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Atmospheric
Optical
Phenomena
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AtmosphericOpticalPhenomenaareproducedbythereflection,refraction,dispersionandscattering/diffusionofraysofsunlight.
Theseprinciplesinphysicsexplaincommonlyaskedquestionsas
1. Whyistheskyblue?2. Whyaresomecloudswhiteandothersdark?3.
Howarerainbowsformed?4.
Whyarerainbowsatdifferentlocationsatdifferenttimesoftheday?
Thefollowingopticalphenomenaarepresentedwithbriefformationprocesses.
BlueSkiesandWhiteClouds 3
RedSunsandBlueMoons 4
Mirage 5
Rainbow 6
HaloandSunDogs 7
Corona 8
IrisationandCloudIridescence 9
LightPillars 10
CrepuscularRays 11
Lightning 12
AuroralLights 13
Glory 14
Sources 15
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BlueSkiesandWhiteClouds
Nitrogenintheatmosphereisaselectivescattererandscattersshorterwavelengthsofvisiblelight(violet,blueandgreen)moreefficientlythanlongerwavelengths(yellow,
orangeandred).
Oureyesaremoresensitivetobluelight,sotheskyappearsblue.
Hazyatmosphereoccurswhensmallparticles(dustandseasalt)aresuspendedintheatmosphere.Theseparticlesscatterlightandweseeawhitishtingeinsteadofclear
blue.
Themoreparticlesthataresuspendedintheatmosphere,thewhitertheskyappears.
Cloudsappearwhitebecausecountlessnumbersofclouddropletsscatterallthewavelengthsofvisiblelightinalldirections.Thelightpenetratesthecloudthroughto
thebaseofthecloud.
Darkcloudsareseenwhenthecloudgrowsthicker,largerandtallerandtheverylittlelightpenetratesthroughtothebaseofthecloud.Theraindropsatthecloudbasealso
aresolargethattheycannotscatterefficientlybutratherabsorbtheenergy.
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RedSunsandBlueMoons
Nearsunriseorsunset,theangleofthesunsraysstriketheatmosphereataloweranglethanduringtheday.Thusthelightpenetratedadeeperlayerofatmosphereand
mostoftheshorterwavelengthofvisiblelighthavebeenscatteredaway.Therefore,
whenthelightfinallyreachesoureyes,weseethelongerwavelengths(orangeyellow).
Brightyelloworangesunsetsoccurwhentheatmosphereisfairlyclean.Amorepollutedatmospheregivesaredsunrise/sunset.Eg.Volcanicsunsets
Bluemoonsoccurwhenrelativelylargeparticles(0.011.0m)particlesscatterlongerwavelengthsofvisiblelight(red,orange,yellow)andtheshorterwavelengthsreaches
oureyes.Thusweseeabluemoon.
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Mirages
Miragesareformedwhenlightisbentasitpassesthroughtheatmospherehavingvaryingdensities.
Whenthesunheatstheearthssurfacetheairclosertothegroundiswarmerandlessdensethanlayersofairhigherintheatmosphere.
Aslighttravelsfromalessdensemediumtoadensermediumatanangle,thelightisbenttowardthenormal.
Aslighttravelsfromadensemediumtoalessdensemediumatanangle,thelightisbentawayfromthenormal.
Weseemiragesasshimmeringorwateryspotsonhotsurfaces,invertedreflectionsofdistantobjects,andotherobjectsappeartallerthantheyreallyare.
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Rainbow
Arainbowisvisiblelightbrokenintowhatweseeassevencolors:red,orange,yellow,green,
blue,
indigo
and
violet.
They
always
appear
in
the
same
order,
with
red
on
top
andvioletonthebottom,inamainrainbow.
Inasecondaryrainbow,theyappearintheoppositeorder.
Rainbowsarelight.Youcan'ttouchthem.Youcan'treacharoundbehindthem.Theyexistonlyintheeyesandsometimesthephotographsofthepeoplewhoseethem.
Threethingsmusthappenforyoutoseearainbow'scolors.First,thesunmustbeshining.Second,thesunmustbebehindyou,andthird,theremustbewaterdropsin
the
air
in
front
of
you.
Sunlight
shines
into
the
water
drops,
which
act
as
tiny
prisms
that
bendor"refract"thelightandseparateitintocolors.
Eachdropreflectsonlyonecoloroflight,sotheremustbemanywaterdropstomakeafullrainbow.You'llseethebrightestrainbowswhenthewaterdropsarelarge,usually
rightafterarainshower.
Oftenyoucanseeasecondrainbowaboveabrighterone.Thisiscausedbyextrareflectioninsidetheraindrops.Becausethesecondaryrainbowisformedfromtwo
reflectionsinsteadofone,ithasawiderradiusthanthefirstrainbowanditscolorsare
reversed,with
red
on
the
bottom
and
indigo
on
the
top.
Becauserainbowsarelightandbecauselightraysstrikeeveryone'seyesalittledifferently,therainbowyouseewillbealittledifferentfromtheonesomeoneelse
sees,evenifheorsheisstandingrightbesideyou.Someoneelseashortdistanceaway
orlookingfromadifferentanglemayseeamuchdifferentrainbow
ornorainbowat
all.
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HaloandSunDogs
Ahaloisanopticalphenomenonproducedbyicecrystalscreatingasingleringofcoloraroundthesunormoon.Itoccursmorefrequentlythanarainbow.
Theyareproducedbytheicecrystalsincirruscloudshigh(510km,or36miles)intheuppertroposphere.
Theshapeandorientationofthecrystalsisresponsibleforthetypeofhaloobserved.Sixsidedicecrystalsthatmakeupthosecloudsactastheprismsthatseparatethesun's
lightintocolours.
Thelightisreflectedandrefractedbytheicecrystalsandsplitupintocoloursbydispersion.Thecrystalsbehavelikeprismsandmirrors,refractingandreflectingsunlight
betweentheir
faces,
sending
shafts
of
light
in
particular
directions.
Atmosphericphenomenasuchashaloswereusedaspartofweatherloreasanempiricalmeansofweatherforecastingbeforemeteorologywasdeveloped.
Toseeahalo,don'tlookdirectlyintothesun.Instead,blockthesunfromyourviewwithyourhand,soyoucanjustseethecloudsaroundit.Sunglassesalsomayhelpyou
seeahalo
butevenwithsunglasses,you'llneedtoblockthesunfromyoureyes.
Sundogsorparheliaareatmosphericphenomenathatcreatebrightspotsoflightinthesky,oftenonaluminousringorhalooneithersideofthesun.
Whenlightrefractsthroughflathorizontalicecrystals,youmightseebrightspotsoflightalongtherightandleftsidesofahalo.
Sundogsmayalsoappearasacolouredpatchoflighttotheleftorrightofthesun.Theycanbeseenanywhereintheworldduringanyseason,buttheyarenotalwaysobvious
orbright.Sundogsarebestseenandaremostconspicuouswhenthesunislow.
Halo
Sundog
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Corona
Theyaresimilartohalosbutareformedfromacloud'swaterdropsratherthanfromicecrystals.
Lightshinesthroughclouddroplets,curvesaroundthecirculardropsandgetsdiffracted.Thismeansitspreadsout,creatinganareaoflightlargerthanthesunor
moon.This"crown"ofmilkylightaroundthesunormoonisthecorona.
Whenalltheclouddropletsareaboutthesamesize,thediffractedlightcanmakethecoronaseparateintocolors.Thesecolorsmayrepeatthemselves.
Commonlyseeninrecentlyformedaltostratusandaltocumulus.
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Irisation/CloudIridescence
Cloudiridescenceistheoccurrenceofcoloursinacloudandissimilartoirisation.
Itisafairlyuncommonphenomenonandisusuallyobservedinaltocumulus,cirrocumulusandlenticularclouds.
Thecoloursareusuallypastelshades.
Iridescenceismostfrequentlyseenneartothesunwiththesun'sglaremaskingit.Itismosteasilyseenbyhidingthesunbehindatreeorbuilding.Otheraidsaredarkglasses
orobservingtheskybyitsreflectioninaconvexmirrororinapoolofwater.
Iridescentcloudsareadiffractionphenomenon.Smallwaterdropletsorevensmallicecrystalsincloudsindividuallyscatterlight.
Largeicecrystalsproducehalos,whicharerefractionphenomenaratherthaniridescence.Iridescenceshouldsimilarlybedistinguishedfromtherefractioninlarger
raindropsthatgivesarainbow.
Thecloudmustbeopticallythinsothatmostraysencounteronlyasingledroplet.Iridescenceisthereforemostlyseenatcloudedgesorinsemitransparentclouds.
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CrepuscularRays
Crepuscularraysareraysofsunlightthatappeartoradiatefromasinglepointinthesky.
Theserays,whichstreamthroughgapsincloudsorbetweenotherobjects,arecolumnsofsunlitairseparatedbydarkercloudshadowedregions.
Thenamecomesfromtheirfrequentoccurrencesduringcrepuscularhours(thosearounddawnanddusk),whenthecontrastsbetweenlightanddarkarethemost
obvious.
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Lightning
Lightningisthedischargeofelectricitythatoccursinamaturethunderstorm(cumulonimbuscloud).
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AuroralLights
Anauroraisanaturallightdisplayinthesky,particularlyinthepolarregions,causedbythecollisionofchargedparticlesdirectedbytheEarth'smagneticfield.
Anauroraisusuallyobservedatnightandtypicallyoccursintheionosphere.
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Glory
Agloryisanopticalphenomenon,appearingmuchlikeaniconicSaint'shaloabouttheheadoftheobserver,producedbylightbackscattered(acombinationofdiffraction,
reflectionandrefraction)towardsitssourcebyacloudofuniformlysizedwater
droplets.
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Sources
http://en.wikipedia.org
Duncan,TomandDenizna,PhysicsfortheCaribbeanACXCCourse
Ahrens,C.Donald,MeteorologyToday
Wallace,J.M.andPeterHobbs,AtmosphericScience
CompiledbyPaulaWellington
FortheTrinidadandTobagoMeteorologicalServices
2011
TrinidadandTobagoMeteorologicalServices
RawinsondeBuilding,PiarcoInternationalAirport,
Piarco
Website:www.metoffice.gov.tt
Email:[email protected]
Telephone:6695465
Fax:6694009
