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ExamReview3
Midla/tudinalCyclonesandCyclone
Development
Commashapeindica/onstormis
s/llintensifying
LifeCycleofa
Developing
Midla/tudinal
Storm:
PolarFront
Theory(the“Norwegian”
model)
Stage1:
Sta/onary
Front
LifeCycleofa
Developing
Midla/tudinal
Storm:
Stage2:
FrontalWave
Kinkformsonthefront
causedbyupperlevelshort
wave,
Coldairbeginstomove
south-ward,warmairnorth
-ward,
Precipita/onstartstoforms
LifeCycleofa
Developing
Midla/tudinal
Storm:
Stage3:
OpenWave
Lowpressureis
deepening
(pressureatcenteris
becominglower)
LifeCycleofa
Developing
Midla/tudinal
Storm:
Stage4
Mature(ini/al
occlusion)
Energysource:Risingwarmairand
sinkingcoldairaswellascondensa/on
releaseenergyforthestorm,
systemismostintense
LifeCycleofa
Developing
Midla/tudinal
Storm:
Stage5
AdvancedOcculsion
Coldfrontovertakesthewarmfront,
occlusionforms,warmsectoris
removedfromcenterofthelow,
takingawayrisingwarmairasan
energysource
Startsweakening,dyingoff
Fig.10.1,p.271
TriplePoint:
mightleadto
forma/onof
newlow.
LifeCycleofa
Developing
Midla/tudinal
Storm:
Stage6:
Cutofflow
lifecyclelastsfroma
fewdaystooveraweek.
Iden/fythestageofeachmidla/tudecyclone
Namesandareasofforma/onforwinter
/mecyclones
Lee-sidelows:Mountainrangescausethe
westerlywindtocurvecyclonically
Typicaltracks:
eastwardornortheastward
Nor’easters:warmth
andmoistureofthe
gulfstreamtriggers
storm
Prevailingwesterliessteerthecyclones
Fig. 8.2, p. 210
Cyclogenesis:dependentonupper
atmosphere’sJetStream(~10kmup)
Ridge
Trough
Whystormstendtoonlyhappeninwintermonths?
Posi/onandstrengthofJetStream
Summer:jetisfarnorth,cyclonesmostly
impactCanada
Winter:jetstreamgetsstronger
andmovesfurthersouth,
enhancedimpactonU.S.
UpperLevelLongandShortWaves
Blackcontours:heightisolinesofthe500hPapressurelevel
Redcontours:isotherms(linesofconstanttemperature)
•  Upper-levellongwaves(typically4-5)moveslowlyfromwesttoeast
•  Upperlevel“shortwaves”moverela/velyquicklyfromwesttoeast.
•  Shortwavesareassociatedwith“coldairadvec/on”(bluearrows)
and“warmairadvec/on”(redarrows),thisamplifieswave.
ConvergenceandDivergence
Divergentmo/onmoredominant
thansurfaceconvergence:leads
tofallingsurfacepressure
Upward
mo/on
Convergenceatthesurface
Forastormtointensify,theupperleveltroughoflowpressuremust
belocatedtothewest(lec)ofthesurfacelow
SIDEVIEW
FlowSlows
FlowSpeeds
Backup
Pudngitalltogether
tothewestof
surfacelow
Fig. 10.11, p. 282
OpenWave:GrowingStage
Occlusion:Dissipa/ngStage
Aslongasmoreairleavesthe
ver/calcolumnthanentersfrom
below,thesurfacelowwill
deepen.Upper-levellowlies
westofsurfacelow.
Oncetheupper-levellowliesabove
thesurfacelowmoreairentersthe
ver/calcolumnbelowthanleaves
intheupperlevels,thelow“fills”
andpressureincreases. Fig. 10.9, p. 278
CloudsandStability
AbsolutelyStable
The atmosphere is
absolutely stable
(absolument stable)
when the air at the
surface is either
cooler than the air
aloft (an inversion), or
the temperature
difference between
the warmer surface
air and the air aloft is
not very great, (i.e.,
the environmental
lapse rate is less than
the moist adiabatic
rate).
AbsolutelyUnstable
The atmosphere is
absolutely unstable
(absolument instable)
when the surface air is
much warmer than the air
aloft, I.e., the
environmental lapse rate
is greater than the dry
adiabatic rate (i.e., the
environmental lapse rate
is greater than the dry
adiabatic rate).
Condi/onallyUnstable
The atmosphere is
conditionally unstable
(conditionnellement
instable) when
unsaturated air can be
lifted to a point where
condensation occurs and
the rising air becomes
warmer than the air
around it. This takes lace
when the environmental
lapse rate lies between
the moist adiabatic rate
and the dry adiabatic
rate.
KeyTerms
•  EnvironmentalLapseRate:Therateatwhichthe
temperatureintheenvironmentchanges.Isdeterminedby
measurement
•  DryAdiaba/cLapseRate:rateatwhichthetemperatureof
unsaturatedairchangeswithheight:10oC/km
•  MoistAdiaba/cLapseRate:rateatwhichthetemperature
ofsaturatedairchangeswithheight(dependenton
temperature):Canuseaveragerateof6oC/km
•  DewPointLapseRate:Rateatwhichdewpointchanges
withheightinunsaturatedair:2oC/km
–  InsaturatedairdewpointchangesatMoistAdiaba/cRate
CausesofInstability
•  Anythingthatsteepenstheenvironmental
lapserate(makestemperaturechangegreater
withheight)
–  Coolingairalo4
•  coldairadvec/on(windsbringincoldair)
•  radia/onalcooling(cloudsemidngradia/ontospace)
–  Warmingairatthesurface
•  day/mesolarhea/ng
•  warmairadvec/on
•  airmovingoverwarmsurface
CloudDevelopment
• 
• 
• 
Cloudsdevelopasanairparcelrisesandcoolstothedewpointtemperature.
Usuallyatriggerorprocessisneededtoini/atetheriseofanairparcel
Convec/on
–  Differen/allandsurfacehea/ngcreatesareasofhighsurfacetemperature
–  Airabovewarmlandsurfaceheats,forminga‘bubble’ofwarmairthatrises
(convec/on)
–  Cloudbaseformsatlevel of free convection
CloudDevelopment
PossibleTriggers
Rising Air (air ascendant)
Temperature
drops at moist
adiabatic
lapse rate
(~6°C/km)
Dew point
temperature
drops at moist
adiabatic lapse
rate (~6°C/km)
Saturated
Unsaturated
Temperature drops at
dry adiabatic lapse
rate (~10°C/km)
Dew point
temperature drops
at 2°C/km
Subsiding Air (l'air diminuant)
Temperature
rises at moist
adiabatic lapse
rate (~6°C/km)
Dew point
temperature rises
at moist
adiabatic lapse
rate (~6°C/km)
Saturated
Unsaturated
Temperature rises at dry
adiabatic lapse rate
(~10°C/km)
Dew point temperature
rises at 2°C/km
Red:DryAdiaba/cLapseRate Blue:MoistAdiaba/cLapseRate
Grey:DewPointLapseRate
Notehowthelapserates
comeintoplay
Lake-EffectSnowstorms:Mechanism
Idealcondi/ons:
•  Unfrozenlake
•  Airtemperatureatleast13°Ccolderthanwatertemperature
•  Distanceacrossthelakeatleast100+km
Thunderstorms
Thunderstorms develop in Absolutely Unstable or Conditionally Unstable Conditions
Airmass Thunderstorm:
Mature Stage
Airmass Thunderstorm:
Beginning Cumulus Stage
(Growth Stage)
Fig.11.2,p.299
Airmass Thunderstorm:
Dissipating Stage
•  After thunderstorm enters mature
stage it begins to dissipate after
15-30 minutes
•  Downdracsdominate,cudngoff
thestorm’sfuelsupply(rising
warmhumidair)
•  Allthreestagesmightbeoverin
lessthan1hour
•  Lower-levelcloudsquickly
evaporate
Third(Dissipa/ng)Stage
ThunderstormCategories:
MulLcellThunderstorms
ShouldbeabletoIden/fythepartsofathunderstorm
MulLcellThunderstorms
–  Contain a number of convection cells, each in a
different stage of development
–  Moderate to strong vertical wind shear is present
–  Tilt: updrafts rides up and over the downdrafts
–  Overshooting top possible (intrusion into stratosphere)
–  The longer the storm persists, the more severe
–  Cold downdrafts under the storm reach the surface,
leading to sharp temperature drops
–  Leads to gust front and downbursts & microbursts
CharacterisLcsofaMulLcellThunderstorm
Updrafts and downdrafts are tilted, supplying additional energy.
MulLcellThunderstorms
Gust Front
–  leading edge of the cold outflowing air (like a mini cold front)
–  around 1000 feet deep with average speeds of 10-30 mph,
wind gusts (called straight line winds) around 60 mph
–  May form shelf cloud (if atmosphere is very stable at the
ground) and roll cloud (spinning around horizontal axis)
–  Many gust fronts can merge into an ‘outflow boundary'
Microbursts
–  localized downburst (less than 2.5 miles wide) that hits the
ground and spreads horizontally in a radial burst of wind
–  Winds as high as 150 mph, severe damage possible
ExampleofaRollCloud
formingbehindagustfront
Outflow Boundary from Multicell Storm System
Radarimageofa
oujlowboundary,
ocentriggeringnew
thunderstorms
ThunderstormCategories:
Squall-LineThunderstorms
– Line of multicell Fig.11.13,p.306
thunderstorms
with strong
gusty winds
directly along
or ahead of a
cold front
Radarimageofaprefrontalsquallline
fromIndianato
Arkansas(red:most
intense)
Causes severe weather over much
of the length of the
squall line
Mostcommon:Pre-frontalsquallline
thunderstormsaheadofcoldfront
Triggered by upper-air gravity waves that originate at the cold front
Squall-LineThunderstorms
Squall-line thunderstorm with a trailing stratiform cloud layer and
tilted updrafts:
•  tilt promotes development of new cells as storm system moves
•  rear inflow jet may exceed 100 mph
Fig.11.15,p.307
ThunderstormCategories:
MesoscaleConvecLveComplexes(MCCs)
of individual multicell
–  During the summer a number
thunderstorms may occasionally grow in size and organize
into a large circular convective weather system
–  Can be 1000 times larger than an air mass thunderstorm
–  Can cover entire states, an area bigger than 100,000 km
–  Move slowly (< 20 mph) and last for over 12 hours
–  Provide significant portions of the growing season rainfall in
2
the Central U.S., reach their max intensity at night
–  Fueled by moist low-level jet from the south (brings
moisture)
–  Severe weather threat: hail, high winds, flash floods &
tornadoes
Fig.11.20,p.310
Satellite image showing
the cold cloud tops (dark
red) of an MCC in Kansas
Radar image of an MCC moving SSEward into Kentucky. Red: strong
thunderstorm activity
Fig.11.21,p.311
WhatMakesaThunderstormSevere?
A thunderstorm is considered “severe” if it contains
one of the following:
•  Hailwithadiameterofthree-quarterinchorgreater
•  Winddamageorgustsof50knots(58mph)orgreater
•  Atornado
SupercellThunderstorm:
A large, long-lasting thunderstorm with a single rotating updraft
Key Difference is Supercells have Rotation
Fig.11.23,p.313
Lightening
•  DevelopmentofLightening
•  HowtoProtectyourselffromLightening
•  TypesofLightening
Developmentofa
lightningstroke
Stage1:
Whenthenega/vechargenearthe
bomomofthecloudbecomeslarge
enoughtoovercometheair’s
resistance,aflowofelectrons—the
steppedleader—rushestoward
theearth
Developmentofa
lightningstroke
Stage2:
Astheelectronsapproachthe
ground,aregionofposi/vecharge
movesupintotheairthroughany
conduc/ngobject,suchastrees,
buildings,andevenhumans(a
streamer).
Developmentofa
lightningstroke
Stage3:
Whenthedownwardflowof
electrons(steppedleader)meetsthe
upwardsurgeofposi/vecharge
(streamer),astrongelectriccurrent
—abrightreturnstroke—carries
posi/vechargeupwardintothe
cloud(travelsat60,000mph).
Thefirstlightningstrikeisocen
repeatedbysubsequentdartleaders
inthesameionizedchannel(upto
30/mes,flickers).
Various Forms of Lightning
Fig.11.37,p.324
Groundtocloudlighteningcausedbyman-made
structures
Hail
•  Strongupdracskeepicecrystalsalocandtheycollide
withsupercooledwaterdropletsincreasingtheirsize
•  Eventuallygetsoheavy,theyfallout
OtherFormsofPrecipita/on
snow
Rain
(≥0.5mmdiameter)
icepellets(sleet)
drizzle
(<0.5mmdiameter)
snowpellets(graupel)
Warmprocess:Dropletsarerisingintheupdracs
ofacloudandgrowviacollisions
Assoonasthedroplets
gettooheavytobe
keptintheairinthe
updracs,theyfalldown
asrain,partly
evapora/ngwhenfalling
throughunsaturated
air.
Thebiggestraindrops
reachasizeof5mm.
Largersizesarenotlong-
livedandbreakapart
whenfalling.
Fig.6.3,p.147
Distribu/onofIceandWaterinClouds
Fig. 6.4, p. 148
Despite the subfreezing temperatures many
supercooled liquid droplets are present here
due to the lack of ice nuclei that initiate the
growth of ice.
Ice will form on particles that possess crystalline characteristics similar to
ice (small aerosol or soil particles or pre-existing ice particles).
Ver/caltemperatureprofilesoftheenvironment
thatleadto(a)snowand(b)rain
Remember….Temperaturesinthesnowforminglayermustbeless
than-10°C(thatis,10°F)inorderforsignificantsnowgrowthtooccur.
Temperature
Temperature
greater
greater
than34°F
than34°F
Fig. 6.20, p. 159
Ver/caltemperatureprofilesoftheenvironment
thatleadto(c)freezingrainand(d)rain
TornadoOverview
Condi/onsNeed:
1. Convec/veInstability
2. WindShearandStrongJet
Stream(forrota/on)
3. Trigger
TornadoOverview
•  What/meofyeardotornadosoccurandwhy
Timewhenwindshearisthe
greatest,andcontrast
betweenwarmandcoldair
massesarelarge
Polarjetdropssouthand
provideswindshearoverthe
Gulf
Hurricanes
4.Trigger
Totalwindspeedsofa
movingtornado:Recall
similarexamplewith
hurricanes