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5/2/11
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Chapter 14
1 Concordia University Geog/Sci-381 Chapter 14
Thunderstorms
A storm containing lightning and thunder; convective storms
Severe thunderstorms: At least one: large hail wind gusts greater than or equal to 50 kt Tornado
2 Concordia University Geog/Sci-381 Chapter 14
Ordinary Cell Thunderstorms
AKA Air-mass thunderstorms: form with limited wind sheer, vertically stacked Stages: cumulus, mature, dissipating Cumulus Stage (Growth Stage) ○ Warm air rises. Cloud droplets evaporate at
top and make air more humid, allowing higher growth.
○ Condensation releases heat, which causes more rising.
○ No precipitation (held aloft by updrafts), no lightning.
3 Concordia University Geog/Sci-381 Chapter 14
Ordinary Cell Thunderstorms Mature Stage
Droplets grow large enough to fall. Drier air is drawn into cloud from sides (entrainment). This
causes evaporates some drops, which cools air. Cooling air leads to downdrafts, also enhanced by falling
precipitation. Anvil-Shaped cumulonimbus is formed, cloud top can be
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.
4 Concordia University Geog/Sci-381 Chapter 14
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.
8 Concordia University Geog/Sci-381 Chapter 14
9 Concordia University Geog/Sci-381 Chapter 14 Fig. 14-4, p. 375 10 Concordia University Geog/Sci-381 Chapter 14
Fig. 14-4, p. 375 11 Concordia University Geog/Sci-381 Chapter 14 12 Concordia University Geog/Sci-381 Chapter 14
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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
Damaging straight-line winds Meso-scale Convective Complex (MCC): a number of
individual multi-cell thunderstorms grow in size and organize into a large circular convective weather system 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
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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
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 Organizing, mature, shrinking, decay stage
Tornado outbreaks Families, super outbreak
43 Concordia University Geog/Sci-381 Chapter 14
Tornadoes Tornado Occurrence
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
fastest? Multi-vortex tornados
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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
49 Concordia University Geog/Sci-381 Chapter 14 Table 14-1, p. 399 50 Concordia University Geog/Sci-381 Chapter 14
Table 14-2, p. 400 51 Concordia University Geog/Sci-381 Chapter 14 Table 14-3, p. 400 52 Concordia University Geog/Sci-381 Chapter 14
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 Art Fig. 14-46, p. 402
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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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Fig. 14-49, p. 405 61 Concordia University Geog/Sci-381 Chapter 14 Fig. 14-50, p. 405 62 Concordia University Geog/Sci-381 Chapter 14
Waterspouts
Rotating column of air that is connected to a cumuliform cloud over a large body of water
Tornadic waterspout
63 Concordia University Geog/Sci-381 Chapter 14 Fig. 14-51, p. 406 64 Concordia University Geog/Sci-381 Chapter 14
Homework for Chapter 14 Chapter 14 Questions for Review, p. 407
#2-6, 19, 25, 29
Chapter 14 Questions for Thought, p. 408 #7
Chapter 14 Problems and Exercises, p. 409 #4
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Project for Chapter 14 None
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