本科气象双语标准课件第一章

MeteorologyMeteorology-asciencethatdealswiththeatmosphereanditsphenomenaandespeciallywithweatherandweatherforecasting.

Onaday-to-day

basis,wewanttoknowmanythingsabouttheweatherwewillencountertoday.

Weather

isthestateoftheearth’satmospherewithrespecttotemperature,humidity,precipitation,visibility,cloudiness,andotherfactors.Byweather

wemeanwhatishappeningintheatmospheretoday,tomorrow,orevennextweek.OceanographyTheapplicationofthesciencestothephenomenaoftheoceans.Themarinersconcernmuchaboutthewave,tide,seacurrentandice.Thegoalofthiscoursewillbetodiscussareasonablemixofweather:thekindyoucanutilizeandenjoy,aswellasthatfromwhichyouronlythoughtmaybetosurvive.Weatherisapresencetobeaddressed,understood,respectedand,aboveall,livedwith.

ChapterV,Regulation4oftheSOLASstatesthatcontractinggovernmentsundertaketowarnshipsofgalesandtropicalstorms;toissuedailyweatherbulletinsbyradio,containingdataofexistingweatherconditions,forecastsandinformationtoenablesimpleweatherchartstobepreparedatsea;toarrangeforthereceptionandtransmissionbycoastradiostationsofweathermessagestoandfromships;toarrangeforcertainshipstobeequippedwithtestedmeteorologicalinstruments,andtotakemeteorologicalobservationsatstandardsynoptichours,andtoencourageothershipstotakeobservationsinamodifiedform,particularlyinareaswhereshippingissparse.CourseoutlineCompletionofthistrainingwillsatisfythemeteorologytrainingrequirementsasspecifiedintheSTCWCodeforcertificationasOfficerInChargeofaNavigationalWatchonvesselsof500ormoregrosstonnage.

This54-hourcourseisdesignedtofamiliarizeandassistthetraineeinbecomingproficienttouseandinterpretrecordedmeteorologyinstrumentinformation;gainknowledgeofweathersystems;report/recorddata;applytheavailablemeteorologicalinformationtofacilitateinmaintainingasafenavigationalwatch.LearningobjectivesWeatherElements GeneraldescriptionoftheatmosphereandatmosphericpropertiesWeatherObservationsandinstruments Basicweatherobservations/Windandwaves/Clouds/Otherobservations/Ship'sWeatherObservationsandReportingMajorwindpatternsAirMassesandgeneralcirculationintheatmosphere(cyclonesandanticyclones)

TropicalCyclones Descriptionandcauses/ Anatomyoftropical/Avoidingtropicalcyclones/ConsequencesoftropicalcyclonesWeatheranalysisandforecastingDeterminingbarometricpressure/truewindspeed,direction/weathertobeencounteredusingsurface,andseastateanalysisweathermapsWeatherRoutingandServicesCHAPTER1BASICKNOWLEDGEOFTHEATMOSPHEREANDMETEOROLOGICALELEMENTS

CompositionoftheAtmosphereGasName

ChemicalFormulaPercentVolumeNitrogenN278.08OxygenO220.95WaterH2O0-4CarbonDioxideCO20.036OzoneO30.000004GreenhousegasesGasesinanatmospherethatabsorbandemitradiationwithinthethermalinfraredrange.Whenthesegasesarerankedbytheircontributiontothegreenhouseeffect,themostimportantare:watervapor,whichcontributes36–72%carbondioxide,whichcontributes9–26%methane（甲烷）,whichcontributes4–9%ozone,whichcontributes3–7%VerticalStructureoftheAtmosphere

TheEarth'satmospherecontainsseveraldifferentlayersthatcanbedefinedaccordingtoairtemperature.TheTroposphere11kilometersinaltitude

About75%ofthetotalmassoftheatmospherecontainedhere

Wherethemajorityofourweatheroccurs

Airtemperaturedropsuniformlywithaltitudeatarateofapproximately6.5degreesCelsiusper1000meters.(EnvironmentalLapseRate)Tropopause&StratosphereThetropopause,extendingfrom11to20kilometers,isanisothermallayerinthe

atmospherewheretemperature

remainsconstantover

adistanceofabout9kilometers.Abovethetropopause,isthestratosphere.20to48kilometersabovetheEarth'ssurface.Temperatureincreaseswithaltitudebecausealocalizedconcentrationofozonegasmoleculesabsorbsultravioletsunlightcreatingheatenergy.Thislayerofozoneisalsocalledtheozonelayer.Mesosphere&thermosphere

Themesospherecontainslittleinthewayoftracegasessothereislittletoabsorbheatfromthesun.thelowesttemperaturesreachedinthislayerareabout-90°C(-104°F).Mesopause

isabout80kilometers(50miles)aboveEarth'ssurface.Thermosphere-Temperaturescangohigherthan1000°C(1830°F).Theionosphereextendsfrom80to400kilometers(50to250miles)aboveEarth'ssurface.Exchangesofenergybetweenouterspace,theEarth'satmosphere,andtheEarth'ssurfaceRedistributionofenergyacrosstheEarth'ssurfaceSensibleheatflux:heatenergyistransferredfromtheEarth'ssurfacetotheatmospherebyconductionandconvection.Energyisthenmovedfromthetropicstothepolesbyadvection,creatingatmosphericcirculation.Warmtropicalairmovestothepolarregionsandcoldairfromthepolestotheequator.Latentheatfluxmovesenergygloballywhensolidandliquidwaterisconvertedintovapor.LargequantitiesofradiationenergyaretransferredintotheEarth'stropicaloceans.Theenergyentersthesewaterbodiesatthesurfacewhenabsorbedradiationisconvertedintoheatenergy.Thewarmedsurfacewateristhentransferreddownwardintothewatercolumn.Horizontaltransferofthisheatenergyfromtheequatortothepolesisaccomplishedbyoceancurrents.Q=H(Sensibleheat)+L(Latentheat)+S(Surfaceheatfluxintosoilorwater)

Temperatureisthemeasureofheatintheairinagivenplace.Severaldifferenttemperaturescalesareinuse.MeasurementScaleSteamPointofWaterIcePointofWaterAbsoluteZero

Fahrenheit21232-460Celsius1000-273

Kelvin3732730K=273.16+°C°C=5(°F-32)/9MeasurementofAirTemperatureTemperaturemeasurementsaretypicallytakenat1.5mabovegrassysurfaces.Thethermometerishousedinaninstrumentshelterthatisawayfrommaterialsthatmayabsorbheatandaffectanaccurateairtemperaturereading.WindChillTheapparenttemperaturefeltonexposedskinduetowind.Thedegreeofthisphenomenondependsonbothairtemperatureandwindspeed.

DiurnalandAnnualVariationsofAirTemperature

Diurnalvariationofairtemperaturemeansthesystematicchangeofthetemperatureoftheatmosphereduringanaverage24hourperiod.Usually,theminimumandmaximumairtemperaturesareattainedatabout1houraftersunriseand1500LMT,andinwinterdaysthemaximumairtemperatureoccursat1400LMT.Theannualvariationofairtemperatureislinkedtotheannualcycleofnetradiation.Nearequator,thereisnotemperatureseasons,netradiationispositiveandshowsonlytwominorpeaks(equinoxes).Inhigherlatitudes,thepeakoccursinJulyonlandandAugustatseaandinJanuaryandFebruary,thetemperaturesaretheaccordinglylowest.Theannualrangeincreaseswithlatitude,especiallyovernorthernhemispherecontinents.Continentallocationshavealargerrangeofdailyandannualtemperaturethancoastallocations,eveniftheyarelocatedatthesamelatitude.WeatherstationDALIANisatabout38.90°N121.63°E.Heightabout93mabovesealevel.AverageMaximumTemperatureJanFebMarAprMayJunJulAugSepOctNovDecYear

°C-14.027.523.914.3

AverageMinimumTemperatureJanFebMarAprMayJunJulAugSepOctNovDecYear

°C-8.3-6.5-1.15.611.616.520.721.416.710.11.8-4.96.9

918monthsbetween1905and1990GlobalSurfaceTemperatureDistributionTwofactorsimportantininfluencingthedistributionoftemperatureontheEarth'ssurface:Thelatitudeofthelocationdetermineshowmuchsolarradiationisreceived.Surfaceproperties-Thespecificheatofwaterissomefivetimesgreaterthanthatofrockandthelandsurface.Ingeneral,thesurfaceofanyextensivedeepbodyofwaterheatsmoreslowlyandcoolsmoreslowlythanthesurfaceofalargelandbody.Meanairtemp.fortheEarth'ssurfaceThespatialvariationsoftemperatureonthesefiguresismostlylatitudinalThehorizontalbandingofisothermsissomewhatupsetbythefactthatwaterheatsupmoreslowlyinthesummerandcoolsdownmoreslowlyinthewinterwhencomparedtolandsurfaces.AtmosphericpressureTheweight(force)oftheairpushingoneachunitofsurfaceareaontheground.

P=ρghs/s=ρghUnits:1hPa=1mb=3/4mmMeasuringAtmosphericPressureaneroidbarometer['ænə,rɔid

]takeintoaccount:

1.thebarometricreadingatthestation,

2.theelevationabovesealevelerror,and3.thetemperature.Variationofpressurewithheight

ThepressurefallsasyougohigherThepressuredropsabout1hPaforeach8metersaltitudenearMSLAirPressuresAverageAltitudes850hPa1500m700hPa3000m500hPa5500m300hPa9000mBarograph

Variationofpressureduringone24-hperiod

Fromamaximumat1000h,thepressurefallstoaminimumat1600h,risestoanothermaximumat2200h,andfallsagaintoasecondminimumat0400hlocaltime.Annualcoursereferstotheperiodicvariationofthemonthlymeanpressureduringayear.Thereisonemaximumandoneminimumpressureeveryyear.Thedifferenceofthetwovaluesiscalledannualrange.Forcontinentalareasthepressurerisestoamaximuminthewinterandfallstoaminimuminthesummer,foroceanicareasthepressurerisestoamaximuminthesummerandfallstoaminimuminthewinter.Theannualrangeonlandisgreaterthanthatatsea.AnnualVariationsofAirPressure

IsobarsandpressurefieldsCentersofsurfacehighand

lowpressureareasarefoundwithinclosedisobarsonasurfaceweatheranalysiswheretheretheabsolutemaximaandminimainthepressurefield

Aridgeisanelongatedregionofrelativelyhighatmosphericpressure,theoppositeofatrough

Theareaofalmostconstantpressure(andthereforefewisobars)betweentwohighsandtwolowsisknownasacol.（鞍形场）

Betweentwohighsisalowbeltandbetweentwolowsisahighbelt.

Standardsealevelpressureis1013.25hectopascals(hPa)(760mmHg)Typicalvaluesofairpressureforlocationsnearsealevelvaryfromabout960mbarforextremelystormyconditionstoabout1050mbarforstronghighpressureconditions.Isobarspacingandthemagnitudeofthepressuregradient

Isobar:alineconnectingpointsofequalatmosphericpressurePressuregradient:inwhichdirectionandatwhatratethepressurechangesthemostrapidlyaroundaparticularlocation.

Themagnitudeofthepressuregradientcanbeassessedbynotingthespacingoftheisobarsiftheisobarsareclosetogether,thepressuregradientislargeiftheisobarsarefarapart,thepressuregradientissmallStabilityoftheAtmosphere

Stableandunstableifyoupushthemarbleupthesideofthebowl,itwillfallbackdowntothebottom,toit'soriginalpositionStableair(parcel)-verticalmotionisinhibitedIfthemarbleisonthetopofthebowlandyougiveitalittlepush,itrollsoffthebowl....doesNOTcomebacktoit'soriginalpositionUnstableair(parcel)-verticalmotionoccursRulesaboutrisingandsinkingair

Pressuredecreasesvertically

Asairrises,itexpandsAsairexpands,itcoolsataratesetbytheexpansion1000hpa500hpaAdiabatic（绝热）LapseRatesRateofchangeoftemperaturewithheightassuming:

noexternalheatingofparcel

nomixingofparcelwithsurroundingair

DryAdiabaticLapseRate-γd:1oC/100mDryairandunsaturatedairMoistAdiabaticLapseRateγm0.5~0.6°C/100msaturated

airDryVersusMoist-AdiabaticProcessThemoistadiabaticlapserateislessthanthedryadiabaticlapseratebecauseasvaporcondensesintowater(orwaterfreezesintoice)forasaturatedparcel,latentheatisreleasedintotheparcel,mitigatingtheadiabaticcooling.Foehn/ChinookArapiddownslopemovementofwarmingairontheleewardsideofamountainrangeEnvironmentalLapseRatesγActualmeasuredrateofchangeoftemperaturewithheight

MeasuredbyballoonsEquilibriumofStabilityTodeterminetheenvironmentalstability,onemustcalculatethelapserateforasounding.thestabilitycriteria:γ<γmAbsolutelystableγm<γ<γdConditionalunstableγd<γAbsolutelyunstableAirHumidity

Waterintheatmosphereexistsin3mainstates:

Watervapor

clouddroplets(sometimesfrozenicecrystals)liquidraindropsAirnearthegroundoroceanusuallyhasmorewaterinitthanthecolderairuphigh.Roleofwatervaporintheclimatesystem

TotalwatervaporabovetheEarth'ssurface

Therewouldbeabout50mmneartheequatorandlessthanone-tenthasmuchnearthepoles.ThemosthumidregionisinthewesternequatorialPacific,abovetheso-called"oceanicwarmpool,"wherethehighestseasurfacetemperaturesarefound.AbsoluteHumiditya&VaporPressureea:themassofwatervaporpresentperunitvolumethedensityofthewatervapor,expresseding/m3

Thegaslawequation:PV=nRTP=DRT,where

P=pressureD=densityT=temperatureindegreesKelvin

R=gasconstantforair=287(J/kg*Kelvin)Soa=e/(TR)WhenPispressureinhectopascals,a=217e/TWhenPispressureinheightofmercury(inmillimeters),a=289e/T

Thenwhent=16°C,a=ee:pressureexertedbythemoleculesofagivenvapor

thepartoftotalatmosphericpressureduetothewatervaporcontentWatervaporconcentrationsdecreaserapidlywithheightNearlyhalfthetotalwaterintheairresidesbelowanaltitudeofabout1.5km.Lessthan5%isintheuppertroposphere(above5km),andlessthan1%isinthestratosphere.SaturationVaporPressureE

Foragiventemperature,ifaircontainsasmuchwateraspossible,itissaturated,andthevaporpressureisSaturationvaporpressure.

Thesaturationpoint(moisture-holdingcapacityoftheair)increasesrapidlyasthetemperaturerises.Thevalueat90°F(32°C)isaboutdoublethevalueat70°F(21°C).RelativeHumidity

fconsiderstheratiooftheactualvaporpressureoftheairtothesaturationvaporpressuref=e/Eusuallyexpressedinpercentagetellshowmuchwatertheairisholdingcomparedtohowmuchitcouldholdatacertaintemperaturecanchangeifthemoisturechangesorifthetemperaturechanges.CondensationCondensationistheformation

ofliquiddropsfromwatervapor.Itistheprocesswhichcreatesclouds,andsoisnecessaryforrainandsnowformationaswell.Condensationusuallyoccurswhenaparcelofrisingairexpandsandcools.Anotherwayinwhichcondensationoccursisduringtheformationofdew.DewPoint

tdThetemperatureatwhichtheairwillbeholdingallthemoistureitcanifcooled.Whenmoistureisdepositedonman-madeobjects,itisusuallycalledsweat.ofparticularconcerntothemarinerbecauseofitseffectuponhisinstruments,andpossibledamagetohisshiporitscargoHygrometersPsychrometer

Dry-bulbandwet-bulbevaporationcoolsthebulbofthethermometer,causingittoindicatealowerreadingthantheother.evaporationisslowerwhentheairisalreadyfullofwatervaporthedifferenceinthetemperaturesindicatedbythetwothermometersgivesameasureofatmospherichumiditySlingpsychrometer

Windisthehorizontal

motionoftheairpastagivenpointDirection.Thedirectionfrom

whichthewindisblowing.Speed.Therateatwhichthewindpassesagivenpoint.Knot,m/sTheBeaufortscale零级烟柱直冲天，一级轻烟随风偏。二级轻风吹脸面，三级叶动红旗展。四级枝摇飞纸片，五级带叶小树摇。六级举伞步行难，七级迎风走不便。八级风吹树枝断，九级屋顶飞瓦片。十级拔树又倒屋，十一十二陆上很少见。Force0.WindSpeedlessthan1knot.

Sea:Sealikeamirror.Force1:WindSpeed1-3knots.

Sea:Waveheight0.1m;Rippleswithappearanceofscales,nofoamcrests.Force2:WindSpeed4-6knots.

Sea:Waveheight0.2-0.3m;Smallwavelets,crestsofglassyappearance,notbreaking.Force3:WindSpeed7-10knots.

Sea:Waveheight0.6-1m;Largewavelets,crestsbegintobreak,scatteredwhitecaps.Force4:WindSpeed11-16knots.

Sea:Waveheight1-1.5m;Smallwavesbecominglonger,numerouswhitecaps.Force5:WindSpeed17-21knots.

Sea:Waveheight2-2.5m;Moderatewaves,takinglongerform,manywhitecaps,somespray.Force6:WindSpeed22-27knots.

Sea:Waveheight3-4m;Largerwavesforming,whitecapseverywhere,morespray.Force7:WindSpeed28-33knots.

Sea:Waveheight4-5.5m;seaheapsup,whitefoamfrombreakingwavesbeginstobeblowninstreaksalongdirectionofwind.Force8:WindSpeed34-40knots.

Sea:Waveheight5.5-7.5m;Moderatelyhighwavesofgreaterlength,edgesofcrestsbegintobreakintospindrift,foamisblowninwellmarkedstreaks.Force9:WindSpeed41-47knots.

Sea:Waveheight7-10m;Highwaves,seabeginstoroll,densestreaksoffoamalongwinddirection,spraymayreducevisibility.Force10:WindSpeed48-55knots.

Sea:Waveheight9-12.5m;Veryhighwaveswithover-hangingcrests,seatakeswhiteappearanceasfoamisblowninverydensestreaks,rollingisheavyandshocklike,visibilityisreduced.Force11:WindSpeed56-63knots.

Sea:Waveheight11.5-16m;Exceptionallyhighwaves,seacoveredwithwhitefoampatches,visibilitystillmorereduced.Force12:WindSpeedover65knots.

Sea:Theairisfilledwithfoamandspray;seacompletelywhitewithdrivingspray;visibilityveryseriouslyaffected.

Directionismeasuredbyawindvane,andspeedbyananemometer,averagingovera2-minuteperiod.

TrueandApparentWindAnobserveraboardavesselproceedingthroughstillairexperiencesanapparentwindwhichisfromdeadaheadandhasanapparentspeedequaltothespeedofthevessel.Sincewindvanesandanemometersmeasureapparentwind,theusualproblemaboardavesselequippedwithananemometeristoconvertapparentwindtotruewind.SustainedwindspeedsVS.Gust

Sustainedwindspeedsarereportedgloballyata10metersheightandareaveragedovera10minutetimeframe.TheUnitedStatesreportswindsovera1minuteaveragefortropicalcyclones,anda2minuteaveragewithinweatherobservations.Indiatypicallyreportswindsovera3minuteaverage.Knowingthewindsamplingaverageisimportant,asthevalueofaone-minutesustainedwindistypically14%greaterthanaten-minutesustainedwind.Ashortburstofhighspeedwindistermedawindgust,onetechnicaldefinitionofawindgustis:themaximathatexceedthelowestwindspeedmeasuredduringatenminutetimeintervalby10knots(19km/h).Asquallisadoublingofthewindspeedaboveacertainthreshold,whichlastsforaminuteormore.Newton’sLawsandAtmosphericForces

firstlaw:Whenaparticleissubjecttononetforce,theparticlehasconstantvelocity.

secondlaw:Whenaparticleissubjectedtoaresultantforce,theparticleisaccelerated.Thesumoftheforcesisequaltothemasstimestheacceleration.F=ma

Theforcesthataffectthemovementofairare:

(1)thepressuregradientforce

(2)theCoriolisforce(effect)

(3)thecentrifugalforce(4)thefriction

forceDrivingForce-thepressuregradientforce,GD=densityofairP2,P1=pressureatpointn=distancebetweenthetwopointsinmeterspressurechangeoveraunitdistancethegreateristheforcethefasterwillthewindsblowtheequationActsatrightanglestotheisobarsinthedirectionofthelowerpressureCoriolisEffectDeviationofmovingobject(airorwaterparcel)TorightinN.HemisphereToleftinS.HemisphereDuetorotationofearthTheCoriolisForce,AFCoriolis=2ωVsinφstrongestatthepolesanddecreasesprogressivelytowardtheequator,whereitiszeroalsoproportionaltothespeedoftheobjectinfluencesdirectionofmovementonly,ithasnoinfluencesonspeedGeostrophicWind-VgGeostrophicwindscomeaboutbecausepressuregradientforceandCoriolisforcecomeintobalanceaftertheairbeginstomovewherefrictioncanbeneglectedandisobarsarestraight.CentrifugalForce,CThesmalleristhecirculation(e.g.,tornado),themoreimportantistheforce.experiencedbyrotatingfluidasanoutward,apparentforceinfluencesthedirectionofwindonlyproducesacircularpatternofflowaroundcentersofhighandlowpressure.C=mv2/rGradientWind-Va,VcAwindthatblowsaroundcurvedisobarsabovetheleveloffriction,duetoacombinationofpressuregradientforceCoriolisforcecentrifugalforce

G=A+CforLG+C=AforHGradientWind-Va,Vc

Thegeneralruleisthatwindsarestrongestwheretheisobarsareclosesttogether.Theoretically,thereisnospeedlimitswithinlows.Themaxwindsoccurnearthecenterinalow.Thereisaspeedlimitinahighandthereisalwaysastronggradientnearahigh’sperimeter.

BuyBallot'sLawWhenstandwithyourbacktotheupperairwindsintheNorthernHemispherethecenteroflowpressurewillbetoyourleftandthehighpressuretoyourright.TheoppositeistruefortheSouthernHemisphere.Windsintheupperair

HLVaVgVcGGGCCAAAG+C=AhG=AgG-C=ALWhenairblowsaroundcurvedisobarswillthewindspeedbethesameasofstraightisobars?assumingthepressuregradientisthesameFriction

-FFrictioncanexertaninfluenceonwindonlyaftertheairisinmotion.Frictionaldragactsinadirectionoppositetothepathofmotioncausingthemovingairtodecelerate.theeffectoffrictiononwindisdominantnearthesurface,inthepartoftheatmospherewecalltheboundarylayer(extendsfromthesurfacetoabout1-1.5km)FrictionLayerWind

Surfacewindstendtocrosstheisobarsatananglevaryingfrom10(sea)to45

(roughland)degrees.Surfacewindspeedonlandusu.is1/3-1/2thatofthecorrespondinggeostrophicwind,whiletheratioatseais3/5-2/3.Windsblowingcounterclockwiseandinward

intoasurfacelow,andclockwiseandout

ofasurfacehighintheNH.ThewindsoftheSHblowclockwiseandinwardaroundsurfacelows,andcounterclockwiseandoutwardaroundsurfacehighs.Ekmanspiralwindvelocitieschangewithheight,blowingalongtheisobarsabovetheboundarylayerthendecreasinginspeedandveeringasheightdecreasesandfrictioneffectsincrease,untilthesurfacevectorliesacrosstheisobarsatlowspeed.

StreamlineAnalysisTheweakhorizontalpressuregradientsofthetropicsdonotadequatelyexplainmuchoftheweatheroccurringthere.Therefore,windisusedtoexplaintropicalweatherpatterns.Cloudsconsistofinnumerabletinydropletsofwater,oricecrystals,formedbycondensationofwatervaporaroundmicroscopicparticlesintheair.

Theshape,size,height,thickness,andnatureofaclouddependupontheconditionsunderwhichitisformed.Therefore,cloudsareindicatorsofvariousprocessesoccurringintheatmosphere.

Cirrus(Ci)-detachedhighcloudsofdelicateandfibrousappearance,withoutshading,generallywhiteincolor,andoftenofasilkyappearance.

generallyassociatedwithfairweather,butiffollowedbylowerandthickerclouds,theyareoftentheforerunnerofrainorsnow.Cirrocumulus(Cc)-highcloudscomposed

ofsmallwhiteflakesorscales,orofverysmallglobularmasses,usuallywithoutshadowsandarrangedingroupsoflines,ormoreofteninripplesresemblingsandontheseashore.

generallyassociatedwithfairweather,butmayprecedeastormiftheybecomethickenandlower

Cirrostratus(Cs)-thin,whitish,highcloudssometimescoveringtheskycompletelyandgivingitamilkyappearanceandatothertimespresentingaformationlikeatangledweb.Theicecrystalsofwhichthecloudiscomposedrefractthelightpassingthroughtoformhalos

withthesunormoonatthecenter.Ifitcontinuestothickenandlower,theicecrystalsmeltingtoformwaterdroplets,thecloudformationisknownasaltostratus.Whenthisoccurs,rainmaynormallybeexpectedwithin24hours.Altostratus(As)-middlecloudshavingtheappearanceofagrayishorbluish,fibrousveilorsheet.Thesunormoon,whenseenthroughtheseclouds,appearsasifitwereshiningthroughgroundglass,withacorona

aroundit.

Halosarenotformed.Altocumulus(Ac)-middlelevelcloudsconsistingofalayeroflarge,ball-likemassesthattendtomergetogether.mayappearasdistinctpatchessimilartoCc,butcanbedistinguishedbyhavingindividualpatcheswhicharegenerallylarger,showingdistinctshadowsinsomeplaces.IfActhickensandlowers,itmayproducethunderyweatherandshowers,butitdoesnotbringprolongedbadweather.Stratocumulus(Sc)-lowcloudsappearingassoft,gray,roll-shapedmasses.Theymaybeshapedinlong,parallelrollssimilartoaltocumulus,movingforwardwiththewind.Stratus(St)-alowcloudinauniformlayerresemblingfog.Nimbostratus(Ns)-alow,dark,shapelesscloudlayer,usuallynearlyuniform,butsometimeswithragged,wet-lookingbases.Thetypicalraincloud.Theprecipitationwhichfallsfromthiscloudissteadyorintermittent,butnotshowery.Cumulus(Cu)-densecloudswithverticaldevelopment

ahorizontalbaseanddome-shapeduppersurface,withprotuberancesextendingabovethedome.appearinsmallpatches,andnevercovertheentiresky.called“fairweather”cumulusbecausetheycommonlyaccompanygoodweather.However,theymaymergewithAc,ormaygrowtoCbbeforeathunderstorm.Cumulonimbus(Cb)-amassivecloudwithgreatverticaldevelopment.Theupperpartspreadsoutintheshapeofananvilwhichmaybeseenatsuchdistancesthatthebasemaybebelowthehorizon.Oftenproducesshowersofrain,snow,orhail,frequentlyaccompaniedbylightningandthunder.Thecloudisoftenpopularlycalleda“thundercloud”or“thunderhead.”Ci

卷高云Nibmo

雨Cu积St层Highclouds:posedprincipallyoficecrystals.Middleclouds:2.5posedlargelyofwaterdroplets,althoughthehigheroneshaveatendencytowardiceparticles.Lowclouds:posedentirelyofwaterdroplets.Withinthese3familiesare10principalcloudtypes.Thenamesare