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4/10/19
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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
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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.
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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.
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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. 375 10 Concordia University Geog/Sci-381 Chapter 14
Fig. 14-4, p. 375 11 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
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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
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Table 14-1, p. 399 54 Concordia University Geog/Sci-381 Chapter 14
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Table 14-2, p. 400 55 Concordia University Geog/Sci-381 Chapter 14
Table 14-3, p. 400 56 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 65 Concordia University Geog/Sci-381 Chapter 14
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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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Fig. 14-51, p. 406 68 Concordia University Geog/Sci-381 Chapter 14
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Homework for Chapter 14 � Chapter 14 Questions for Review
� #2-6, 19, 25, 29
� Chapter 14 Questions for Thought � #7
� Chapter 14 Problems and Exercises � #4
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Project for Chapter 14 � None
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