Class #9: Monday, July 19
Thunderstorms and tornadoes
Chapter 14
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Fig. 14-CO, p. 370
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Fig. 14-1, p. 372
Chapter 14
Thunderstorms and Tornadoes
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Thunderstorms
• A storm containing lightening and thunder;
convective storms
• Severe thunderstorms: one of large hail, wind
gusts greater than or equal to 50kts, or
tornado
• Ordinary Cell Thunderstorms
– Air-mass thunderstorms: limited wind sheer
– Stages: cumulus, mature, dissipating
– Entrainment, downdraft, gust front
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Fig. 14-2, p. 373
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Fig. 14-2, p. 373
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Fig. 14-2, p. 373
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Fig. 14-4, p. 375
Thunderstorms
 Multi-cell Thunderstorms
 Thunderstorms that contain a number of convection
cells, each in a different stage of development,
moderate to strong wind shear; tilt, over shooting top
 Gust Front: leading edge of the cold air out-flowing
air; shelf cloud, roll cloud, outflow boundary
 Micro-bursts: localized downdraft that hits the ground
and spreads horizontally in a radial burst of wind; wind
shear, virga
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Fig. 14-6, p. 376
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Thunderstorms
• Multi-cell Thunderstorms
– Squall-line thunderstorms; line of multi-cell
thunderstorms, pre-frontal squall-line, derecho
– Meso-scale Convective Complex: a number of
individual multi-cell thunderstorms grow in size
and organize into a large circular convective
weather system; summer, 10,000km2
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Fig. 14-10, p. 378
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Fig. 14-11, p. 378
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Stepped Art
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Fig. 14-11, p. 378
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Thunderstorms
 Supercell thunderstorms
 Large, long-lasting thunderstorm with a single rotating
updraft
 Strong vertical wind shear
 Outflow never undercuts updraft
 Classic, high precipitation and low precipitation
supercells
 Cap and convective instability
 Rain free base, low-level jet
 Surface, 850mb, 700mb, 500mb, 300mb conditions
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Fig. 14-23, p. 384
Thunderstorms
 Thunderstorms and the Dryline
 Sharp, horizontal change in moisture
 Thunderstorms form just east of dryline
 cP, mT, cT
 Floods and 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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Fig. 1, p. 386
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Thunderstorms
• Topic: Big Thompson Canyon
– July 31, 1976, 12 inches of rain in 4 hours created
a flood associated with $35.5million in damage
and 135 deaths
• Distribution of Thunderstorms
– Most frequent Florida, Gulf Coast, Central Plains
– Fewest Pacific coast and Interior valleys
– Most frequent hail Central Plains
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Fig. 14-25, p. 387
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Thunderstorms
• Lightening and Thunder
– Lightening: 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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Fig. 2, p. 390
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Thunderstorms
• Observations: Elves
– Blue jets, red sprite, ELVES
• The Lightening Stroke
– Positive charge on ground, cloud to ground
lightening
– Stepped leader, ground stroke, forked lightening,
ribbon lightening, bead lightening, corona
discharge
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Thunderstorms
• Observation: Apple tree
– DO NOT seek shelter during a thunderstorm under
an isolated tree.
• Lightening Detection and Suppression
– Lightening direction finder detects radiowaves
produced by lightening, spherics
– National Lightening Detection Network
– Suppression: seed clouds with aluminum
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Fig. 14-32, p. 392
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Fig. 14-33, p. 393
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Fig. 14-34, p. 393
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Fig. 14-35, p. 394
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Fig. 14-36, p. 394
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Fig. 3, p. 395
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Fig. 14-37, p. 396
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Tornadoes
• Rapidly rotating column of air that blows
around a small area of intense low pressure
with a circulation that reaches the ground.
• Tornado life cycle
– Organizing, mature, shrinking, decay stage
• Tornado outbreaks
– Families, super outbreak
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Tornadoes
 Tornado Occurrence
 US experiences most tornadoes
 Tornado Alley (warm, humid surface; cold dry air aloft)
 Highest spring, lowest 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
 Multi-vortex tornados
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Tornadoes
• Seeking shelter
– Basement or small, interior room on ground floor
– Indoor vs. outdoor pressure
• The Fujita Scale
– Based upon the damage created by a storm
– F0 weakest, F5 strongest
– Enhanced Fujita Scale
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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, rear flank downdraft,
vertical stretching, funnel cloud, rotating cloud,
wall cloud
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Table 14-1, p. 399
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Table 14-2, p. 400
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Table 14-3, p. 400
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Fig. 14-42, p. 400
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Fig. 14-43, p. 401
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Fig. 14-46, p. 402
Tornadic Formation
• Nonsupercell Tornadoes
– Gustnadoes
– Land spout
– Cold-air funnels
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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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Fig. 14-49, p. 405
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Fig. 14-50, p. 405
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Fig. 14-51, p. 406
Waterspouts
• Rotating column of air that is connected to a
cumuliform cloud over a large body of water
• Tornadic waterspout
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Fig. 14-52, p. 409