Chapter 10
Thunderstorms
A storm containing lightning 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
Fig. 10-1, p. 275
Fig. 10-2, p. 276
Fig. 10-2a, p. 276
Fig. 10-2b, p. 276
Fig. 10-2c, p. 276
Stepped Art
Fig. 10-2, p. 276
Fig. 10-3, p. 277
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 outflowing 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
Fig. 10-4, p. 278
Fig. 10-5, p. 278
Fig. 10-6, p. 279
Fig. 10-7, p. 279
Fig. 10-8, p. 280
Fig. 10-9, p. 280
Fig. 10-10, p. 281
Fig. 10-11, p. 281
Stepped Art
Fig. 10-11, p. 281
Thunderstorms

Mutli-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
Fig. 10-12, p. 282
Fig. 10-13, p. 282
Fig. 10-14, p. 282
Fig. 10-15, p. 283
Fig. 10-16, p. 283
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
Fig. 10-17, p. 284
Fig. 10-18, p. 284
Fig. 10-19, p. 285
Fig. 10-20, p. 285
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
Fig. 10-21, p. 286
Fig. 10-22, p. 287
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
Fig. 10-23, p. 289
Fig. 10-24, p. 289
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 lightning
 Electrification of Clouds: graupel and
hailstones fall through supercooled water,
ice crystals become negatively charged
 Upper cloud positive, bottom cloud negative
Fig. 10-25, p. 290
Fig. 10-26, p. 291
Fig. 10-27, p. 291
Thunderstorms

Types of lightning
 Blue jets, red sprite, ELVES

The Lightning Stroke
 Positive charge on ground, cloud to ground
lightning
 Stepped leader, ground stroke, forked
lightning, ribbon lightning, bead lightning,
corona discharge
Fig. 10-28, p. 292
Fig. 10-28a, p. 292
Fig. 10-28b, p. 292
Fig. 10-28c, p. 292
Fig. 10-29, p. 293
Fig. 10-30, p. 294
Fig. 10-31, p. 294
Fig. 10-32, p. 295
Thunderstorms

Lightning Detection and Suppression
 Lightning direction finder detects radiowaves
produced by lightning, spherics
 National Lightning Detection Network
 Suppression: seed clouds with aluminum

Observation: Apple tree
 DO NOT seek shelter during a thunderstorm
under an isolated tree.
Fig. 10-33, p. 295
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
Fig. 10-34, p. 297
Tornadoes

Tornado Occurrence
 US experiences most tornadoes
 Tornado Alley (warm, humid surface; cold dry air
aloft)
 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
 Multi-vortex tornados

Tornado outbreaks
 Families, super outbreak
Fig. 10-35, p. 298
Fig. 10-36, p. 298
Fig. 10-37, p. 299
Fig. 10-38, p. 299
Fig. 10-39, p. 301
Tornados

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
Table 10-1, p. 300
Table 10-2, p. 301
Table 10-3, p. 301
Tornadic Formation
Basic requirements are an intense
thunderstorm, conditional instability, and
strong vertical wind sheer
 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
Fig. 10-41, p. 303
Fig. 10-42, p. 303
Fig. 10-42a, p. 303
Fig. 10-42b, p. 303
Stepped Art
Fig. 10-42, p. 303
Fig. 10-43, p. 304
Fig. 10-44, p. 304
Tornadic Formation

Nonsupercell Tornadoes
 Gustnadoes
 Land spout
 Cold-air funnels
Fig. 10-45, p. 305
Fig. 10-46, p. 306
Fig. 10-47, p. 306
Fig. 10-47a, p. 306
Fig. 10-47b, p. 306
Observing Tornadoes and Severe
Weather
Doppler radar measures the speed of
precipitation toward and away radar unit
 Two Doppler radars can provide a 3D
view
 TVS, doppler lidar
 NEXRAD

Fig. 10-48, p. 307
Fig. 10-49, p. 308
Waterspouts
Rotating column of air that is connected
to a cummuliform cloud over a large
body of water
 Tornadic waterspout

Fig. 10-50, p. 308