Chapter 10: Chapter 10: Thunderstorms Thunderstorms and Tornadoesand Tornadoes

ThunderstormsThunderstorms TornadoesTornadoes Tornadic thunderstormsTornadic thunderstorms Severe weather and doppler radarSevere weather and doppler radar

ThunderstormsThunderstorms

• Convective storms that form with rising air in a conditionally unstable environment• The trigger needed to start air moving upward may be surface heating; topographic lift; convergence zone (e.g., sea breeze leading edge); frontal lift; divergence aloft

Ordinary Cell Ordinary Cell ThunderstormsThunderstorms

cumulus stagecumulus stage mature stagemature stage dissipating stagedissipating stage

• Ordinary cell thunderstorms are Ordinary cell thunderstorms are sometimes calledsometimes called‘air mass thunderstorms’, because they ‘air mass thunderstorms’, because they form in form in conditionally unstable air masses and conditionally unstable air masses and are not necessarily associated with are not necessarily associated with fronts or severe weatherfronts or severe weather

• Shortlived (<1 hr), less than 1 km Shortlived (<1 hr), less than 1 km wide, low wind shear, rarely produce wide, low wind shear, rarely produce strong wind or large hailstrong wind or large hail

• On 7/13/1999 in California, a strong On 7/13/1999 in California, a strong downdraft from a mature thunderstorm downdraft from a mature thunderstorm dropped the air temperature from 97F dropped the air temperature from 97F to a chilly 57F in one hour!to a chilly 57F in one hour!

Fig. 10-1, p. 265

Moisten environment latent heat warms the downdraft cuts off humid cloud layer; updraft; entrained dry air causes gust front is too far away evaporative cooling and to enhance updraft downdraft and gust front

Severe Thunderstorms and the Severe Thunderstorms and the SupercellSupercell

multicell stormsmulticell storms

moderate wind shearmoderate wind shear

Severe thunderstorm is defined as a thunderstorm with at least one of the following: large hail with a diameter > ¾ inch, surface wind gusts >50 knots (58 mi/hr), and/or produces a tornado

Fig. 10-5, p. 268

Supercell:Strong wind shear in speed and direction;Shallow inversion above warm and humidlayer acts as a lid;

Long-lasting (hours);Larger than 1 km in diameter;Single violently rotating updraft;Produces tornado, large hail, strong gusts

Squall Lines and Squall Lines and Mesoscale Convective Mesoscale Convective

ComplexesComplexes squall line: squall line: multicell storms as multicell storms as

a line of thunderstorms extending a line of thunderstorms extending for many kilometers (up to 1000 for many kilometers (up to 1000 km)km)

pre-frontal squall pre-frontal squall lineline

mesoscale convective mesoscale convective complex: complex: multicell storms as a multicell storms as a large circular cluster of storms;large circular cluster of storms;

tend to form in summer in regions tend to form in summer in regions where the upper-level winds are where the upper-level winds are weak;weak;

large size (100,000 square km)large size (100,000 square km)

Dryline ThunderstormsDryline Thunderstorms

drylinedryline• These storms occur frequently in the southernThese storms occur frequently in the southernGreat Plains of the US. Great Plains of the US.

Gust Fronts, Microbursts Gust Fronts, Microbursts and Derechoesand Derechoes

gust front: gust front: leading edge of cold air originating inside leading edge of cold air originating inside a thunderstorma thunderstorm

shelf cloud and roll cloudshelf cloud and roll cloud outflow boundary: outflow boundary: merging several gust frontsmerging several gust fronts downbursts downbursts (intense downdraft)(intense downdraft) and and

microbursts microbursts (< 4km; caused aircraft crash)(< 4km; caused aircraft crash)

Derecho Derecho (day-ray-sho): straight-line (not associated (day-ray-sho): straight-line (not associated

with rotation) strong wind extending for 100’s km with rotation) strong wind extending for 100’s km produced by strong downdrafts; produced by strong downdrafts;

producing large hail and sometimes tornadoproducing large hail and sometimes tornado

Fig. 10-11, p. 271

Fig. 10-13, p. 272

Microbursts present a Microbursts present a severe hazard to severe hazard to aircraft, especially aircraft, especially during takeoff and during takeoff and landing.landing.

Several airports have Several airports have installed microburst installed microburst detection instruments.detection instruments.

Fig. 10-16, p. 273

A squall line thunderstorms appear in the shape of a bow (or bow echo) on a radar screen and produce a straight-line wind (derecho)

Floods and Flash FloodsFloods and Flash Floods

flash floods: flash floods: floods that rise rapidly with little or no floods that rise rapidly with little or no

advance warningadvance warning

The Great Flood of 1993over the upper midwest

1976 Big Thompson flash flood(12 inch rain in 4 hours)

Distribution of Distribution of ThunderstormsThunderstorms

combination of combination of warmth and warmth and moisturemoisture

geographical geographical placementplacement

Great Plains are morefavorable for hails partlybecause evaporation of hail surface water cools the dry air

Lightning and ThunderLightning and Thunder

Lightning: Lightning: a discharge of electricity, a giant spark, a discharge of electricity, a giant spark, which usually occurs in mature thunderstorms (and may which usually occurs in mature thunderstorms (and may also occur in snowstorms and dust storms) also occur in snowstorms and dust storms)

majority of lightning strokes within clouds with only 20% majority of lightning strokes within clouds with only 20% between clouds and surfacebetween clouds and surface

a lightning stroke can heat the air through it travels to a lightning stroke can heat the air through it travels to 30,000C, 5 times hotter than the Sun’s surface30,000C, 5 times hotter than the Sun’s surface

Thunder: Thunder: The extreme heating causes the air to The extreme heating causes the air to expand explosively, thus initiating a shock wave that expand explosively, thus initiating a shock wave that becomes a booming sound wavebecomes a booming sound wave

Do you see the lightning or hear the thunder Do you see the lightning or hear the thunder first?first?

Electrification of CloudsElectrification of Clouds

electrical charges in electrical charges in cloudsclouds

relationships of updrafts relationships of updrafts

and downdrafts to and downdrafts to

electrical chargeselectrical charges

For normal fair weather, the atmosphere is usually characterized by a negatively charged surface and a positively charged upper atmosphere

There is a net transfer of positive ions(charged molecules) from the warmer(and larger) hailstone to the colder(and smaller) ice crystal or supercooleddroplets which are lifted to the upper layer of clouds

The Lightning StrokeThe Lightning Stroke

cloud-to-ground lightningcloud-to-ground lightning stepped leaderstepped leader return strokereturn stroke dart leaderdart leader -subsequent-subsequent

leaderleader

Types of LightningTypes of Lightning forked lightningforked lightning ribbon lightning: ribbon lightning: hanging from clouds hanging from clouds due to windsdue to winds dry lightning:dry lightning: cause forest firecause forest fire heat lightning:heat lightning: seen but not heardseen but not heard St. Elmo’s fire: St. Elmo’s fire: a corona discharge or sparks, a corona discharge or sparks,

can cause the top of a ship’s mast to glow; also can cause the top of a ship’s mast to glow; also seen over power lines and aircraft wingsseen over power lines and aircraft wings

Lightning Detection and Lightning Detection and SuppressionSuppression

lightning direction-lightning direction-finderfinderDetecting the radio

waves produced bylightning

Figure 2, p. 282

Where do you stay under thunderstorm?

• Not under trees;• Avoid elevated places;• Keep your head as low as possible but not touch ground

• Inside a building;• Inside a car

Tornado Life CyclesTornado Life Cycles tornado or twistertornado or twister: : typically 100-600 m, may >1 typically 100-600 m, may >1

mi,mi, usually at 20-40 knotsusually at 20-40 knots funnel cloudfunnel cloud:: not reach the groundnot reach the ground

dust-whirl stagedust-whirl stage mature stagemature stage decay stagedecay stage tornado familiestornado families: : different tornados spawned different tornados spawned by the same thunderstormby the same thunderstorm tornado outbreakstornado outbreaks: : 6 or more tornados over 6 or more tornados over a particular regiona particular region

Tornado OccurrenceTornado Occurrence tornado alleytornado alley time of day: time of day: most frequent at 4-6pm most frequent at 4-6pm

LTLT times of yeartimes of year

Annual number per state;Annual number per 100 mi by 100 mi

Tornado WindsTornado Winds

multi-vortex multi-vortex tornadoestornadoes

suction vorticessuction vortices

Seeking ShelterSeeking Shelter

tornado watch: tornado watch: likely to formlikely to form

tornado warning: tornado warning: spotted visually or by spotted visually or by radarradar• It’s always a good idea to know what to do if a It’s always a good idea to know what to do if a

tornado watch or warning is issued for your area. tornado watch or warning is issued for your area. • Take shelter in the basement or small room in the Take shelter in the basement or small room in the middle of the house at a lower level and cover your headmiddle of the house at a lower level and cover your head• Lie flat on the ground in a ditch Lie flat on the ground in a ditch • Don’t stay under a highway overpassDon’t stay under a highway overpass• Don’t stay near window or wall Don’t stay near window or wall

The Fujita ScaleThe Fujita Scale tornado classification based on damagetornado classification based on damage

• The “F-scale” was named after Prof. The “F-scale” was named after Prof. Ted Fujita.Ted Fujita.• Wind damage is proportional to the Wind damage is proportional to the square of wind speedsquare of wind speed

Tornadic ThunderstormsTornadic Thunderstorms

mesocyclonesmesocyclones bounded weak bounded weak

echo regionecho region hook echohook echo rotating cloudsrotating clouds wall cloudwall cloud

• A rotating wall cloud is an unforgettable sight - justA rotating wall cloud is an unforgettable sight - justask a successful storm chaser. ask a successful storm chaser.

Supercell Tornadoes

Fig. 10-35, p. 290

Nonsupercell TornadoesNonsupercell Tornadoes

Gustnadoes: Gustnadoes: along a along a gust frontgust front

Landspouts: Landspouts: weak and weak and short-lived, from short-lived, from congestuscongestus

cold air funnels: cold air funnels: cold cold air aloftair aloft

Severe Weather and Severe Weather and Doppler RadarDoppler Radar

Doppler shiftDoppler shift tornado vortex signaturetornado vortex signature rapidly changing wind directionrapidly changing wind direction

Doppler lidarDoppler lidar use light beam, higher resolutionuse light beam, higher resolution

NEXRADNEXRAD waterspoutwaterspout