Chapter 14

Wall Cloud associated with a super cell thunderstorm

Thunderstorms

A storm containing lightening and thunder; convective storms may have heavy rain hail

Ordinary Cell ThunderstormsAir-mass thunderstorms: limited wind sheerStages: cumulus, mature, dissipatingEntrainment, downdraft, gust front

A dissipating thunderstorm

Thunderstorms

Multi-cell ThunderstormsThunderstorms that contain a number of

convection cells, each in a different stage of development, moderate to strong wind shear; tilt, over shooting top

Multicell storm

Thunderstorms

Multi-cell ThunderstormsMicro-bursts: localized downdraft that hits

the ground and spreads horizontally in a radial burst of wind; wind shear, virga

Dust clouds from microbursts

Thunderstorms

Multi-cell ThunderstormsGust Front: leading edge of the cold air out-

flowing air; shelf cloud, roll cloud, outflow boundary

Thunderstorms

Severe thunderstorms: one of large hail, wind gusts greater than or equal to 50kts, or tornado, tilted updraft/downdraft

Shelf cloud

Thunderstorms

Multi-cell ThunderstormsSquall-line thunderstorms; line of multi-cell

thunderstorms, pre-frontal squall-line, derecho

Pre frontalSquall line

Pre frontalSquall line

The thunderstorms are producing strong straight-line winds called a derecho

Thunderstorms

Multi-cell ThunderstormsMeso-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

Thunderstorms

Supercell thunderstormsLarge, long-lasting thunderstorm with a

single rotating updraftStrong vertical wind shearOutflow never undercuts updraftClassic, high precipitation and low

precipitation supercellsRain free base

A supercell thunderstormwith a tornado sweeps over Texas

Thunderstorms

Supercell thunderstormsStrong vertical wind shearSurface, 850mb, 700mb, 500mb, 300mb

conditions low-level jet

Thunderstorms

Supercell thunderstormsCap and convective instability

Thunderstorms

Thunderstorms and the DrylineSharp, horizontal change in moistureThunderstorms form just east of drylinecP, mT, cT

Fig. 14-23, p. 384

Thunderstorms

Floods and Flash FloodsLarge floods can be created by training of

storm systems, Great Flood of 1993

Flash floods rise rapidly with little or no advance warning; many times caused by stalled or slow thunderstorm

Fig. 14-25, p. 387

downtown Des Moines, Iowa, during July, 1993

Thunderstorms

Topic: Big Thompson CanyonJuly 31, 1976, 12 inches of rain in 4 hours

created a flood associated with $35.5million in damage and 135 deaths

Fig. 1, p. 386

Flash FloodsSlow moving or Stalled thunder Storm, especially in canyon areas

135 deaths in 1976 flood12 inches of rain in 4 hours(normal ~16 inches /year)

Thunderstorms

Distribution of ThunderstormsMost frequent Florida, Gulf Coast, Central

PlainsFewest Pacific coast and Interior valleysMost frequent hail Central Plains

Thunderstorms

Lightening and ThunderLightening: 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

Thunderstorms

Observations: ElvesBlue jets, red sprite, ELVES

Fig. 2, p. 390

Thunderstorms

The Lightening StrokePositive charge typically on ground, cloud to

ground lighteningStepped leader, ground stroke, forked

lightening, ribbon lightening, bead lightening, corona discharge

Thunderstorms

Observation: Apple treeDO NOT seek shelter during a thunderstorm

under an isolated tree.

Lightening Detection and SuppressionLightening direction finder detects

radiowaves produced by lightening, sphericsNational Lightening Detection NetworkSuppression: seed clouds with aluminum

Fig. 3, p. 395

Fig. 14-32, p. 392

Fig. 14-33, p. 393

Fig. 14-34, p. 393

fulgurite

Fig. 14-35, p. 394

Fig. 14-36, p. 394

sferics