BASIC'THUNDERSTORM'INGREDIENTS
• Moisture'–'most'notably'in'the'
lower'levels'of'the'atmosphere'
• Instability'–'ability'of'air'to'
accelerate'up'(or'down)'when'
given'a'push'
• Li0ing2Mechanism'–'The'“push”'
that'gets'the'whole'thing'
started'
'''
Lecture:
Severe Local Storms
!
Professor Kenny L. Tapp
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What'defines'a'TNStorm?
Cumulus'Stage
• Lightning'
• >'1'inch'of'rain'per'hour'(rainstorm)
What'defines'a'Severe'TNStorm?
• Hail'≥'¾”'Hail'
• Wind'Gusts'50'knots'+'
• And/or'Tornado
!
• Ini[a[on'of'TNStorm'(commonly'
sfc'hea[ng)'
!
• Developing'cumulus'cloud'
!
• No'precipita[on'(only2updra0)
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Mature'Stage
Dissipa;on'Stage
• Cumulonimbus'cloud,'stage'
marked'by'start'of'
precipita[on'at'the'ground'
• Ou_low'dominated'
• Only2downdra02
• Storm'looks'primarily'ver[cal'
• Storm'now'has2updra02and2
downdra02
• Downdra]s'are'recognized'as'
dark'fuzzy'areas'
• Storm'is'now'at'its'greatest'
intensity'
• Severe'weather'is'most'likely'
at'this'stage
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• Heavy'precipita[on'at'
the'beginning'but'trails'
off'
• Eroding'cloud'base
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Visual'Indica[ons'of'
Updra]'Strength'and'Organiza[on
Avg.'Thunderstorm'Life'='30'min.
• With'weak'wind'flow'alo]'
thunderstorm'updra]s'and'
downdra]'may'develop'
adjacent'to'each'other'with'
lifle'separa[on'
• The'cool'air'from'the'downdra]'
undercuts'the'updra]'and'cuts'
off'the'“fuel”'for'con[nued'
storm'development
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Updra]'strength'is'[ed'to'poten[al'severity.
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Overshoo[ng'Top
Thunderstorms'viewed'by'Satellite
– Dome'like'bubble'“overshoo[ng”'the'anvil'cloud'
• Indicates'strong'updra]'
• Updra]'has'punctured'into'lower'stratosphere'
– Most'significant'when'persistent'for'more'than'10'
minutes
Supercell'with'overshoo[ng'top'east'of'Salina,'Ks.
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Wind'Shear
Ver[cal'Wind'Shear
• Defini[on:'
– The'change'in'the'direc[on'or'speed'of'the'wind'over'
a'distance'
• Increases'longevity'and'organiza[on'
• Strong'shear'results'in'stormNscale'rota[on'by'[l[ng'
horizontal'vor[city'into'ver[cal'vor[city
• Ver[cal'Wind'Shear'
– The'change'in'the'direc[on'or'speed'of'wind'with'
height'
• Low'wind'shear'indicates'lifle'change'in'direc[on'or'
speed'of'the'wind'over'a'distance'
• Life;me2of2storm2is2wind2shear2dependent!
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Types'of'Thunderstorms
• The'environmental'condi[ons'dictate'what'
type'of'TNstorm'we'should'expect'and'what'
kind'of'damage'it'might'cause'(Wind'Shear)'
– Air2mass2thunderstorm2
– Mul;cell2Thunderstorms2
• Convec[ve'Complexes'
• Squall'Lines'
– Supercells
Air'Mass'Thunderstorms
• Basic'single'cell'TNstorm'
– Cell'defined'as'one'updra]'and'one'downdra]'
couplet'
•
'
'
'
'
Typical'Characteris[cs'
'Time:'~'30min'to'1'hour'
'Size:' '10’s'of'kilometers'in'width'
'Wind'Shear:'weak'to'zero'
'Severe:'Usually'not'severe
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A]ernoon'Storm'near'UCO
A]ernoon'Storm'near'UCO
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A]ernoon'Storm'near'UCO
Mesoscale'Convec[ve'System
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Mesoscale'Convec[ve'System:''Squall'Lines
• A'specific'organized'line'of'
convec[on'
Mesoscale'Convec[ve'System:''Squall'Lines
Time:'~several'hours'(6'hours?)'
Area:'rela[vely'narrow'in'width'but'can'stretch'
across'several'states'long'
Wind2Shear:'strong'speed'shear,'no'direc[onal'
shear'
Severe:'can'be'but'not'always'(most'notable'
weather'is'heavy'precip'and'fast'straight'line'
winds)
• Usually'occur'during'transi[on'
months'
• Usually'associated'with'cold'
front'and/or'dryline'
• Li]ing'mechanism'is'the'front'
or'dryline'
• A'squall'line'is'any'line'of'
convec[ve'cells.'It'may'be'a'
few'tens'of'km'long'or'1000'
km'long'(>500'nm);'there'is'no'
strict'size'defini[on
Courtesy'COMET
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Some'Basic'Facts
Lightning'is…'
• An'electrical'discharge'
• ~5km'tall'and'2'to'3'cm'in'diameter'
• Temperature'in'lightning'channel'~30000°C'
– Five'[mes'the'temperature'of'the'sun'
• Sonic'boom'created'by'heated'channel'
• Kills'~80'people'per'year
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Supercells
Where'do'Supercells'Form?
• What'makes'a'supercell'different'from'all'other'
TNStorms'on'earth?'
– They'Rotate!!!'
'
'
'
'
Typical'Characteris[cs'
Time:'~several'hours'(4N6'hours)'
Area:'40N60'km'
Wind'Shear:'Yes!'Both'speed'and'direc[on.'
Severe:'Almost'Always.''Hail,'High'Winds'and'
Tornadoes
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Three'Supercell'Classifica[ons
LP'Supercell
• Low2Precipita;on'(LP)'
– Lifle'to'no'precip;'generally'not'a'tornado'
producer'
• Classic2
– What'we'have'been'describing'thus'far'
• High2Precipita;on'(HP)'
– Lots'of'Precip;'rain'wrapped'tornadoes'occur
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Classic'Supercell
HP'Supercell
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Supercell:''March'28,'2007
Lightning
• Lightning'is'a'discharge'of'electrical'energy.'
• May'occur:'
– Between cells in the same storm (Inter-cloud
Lightning)
– Within a cloud (Intra-cloud Lightning)
– Cloud to Air
– Cloud to Ground (CG)
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Dangerous'Condi[ons
©219992Pren;ceJHall2JJ2From:2Aguado2and2Burt,2Understanding+Weather+and+Climate
Lightning'Impacts'on'Humans
Courtesy'www.postgradmed.com
Courtesy'New'England'Journal'of'Medicine
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Lightning
TNStorm'Charge'Separa[on
• Air'is'a'very'good'insulator.'
• To'have'lightning:'
– Need to have the charge centers very
close to each other
– Have very large differences in charge
“strength”
• In'order'to'get'lightning'in'a'thunderstorm'
you'need'to'separate'large'amounts'of'
charge.''How'is'this'done?
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The'(CG)'Lightning'Stroke
• Posi[ve'charge'TN
storm'top'
• Nega[ve'charge'TN
storm'bofom'
• Posi[ve'charge'sfc'
in'shadow'region'of'
storm'
• Eventually'form'
lightning'strike
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Stepped'Leader
• Ini[ally'streams'of'electrons'
surge'from'the'cloud'base'
toward'the'ground'in'steps'of'
50'to'100'm.'
• Start'and'stop'steps'as'the'
stepped2leader'progresses'
toward'ground.'
– Creates'Lightning'Channel'
– Essen[ally'clears'the'way'
for'the'Return'Stroke'
– Follows'the'path'of'least'
resistance
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Stepped'leader'moving'in'~50m'steps.
©219692Mar;n2Uman2JJ2From:2Uman,2Lightning
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The'(CG)'Lightning'Stroke
Streamers
• When'the'stepped'leader'gets'near'the'
ground'(~100'm'or'so):'
– Positive charge moves from the ground up
toward the stepped leader -- these are
called streamers.
– The streamers may come from almost any
pointed object on the ground:
•
•
•
•
Trees
Antennas, Tall Towers
Flagpoles, Telephone Poles
People
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The'Return'Stroke
A'streamer'rising'from'a'part'of'the'
tree.''The'return'stroke'was'created'
when'the'stepped'leader'met'with'a'
streamer'from'another'part'of'the'
tree.
©219982Oxford2University2Press2JJ2From:2MacGorman2and2Rust,2
The+Electrical+Nature+of+Storms
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Thunder
• Due to extreme heating of channel of air
– Lightning is very hot (~30,000 degrees C)
– Air expands because it is heated causing a sonic boom
• Stepped'leader'gets'close'
to'ground,'break'down'
occurs'and'we'get'the'
illuminated'lightning'
stroke'
• How far is that lightning????
– Light travels much faster than sound.
– Count the time between when you saw the flash and heard
the thunder.
• 5 seconds per mile
• 3 seconds per km
• This'is'what'we'see!!'
• Occurs'in'the'stepped'
leader'channel
• Close to lightning strike sounds like a boom
– Farther and farther away is sounds more like rumbling
– Due to higher frequencies being bent more easily; lower
frequencies travel longer distance
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