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 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
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 , p. 281
Fig , p. 281
Stepped Art Fig , 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, 10,000km 2
Fig , p. 282
Fig , p. 282
Fig , p. 282
Fig , p. 283
Fig , 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 , p. 284
Fig , p. 284
Fig , p. 285
Fig , 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 , p. 286
Fig , 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 , p. 289
Fig , 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 , p. 290
Fig , p. 291
Fig , 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 , p. 292
Fig a, p. 292
Fig b, p. 292
Fig c, p. 292
Fig , p. 293
Fig , p. 294
Fig , p. 294
Fig , 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 , 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 , 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
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Fig , p. 298
Fig , p. 299
Fig , p. 299
Fig , 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 , p. 303
Fig , p. 303
Fig a, p. 303
Fig b, p. 303
Stepped Art Fig , p. 303
Fig , p. 304
Fig , p. 304
Tornadic Formation Nonsupercell Tornadoes Gustnadoes Land spout Cold-air funnels
Fig , p. 305
Fig , p. 306
Fig , p. 306
Fig a, p. 306
Fig b, 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 , p. 307
Fig , p. 308
Waterspouts Rotating column of air that is connected to a cummuliform cloud over a large body of water Tornadic waterspout
Fig , p. 308