midbaht

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com6435FACTORSCONTRIBUTINGTOURBANHEATISLANDINBANGKOK,THAILANDSigitD.
ArifwidodoDepartmentofLandscapeArchitecture,FacultyofArchitecture,KasetsartUniversity,Bangkok,ThailandE-Mail:sigit.
d@ku.
ac.
thABSTRACTThestudyfocusesonthecharacteristicsofurbanheatisland(UHI)inBangkok,Thailand.
Hourlyairtemperaturedatafromfourweatherstations-oneinruralsiteandthreeinurbansitesforthelastfiveyearareusedtostudythecharacteristicsandintensitiesofUHIinBangkokarea.
TheresultsindicatesthepresenceofurbanheatislandinBangkokanditisincreasingintermsofintensity.
Thestudyrevealsthemaximumintensityofaround6-7°Cisdetectedduringdryseason.
ThemeanannualairtemperatureinBangkokcityishigherby0.
8°Cthanoutsidethecity.
Theweatherconditions(wind,cloud,andprecipitation),anddifferentlandcovertypesarethemajorfactorsgoverningthenearsurfaceurbanheatisland.
Keywords:urbanheatisland,urbanization,urbanclimate,energyconsumption.
INTRODUCTIONUrbanheatisland(UHI)isdefinedasaphenomenonwheretemperaturesofurbanareasarehigherthansurroundingorruralareas(Oke,1982).
AmeasuretoquantifyurbanheatislandusuallyusesthetermUrbanHeatIslandIntensity(UHII)(Kolokotroni,2005),whichisthemaximumtemperaturedifferencebetweenurbanandruralair.
Generally,thelargesturbanheatislandeffect,ormaximumurban-ruralareatemperaturedifferenceoccursmostatnight,threetofivehoursaftersunset,becausetheroadsandothersurfacesabsorbingsolarradiationindaytimereleaseheatinnighttime.
Thus,theruralareascoolofffasterthanurbanareasatnight.
UHIscanprovidebothnegativeandpositiveimpactsforcities.
Ascitiesgrow,theurbanizationcauseslesstreeandvegetationdisplacedbybuildingsandroads,moreskyscrapersandstreetstrapthewindpath,andmoreheatisreleasedfromvehiclesandair-conditioners.
Besides,UHIincreaseshumandiscomfortandairpollutionconcentration.
Moreover,highertemperaturesinurbanheatislandincreaseenergyuseespeciallyforair-conditioninginbuildings.
Thisincreasesmoreairpollutionandenergycostduetotheuseofmorefuel.
TheUHIconditionsincreasetheriskofclimaticandbiophysicalhazardsintheurbanenvironmentsincludingheatstressandheightenacuteandchronicexposuretoairpollutants.
Climatechange,whichiscausedbyincreasedanthropogenicemissionofcarbondioxideandothergreenhousegases,isalongtermeffectwiththepotentialtoaltertheintensity,temporalpattern,andspatialextentfortheUHIinmetropolitanregions(Cynthiaetal,2005).
Onthecontrary,urbanheatislandmaybebeneficialforreducingheatloadsasaresultofreducedenergyuseforheatingconsumptionreduces.
However,thisbenefitdoesnotcountfordevelopingcountries(Arifwidodo,2012).
UHIsalsohavefurtherimpactsonglobalscale;itinfluencesthelong-termtemperaturerecordleadingtodifficultiestodetectglobalclimatechanges.
Thesurfaceheatislandreferstotherelativewarmthofsurfaces.
Thesurfacetemperatureiseasytochange,andthusitshowsmuchgreaterdifferenceinspatialvariabilityandtemporalvariationbetweendayandnightthanairtemperaturedoes.
Themainfactorsthatcausetheappearanceofurbanheatislandincludeweather,geographiclocation,timeofdayandseason,cityform,andcityfunctions(Voogt,2004).
Weather:Calmandclearweathercanleadtothelargestmagnitudeofheatisland.
Increasingwindsdecreaseheatisland,andalsoincreasingcloudsatnight.
Geographiclocation:Thetopographyoftheareasinfluencestheweathersuchaswind.
Forinstance,coastalurbancitiescomeacrosswithcoolingtemperaturesinthesummerduetothecoolerseasurfacetemperatures.
Inwarmhumidclimates,thewetsurfacescanreducetheheatislandmagnitudes.
Timeofdayandseason:Incitieswhicharelocatedinthemidlatitudeshavethestrongestwinterorsummerseasonscanleadtolargemagnitudeofheatislandintropicalcities.
Cityform:Thematerialsinconstruction,thebuildingdimensionsandspacing,thegreenareasarealltheexampleofcityform.
Somebuildingmaterialscanstorealargeamountofheat.
Thereplacementofimperviousorwaterproofedsurfaceleadtothehigherheatislandformation.
Cityfunctions:Theurbanpollutantscomefromenergyuse,andanthropogenicheatcangenerateheatisland.
Forexample,indenselybuildingcitiesthehigh-energyusehasalargeinfluencetoanthropogenicheating.
BANGKOKANDURBANHEATISLANDDevelopmentofBangkokandUrbanHeatIslandBangkokisthecapitalcityofThailandlocatedinthecentralpartofthecountry.
Itisthecenterofindustries,manufacture,economy,commerce,andconstruction.
Thisdrawsalargeamountofpeoplefromalloverthecountryintothecity,leadingtothehighgrowthofurbanizationandindustrialization.
Thepopulationisabout10millionindaytimewhichis16%ofthetotalpopulationofThailand(theBureauofRegistrationDepartmentofProvincialMinistryofInterior,2004).
ThisrapidurbanizationhasledVOL.
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com6436toseveralenvironmentalproblemssuchasairpollution,waterpollution,landsubsidenceaswellastheproblemsfromthepresenceofurbanheatislandsuchastemperaturerise,highenergyconsumption,andbiophysicalhazardsetc.
In2012,themaximumtemperaturedifferencebetweenurbanandruralareaofBangkokwas7°C,whichhigherthaninthelast10years.
Boonjawatetal(1998)initiallyshowedthepresenceofheatislandinBangkok.
Theurbanheatislandintensity(UHII)betweenChulalongkornUniversity(urbanarea)andAsianInstituteTechnology(ruralarea)wasobservedtobe3.
5°Cduring6.
00-7.
00a.
m.
ThisstudyalsoshowsthesubstantialeffectofseabreezeandsolarradiationonUHI.
SeabreezedecreasestheairtemperatureinthesouthernpartofBangkok.
AnotherstudyshowedtheeffectsoflandcoveronUHIinBangkok.
Komonveeraket(1998)conductedastudybyusingtransformedvegetationindex(TVI).
ThestudyshowedinverserelationshipamongTVI,surfacetemperatures,andlandapplications.
GreenareahadhighTVIandlowsurfacetemperature,onthecontrarythelowTVIandhighsurfacetemperaturecorrespondedtobuildingarea.
ThisstudyalsoshowsthesubstantialeffectofseabreezeandsolarradiationonUHI.
SeabreezedecreasestheairtemperatureinthesouthernpartofBangkok.
However,therearenofurtherdetailempiricalevidencesonthecurrentstatusofUHIseffecttourbanareainBangkok,especiallynowthatthecityisintheprocessofimplementingnewmasterplantoguideitsfuturedevelopment.
Thus,tohaveaspatialdistributiondataandfurtherupdateofmagnitudeofBangkokheatisland,morestudiesshouldbecarriedouttounderstandcurrentUHIscharacteristicsandeffectstourbanareaofBangkok.
Correspondingly,duetotheappearanceofurbanheatislandmanyproblemsarise,hencetheimpactsofurbanheatislandshouldbealsotakenintoconsideration.
IncaseofBangkok,theairconditioningloadisconsideredtohavethelargestshare(almost60%)ofelectricityuse(Arifwidodo,2014).
Therefore,itisimportanttoassessthecurrentstatusofUHIcharacteristicsanditseffectstourbanareafromthemicroclimateperspective.
MeasuringUrbanHeatIslandToidentifyurbanheatisland,thecommonlyusedmethodsaretheurban-ruralweatherstations,auto-traversemethods,computermodelling,andremote-sensingtechniques(Henryetal,1989).
Theurban-ruralstationisthesimplestandmostfrequentlyusedmethodwhichpresentstheairtemperaturebelowtheurbancanopylayer.
Remote-sensingtechniquesofferhighspatialresolutionandeasyrepeatability(Henryetal,1989),butthederivedtemperaturemaybedifferentfromthetruesurfacetemperature.
TheairtemperaturedataarecollectedfromTheMeteorologicalDepartmentinBangkok(3stations)andPathumthaniProvince(1station)tocomparetheurban-ruralcondition.
Table-1showsthetypeofstationanddatacollectedforthestudy.
Table-1.
Weatherstationinthestudy.
Theairtemperaturefrom2008-2012fromallfourstationsarecollectedtounderstandthetemperaturevariationtrendinBangkokin5yearsmovingaverage.
ThesedataarealsousedtounderstandthedailyandannualcourseofUHI.
Table-2summarizedthedataandanalysisdoneinthestudy.
Table2.
TypeofAnalysisandDataSourceUsedintheStudy.
UrbanizationandUHIBangkokisthecapitalofThailandsituatedinthecentralpartofthecountryonthelow-flatplainoftheChaoPhrayaRiverwhichextendstotheGulfofThailand.
Itslatitudeis13°45'northandlongitude100°28'east.
Theelevationisabout2.
31m.
MSL.
Thecityisdividedinto50districtsand154sub-districts.
TotalareaofBangkokisaround1568.
737squarekilometers.
Generally,theclimateofBangkokistropical.
TheweatheriswarmVOL.
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com6437andhumid,anditisaffectedbymonsoonseason.
Therelativehumidityishighthroughouttheyeararound60to80percent.
Therearethreemainseasons:Rainy(May-October),winter(November-January)andsummer(February-April).
Theaveragewindvelocityis1.
2m/sec(4.
3km/hr).
Theaveragerelativehumidityis73%andtheyearlyaverageprecipitationis1,652mm.
Theannualaverageambienttemperatureisaround33-38°C.
Theabsoluteminimumtemperatureisabout20°andtheabsolutemaximumtemperatureisabout30°C.
Therainyseasontemperatureisaround25-32°C.
Thedryseasontemperatureisaround20-25°Candhotseasontemperatureisaround40-42°C.
TheregisteredpopulationinBangkokincreasedfrom1.
6millionin1958to5.
4millionin1986and5.
6millionin1999.
ThepopulationofBangkokiscloseto5.
78millionaccordingtohouseholdregistrationin2004.
Thisis10%ofthetotalpopulationofThailand.
Thepopulationdensityis3,686persquarekmwiththeincreaseof0.
98%peryear(ArifwidodoandChandrasiri,2013).
Thelong-termannualairtemperaturerecordinBangkokfrom1980-2010showsthatthetemperaturehadbeencoolerincoolseasonandwarmerinhotseason.
Forexample,BangkokMetropolisweathershowsthatthemeanmaximumandminimumannualairtemperaturefrom1980-2012was33°Cand24°Crespectively,andincreasinglinearlyby0.
95°Cand1.
97°C.
OnesignificantfactoraffectingthisincreaseisprobablytherapidurbanizationinBangkok.
Usingthedatafromthesameweatherstation,thedailytemperaturefrom2008-2012iscomparedtoPathumthaniweatherstationtounderstandthedailytemperaturevariationsbetweenurbanandnon-urbanarea.
Theresultshowsthatthetemperaturedifferencesbetweeneachyeararemoreobviousduringwinter.
Thetemperatureseemstobehighereachyearinthesummer,anddecreaseduringwinter.
Table-3showstemperaturedifferencebetweenurbanandruralBangkok.
Theresultindicatesthatthedailyvariationsofurbanheatislandinalldifferentthreeseasonshavesimilartrends.
TheUHIeffectishighaftersunsetaround6-7PMandbeginstoriseduringthenighttimeandreachesitsmaximumvalueat2-4°Cdependingontheseasons.
Table-3.
TemperaturedifferencebetweenurbanandruralBangkok.
PrecipitationandUHIFigure-1showstheeffectofrainfalltoUHIintensityinBangkok.
ItisobservedthattheUHIintensityvarieswithprecipitation.
TheincreaseoftheprecipitationwhichhasthelargestvalueinAugustat230mmcausesagradualdeclineoftheUHIintensitytothelowestintensityof2°C.
Afterthat,astheraindecreasestominimuminDecember(0mm),themagnitudesofheatislandreachtheirmaximum(5°C)inDecember.
Thus,theprecipitationcanbeconsideredasoneofthemostsignificantfactorsgoverningUHIdevelopment,especially,inrainyseason(fromMaytoOctober).
Ontheotherhand,theUHImaycreatetheprecipitationinurbanarea.
Therisingwarmairinurbanareahelpstocreatethecloudswhichresultsinmorerainfallinurbanareaandareaindownwindcities.
Figure-1.
MonthlymaximumUHIintensitybetweenBangkokMetropolis(urban)andPathumthani(non-urban)stationandmonthlyrainfall(mm)in2012.
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com6438PopulationDensityandUHIInsummerandinwinter,3weatherstationsaroundthecenterismuchwarmerthanitssuburb(Pathumthanistation).
Thisisbecausetheyarelocatedinthemostdenselybuild-upareawhichissurroundedbyanumberofbuildingsincludinghotels,hospitals,andgovernmentinstituteswhilethePathumthanistationissituatedawayfromthebuild-uparea.
Figure-2.
RelationshipbetweenpopulationdensityandUHIvariation.
ToshowtherelationshipbetweenpopulationdensityandUHImagnitude,thepopulationdensitydataindistrictofthosestationsarepresentedandplottedinFigure-2.
Therelationshipbetweenthepopulationincityandthemagnitudeofheatislandshowthemaximumurbanheatislandintensityintropicalhotandwetregionis4-9°Cwhenthepopulationisaround1millionto10million.
ThetotalpopulationinBangkokaccordingtohouseholdregistrationwas5.
78millionabout10%ofthetotalpopulationinThailand.
However,thisdoesnotincludethosewhocommuteandliveinBangkokwithoutregistration.
Thesenumbersareestimatedataround3.
2million(DepartmentofCityPlanning,2004).
ThismakesthetotalpopulationofBangkokcitytobealmost9million.
TheBangkokheatislandmagnitudeshouldbeapproximately6.
5°C.
HouseholdEnergyConsumptionandUHIAsurveyconductedbytheNationalStatisticalOfficeofThailand(NSO,2013)showsthattheaverageenergyexpenditureis2,084THBor10.
9%ofthetotalexpenditurewiththeexpenditureonelectricityis607THB(29.
1%ofthetotalenergyexpenditure).
TheaverageelectricityexpenditureinBangkokMetropolitanAreais1,133THB,higherthanotherregioninthecountry.
Thenumberisslightlydifferentwiththeresultfromthesurvey(854.
35THBfortheelectricityexpenditure).
72%ofhouseholdsinthestudyhasAirConditioning(AC)equipmentintheirhousingunits.
ThereisapositivecorrelationbetweenincomeandthenumberofACunitownedinthehouse(two-tailedt-statistics,pBaht)from400householdsamplesinBangkok.
Asexpected,thereisapositivecorrelationbetweenhouseholdenergyconsumptionandCDD.
RegressingmonthlyelectricityconsumptiontogeneratedCDDprofilesresultinginhighcoefficientsofR2(adj.
R2=0.
840;S.
E.
=1.
216;F=0.
000017;p-value=0.
000017).
CONCLUSIONSThisstudyusedhourlyairtemperatureofBangkokarea(urban)andPathumthani(non-urban)toestimatetheurbanheatislandcharacteristics.
Theresultsfromyearlydataofairtemperatureshowsthefollowings.
Duringthelast30yearofairtemperatureobservations(1980-2012),themean,maximum,andminimumannualairtemperatureappeartohaveincreasedlinearlyby1.
74,0.
95,1.
97°Crespectively.
Thisshowsaslightincreaseinmaximumtemperaturesandasignificantincreaseinminimumtemperature.
TheUHIseverityinBangkokisfoundtobehighercomparedtoothermajorcitieswithUHIproblemssuchasShanghai,SanDiego,andSanFrancisco,andisofsimilarrangetoTokyo.
TheUHIeffectismostpronouncedduringthenighttime.
Itbeginstoriseaftersunsetandreachesitsmaximumataboutsunriseduring6-7am.
Itcontinuestodecreasetothelowestmagnitudeoroftenbecomecoolislandphenomenonaround3-6pm.
ThisdevelopmentofUHIsisobservedinallseasons.
Forseasonalvariation,theUHIeffectismostintensiveduringthedryseason,followedbyhotandwetseason.
ThehighestintensitycanbeobservedinDecember(midofcoolseason)around5°Candinsummeraround2-3°C.
Unlikeinwinterandsummer,theUHIintensitybetweenthenighttime(7pm-7am)andmorningtime(7-12am)duringrainyseasonarealmostthesame.
ThestudyshowsthatmanyfactorsgoverntheUHIvariationsincludingcitystructures,population,andweather.
PrecipitationisfoundtobeinverselyproportionaltotheUHImagnitude.
TheareawiththemaximumUHIintensityiscorrespondstothemostdenselybuild-upareawithhighestbuildingandpopulationdensityamongotherstationsusedinthestudy.
Themaximumintensitycouldrangefrom8-10°Cduringthedaytimeduetothesurfacematerialssuchasroad(concreteorasphalts),buildingwalls,orpavedsurfacewarmingfasterthanthesurfacesinruralarea,whichnormallyarecoveredwithgreenareas.
ThestudyalsoexaminestherelationshipbetweenhouseholdenergyconsumptionandUHI.
Theresultshowsthatthehighertherelationshipispositive.
ItmeansthatenergyconsumptionishighintheareawithhighUHIvariationsandtheotherwayaround.
ThisfindingimpliesthatifnotUHIisnotmitigatedproperly,Bangkokwillexperienceasignificantincreaseofhouseholdenergydemand.
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