cell thunderstorms grow in size and organize
into a large circular convective weather system
  Damaging straight-line winds
  Happens in summer
  Can last 12 hours, cover 10,000 km2
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Supercell Thunderstorms
 
 
 
 
 
 
Large, long-lasting thunderstorm with a single
rotating updraft
Strong vertical wind shear: horizontal rotation
becomes vertical.
Outflow never undercuts updraft
The Mesocyclone and the Overshooting Top
Wall clouds
Three types of supercell:
  Classic
  High precipitation
  Low precipitation
 
 
Rain free base, low-level jet
Surface, 850mb, 700mb, 500mb, 300mb conditions
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Inversion layer caps air, convective instability: breakthrough
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Floods
  Flash
Floods
  Flash floods rise rapidly with little or no
advance warning; many times caused by
stalled or slow thunderstorm
  Large floods can be created by training of
storm systems, Great Flood of 1993
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Thunderstorms
  Distribution
of Thunderstorms
  Most frequent Florida, Gulf Coast, Central
Plains
  Fewest Pacific coast and Interior valleys
  Most frequent hail Central Plains
Concordia University Geog/Sci-381 Chapter 14
Thunderstorms
  Lightning
and Thunder
  Lightning: discharge of electricity in mature
storms (within cloud, cloud to cloud, cloud to
ground)
  Thunder: explosive expansion of air due to
heat from lightening
  Electrification of Clouds: graupel and
hailstones fall through supercooled water,
ice crystals become negatively charged
  Upper cloud positive, bottom cloud negative
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Thunderstorms
  The
Lightning Stroke
  Positive charge on ground, cloud to ground
lightning
  Stepped leader, ground stroke, forked
lightening, ribbon lightning, bead lightning,
corona discharge
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Thunderstorms
  Observation: Apple
tree
  DO NOT seek shelter during a thunderstorm
under an isolated tree.
  Lightning
Detection and Suppression
  Lightning direction finder detects radiowaves
produced by lightning: Spherics
  National Lightning Detection Network
  Suppression: seed clouds with aluminum
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Tornadoes
Tornadoes
  Rapidly
rotating column of air that blows
around a small area of intense low
pressure with a circulation that reaches
the ground.
  Funnel cloud: A tornado that has not
reached the ground
  Tornado life cycle
 
  US experiences most tornadoes
  Tornado Alley (warm, humid surface; cold dry air
aloft, wind shear helped by jet stream)
  Highest in spring, lowest in winter
 
Tornado winds
  Measurement based upon damage after storm or
Doppler radar
  For southwest approaching storms, winds strongest
in the northeast of the storm, 220 kts maximum
  If a tornado approaches, on which side is the wind
  Organizing, mature, shrinking, decay stage
  Tornado
Tornado Occurrence
fastest?
  Multi-vortex tornados
outbreaks
  Families, super outbreak
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Tornadoes
  Seeking
shelter
  Basement or small, interior room on ground
floor
  Indoor vs. outdoor pressure, p. 398
  The
Fujita Scale
  Based upon the damage created by a storm
  F0 weakest, F5 strongest
  Enhanced Fujita Scale
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Table 14-2, p. 51
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Tornadic Formation
  Basic
requirements are an intense
thunderstorm, conditional instability, and
strong vertical wind shear
  Supercell Tornadoes
  Wind sheer causes spinning vortex tube that
is pulled into thunderstorm by the updraft
  Mesocyclone, BWER (bounded weak echo
region), rear flank downdraft, vertical
stretching, funnel cloud, rotating cloud, wall
cloud
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Stepped
Fig. 14-46, p. 402
Tornadic Formation
  Nonsupercell
Tornadoes
  Gustnadoes
○  Form along gust front, short-lived and weak
  Land spout
○  Common over East-Central Colorado
  Cold-air funnels
○  Formed by cold air aloft, common along US
West Coast
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Severe Weather and Doppler
Radar
  Doppler
radar measures the speed of
precipitation toward and away radar unit
  Two Doppler radars can provide a 3D
view
  TVS, Doppler lidar
  NEXRAD
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Waterspouts
  Rotating
column of air that is connected
to a cumuliform cloud over a large body
of water
  Tornadic waterspout
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Homework for Chapter 14
 
Project for Chapter 14
Chapter 14 Questions for Review, p. 407
 
None
  #2-6, 19, 25, 29
 
Chapter 14 Questions for Thought, p. 408
 
Chapter 14 Problems and Exercises, p. 409
  #7
  #4
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