Decreases in the Number of Foggy Days in Thailand and Japan, and Possible Causes

Decreases in the Number of Foggy Days in Thailand and Japan, and Possible Causes

著者 NOMOTO Seiki

出版者 Institute of Comparative Economic Studies, Hosei University

journal or

publication title

Journal of International Economic Studies

volume 17

page range 13‑28

year 2003‑03

URL http://doi.org/10.15002/00002482

JoumaIoflntemationalEcmomicS【udies(2003),No.17,13-28 o2003ThelnstiluteofCompara[iveEconomicS[udics,HoseiUnWcrsity

DecreasesintheNumberofFoggyDaysinThailandand Japan,andPossibleCauses

SeikiNomoto

Depam"e"'q/Geogmpﾉi】FncMjZyq/Etmcajjo",Ｇ”U7ZjVe応jひ

１．Introduction

FOg,deiinedasmoistairwithahorizontalvisibilityoflessthanlkm,isalitmustest

thatmdicates1℃gionalenvironmentalchangaltfbrｍｓｄｕｅｔｏｔｈｅｃｏｏｌｍｇｏｆａｉｒｌａｙｅｒｓ，

anincreaseintheratioofwatervaporsupplytoairvolume，ｏｒｔｈｅｍｉｘｉｎｇｏｆａｉｒｍａｓｓ、

Ａｄｅｃｌｃａｓｅｉｎｔｈｅｃｏｏｌｉｎｇｏｒｔｈｅｓｕｐplyofwatervapor,fOranyIeason，causeschanges

inthepattemoffOgmanifestation；ｔｈｅｎumberoffbggydayscandecrease，thefOgcan

appearlater,orthefOg，sdurationcanbereduced・Humanactivities，suchasdefOrestation orurbanization,affectthethermalconditionsandwaterenvironmentsintheairnearthe ground,whichcanaffectfOgappearance,especiallyinmountainbasmsthatareclosedoff

EarlysmdiesfOundthatbigEuropeancitiesandNorthAmericanindustrialcitieshad morefOggydaysthandidcountrysideareas,duetotherichsupplyofcondensationnuclei

(Geigen959:LandsbuIg,1980）I、Tokyo,thebiggestcityinJapan,thenumberoffOggy

daysinc1℃asedrapidlywithindustrializationinthel920s(YOshino,1975)．

Ontheotherhand,Zhang（1986)studiedtheinHuenceofdefbrestationonlocalcli-

matesinXishuangbanna,atropicalmountainousregionwith49basinsintheSouthernpart ofYimnanProvince,Ｃｈｍａ;andpointedoutthatthenumberofannualfbggydaysdecreased rapidlyduringtheperiodofl954-1980ThedecreaseinannualfOggydaysmayhavebeen

causedbythereductionofevapotranspirationfromthetropicalfOrestduetodefOrestation

ThefOrestcoverageinXishuangbannawasreducedbyhalfintheperiodfroml949tol980；

from6996coverageinl949t０３０％coverageinl980､YOshino(1986)andNomoto(1995）

noticedsimilarphenomena,thoughtheirobservationwasbasedonlyonthelCcoldofone

meteorologicalstationmXishuangbanna･

ThestretchofXishuangbannaislocatedinthetropics・However,asithasabasinto-

poglaphy,ittendstoamasschillyairatnight,andtemperatu1℃scanberatherlow,especially inthebottomsofthebasinsduringthedryseason・WhenfOgfblms,thelatentheatandra- diationfromthefOgdropletswarmtheearth,ssurfaceandthelowestairlayeHarresting thedropintemperatme，TherefblB，thetreesinXishuangbannaguardthemselvesagainst thecoldbyretainingheatviafOgfbrmation．Inmostplacesthelowesttemperaturesare usuallyatdawn,butinXishuangbannaitiscoldestatmidnight,rightbefOrethefOgfOIms (Nomoto,etaL,1988:Nomoto,etaL,1990).TheseasonsofXishuangbannaareclassified intorainyanddryseasons，withthelattermnningapproximatelyhomOctobertoApriL Thereishardlyanyprecipitationdurmgthedryseason;atthistimefOgisavaluablewater

sourcefbrthetlCes(Ren,etaL,1985)．

InXishuangbanna,fOgplaysasignificantroleasaconstituentelementinthemainte-

1３

DecIeasesinthcNumberofFbggyDaysinThailandandJapan,andPossibleCauses

nanceoftheregionalecosystems・TherefOre,therecentdecreaseinthenumberoffOggy daysinXishuangbannamayaddtothefurtherdeteriorationoftheecosystemthathasal- readybeeninjurcdbydefbrestation

lntheareaextendingfromtheSouthernpartofYUnnanProvincetoNorthemThailand，

fＯｇｉsficquenLFurthermore，asatisfactorymeteorologicalobservationnetworkhasbeen establishedalloverThailand,andthedataisrelativelyeasytoacquire・

InJapan，urbanizationhasrecentlyproceededtothecountrysideaTeasaroundbig cities,andpaddyfieldarea,whichmayaddwatervaportotheairneartheground,hasbeen drasticallyreducedinruralareasunderthefbodpolicywhichtheJapanesegovemmenthas pursuedｓｍｃｅｌ９６９、

ThissmdywillexaminethenumberoffOggydaysinThailandandJapan,andaimto clarifyseveralpropertiesofthechanges．

2．ChangeintheNumberofFoggyDaysinThailandandDefbrestation 2.1FoggyRegionsandtheFogSeason

Thedatausedinthisstudy,whichwasprovidedbytheMeteorologicalDepartment ofThailand，iscomprisedmainlyofmonthlyvaluｅｓｆＯｒｔｈｅｐｅｒｉｏｄｆｒｏｍｌ９５１ｔｏｌ９９５・

TheobservationalnetworkinThailandiscomposedof51synopticweatherstations(Me- teorologicalDepartment，1982),asshowninFigureLIntennsofclimatepatternsand geographicalconditions,Thailandcanbedividedintofiveparts：Northern,Northeastem，

Central,Eastern,andtheSouthernparts(RungdilokroajnandNimma,1990).Mostofthe areasintheNorthernpartarehillyandmountainous,ａｎｄthemeteorologicalstationsthe1℃

arelocatedatthebottomsofdeepvalleysorbasins･TheNortheastempartconsistsofa highlevelplaincalledtheNortheastPlateau,andtheCentralpartisalargelowlevelplain runningalongtheChaoPhrayaRiveranditstributaries・TheEastempartisadjacenttothe GulfofThailand,andmostoftheareastherCconsistofplainsandsmallhills・TheSouthem partislocatedonthepeninsula,wherethetopographyishillyandmountainousbecauseof thelongridgeofwestemmountainsextendingfromthenorth,thatframesthepeninsula・

FigurelshowsadistributionofaverageannualfOggydays（1951-1980)inThailand GiventhesuitabletopographicalconditionsfOrfOgfOrmationandrichfOrests,asmentioned lateBthenumbersal己higherfOrtheNorthempart(100daysatMaeHongSon,ｇ１ｄａｙｓａｔ ＭａｅＳｏＬ８９ｄａｙｓａｔＭａｅSariangand87daysatPhrae).Withtheexceptionof94daysat Loei,whichisadjacenttotheNorthempart,theNortheasternpartisalessfOggyregion，

andtheCentralandEastempartshavehardlyanyfOg・Conversely,theSouthempartis consideredanotherfbggyregionofthecountry・ThenumbersamounttolO3daysatSurat Thani,thehighestvaluerecordedinthecountry,６９daysatTrangand51daysatChumphon、

Figure2givesseasonalchangesinmonthlyfOggydaysandrainfallatselectedstations intheNorthem，Northeastem，CentralandSouthemparts，Ｉｔｉｓｗｅｌｌｋｎｏｗｎｔｈａｔｔｈｅｃｌｉ－

ｍａｔｅｓofcontinentalSoutheastAsiaarecontrolledtoaverylargeextentbythesystemof AsianMonsoons､MaythroughOctoberisgenerallytheSouthwestmonsoonseason(rainy season)ｉｎThailand,ａｎｄthemonthlyfOggydaysbegintoinc1℃aseobviouslyattheendof therainyseason，reachingamaximuminthemiddleofthedryseasonlnThailand，ｔｈｅ ｄｌｙｓeasonisalsothefOgseason・ThissuggestsaconnectionbetweenfOgfOnnationand evapotranspirationfiPomthefOrests．

1４

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FigurelDistributionofAverageAImualFbggyDaysinThailalld

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DecreasesintheNumberofFoggyDaysinThlliIandandJapan,andPossibIeCauses

ThereisoneexceptiontothelclationshipbetweenthefOgseasonandthedryseason，

Le.,SuratThaniintheSouthempart,asshowninFigure2(｡).There,thenumberoffbggy dayspermonthhaｓｔｗｏｐｅａｋｓｉｎａｙｅａｒ,andincreasesevenintherainyseasonSeasonal changesinthenumberoffOggydayspermontharecommonatstationsthatfTequentlyob- selvefOgintheSouthemparLThisleadsonetobelievethattheprocessesoffOgfOrmation a１℃differenti、theSouthernpartthaninotherparts．

2.2RecentChangesintheNumberofFoggyDaysinNorthemThailand

Figure3showschangesinannualfOggydaysatselectedstationsintheNorthempart (NorthemThailand)sincel95LAsmentionedpreviously,themeteorologicalstationsin NorthernThailandaTelocatedatthebottomsofdeepvalleysorbasins,andfOgisfrequently observed・ThenumberoffOggydayshasbeendecreasingatmoststations,especiallyChi- angRai,PhraeandNan,asshowninFigure3(a).Assummgthatthetrendsinchangesa1℃

lineaLtherateofdecreasereachedL42daysperyearatChiangRai,2.01days/yatPhrae andL97days/ｙａｔＮａｎ,respectively・Furtbermore,ChiangMaiandPhitsanulokaremete- orologicalstationswherehardlyanyfOgisobservedPhitsanuloｋｈａｓｈａｄａｌｍｏｓｔｎｏｆＯｇ ｓｍＣｅ1965．Ontheotherhand,thetrendsinchangesinthenumberoffOggydaysshown inFiguTe3(b)a1℃quitediffermthomthoseinFigure3(a）ThenumbersatMaeHongSon andMaeSarianghavenotdecreasedcontinuously,buthaveHuctuatedwidelythroughout theperiod．

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Figure3ChangesofAnnuaIFOggyDaysatSelecにdStationsinNorthemThailandsincｅｌ９５１

Figure4showsthedecade-averagedmonthlyfOggydaysatPhrae,ChiangRai,Mae SariangandMaeHongSonThe45-year（1951-1995)periodhasbeendividedintofive

terms,withthelasttennconsistingofjustfiveyears・AtPhraeandChiangRai,where thenumberofannualfOggydaysdecreasedsharplyduringtheperiod,thedecade-averaged monthlyfbggydaysgraduallydeclcasedtowardthelasttermofeachmonththroughoutthe fOgseasonThistendencyisespeciallyclearatPhrae,whe｢ethelargestrateofdecreaseof annualfOggydaysinNorthernThailand,ｏｆ２．０１days/y,wasrecordedOntheotherhand，

ｉｎｔｈｅｃａｓｅｓｏｆＭａｅＳａｒｉａｎｇａｎｄＭａｅＨｏngSon，whichshowedaslightrateofdecrease ofannualfOggydays,thedecreaseinthenumberofmonthlyfOggydayscanbeconliImed

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ThenumberoffOggydayshasalsobeendecreasmgthroughoutthecountry(Nomoto，

1999).OneseesPhraetypefbg,witharecentrapiddeclCaseoffbggydaysorPhitsanulok typefbgwiththenumberoffbggydayshavingfallenandrecentlydisappealm・However，

theMaeSaliangtypefOgwithagreater、umberoffOggydayswithaslightdec1℃ase,ｗａｓ notwimessedinanyoftheseparts・Theratesofdec１℃asereachedl92days/yatLoeiin theNortheastempartand2.04days/yatSuratThaniintheSouthempart,respectively・The stationsmtheEastempartobservedalmostnofbgsincel980．

2.3DefOrestationandUrbanization

Figure5showschangesinthefOrestarearate(ratiooffbrestalcatothewholepだ- fectuIearea)inselectedprefecm1℃sinNorthemThailand､Althoughfbgisaphenomenon thattakesplaceonalocalscale，ｄａｔａｏｎｔｈｅｆＯｒｅｓｔａ1℃arateprovidesinfOrmationabout tｈｅn℃ndsinthenumberoffOggydays・ＭａｎｙｐlCfecturesinNorthernThailandhadrich fOrestsinl973,butsawthefOrestareacutbyhalfby1995．Ｆｏｒexample,NanPrefecture had904％fbrestcoverageml973,butjust48.9％coverageinl982and4L9％ｉｎ1995．

Likewise,PhraePrefectu1℃ｈａｄ66.2％fbrestcoverageinl973，ｂｕｔｏｎｌｙ37.2％、1995．

GiventhesechangesinthefOrestarearatQthenumberoffOggydayshasbeendeclCasing

drasticallyatmoststationsinNorthemThailandOneexceptionisMaeHongSonPrefec- ture,whichcontmuestomaintainhighcoverage・MaeSariangandMaeHongSon,which arelocatedintheprefecture,didnotshowthecontinuousdeclCaseinfOggydays・Onthe otherhand,PhisanulokandPechabunPrefecturehadjust42.7％and39.3％f０１℃stcoverage，

にspectively,eveninl973・Thestationsinbothprefectureshaveseenalmostnofbgsince

themiddleofthel960s．

Figure6givesthedistributionofthefblcstarcarateinThailand，sprefectu妃ｓｉｎｌ９７３ ａｎｄｌ９９５,showingthatdefbrestationhastakenplacealloverthecountlyduringthisperiod．

1７

Dec礎asesintheNumberofFbggyDaysinThailandandJapanandPossibleCauses

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３７.Chainnt，３８.Saraburi，３９.AnIhing，４０.Ayulhaya,４LSuphanBuri，４２Ｐｒ\lchinBuri，４３.Kanchanabun，

４４.Ralchaburi,４５.Phetchaburi,４６.NakhonNayok,４７.SamulSakhon,４８.Pathumthani,４９.NonthabuTi,５０.Samut Pmkan，５１.ChonBuli，５２.ChaCI1oengSao，５３.Rayong，S4ChanlI1aburi155・TTat，５６.Ｓａｍu【Songkhram，

s7PmchuapKhiriKhan,５８，Chumphon,５９.SuTxhIThanｉｖ６０・Nilkl1onSiThammahrat,６１，SongkhIa,６２.PhathaIun，

６３Paltgmi､６４.YaIa,６５.Narathiwal,６６Ranong'６７.Phukel,６８.PhanglMl､６９.ＫｍｂＬ７０・Ｔｍ､9,71.s【oon Figure6PercenlageofFores【Ａ定ａｉｎＥａｃｈＰ｢c化ctulCinl973andl995

1８

SeikiNomoto

Theprefecmre-by-prefecmrefiguresshowthattheonlyareaswithafOrestarearateofmore than６０％ａ1℃thefburprefectureswhichmakeupNorthernThailandTherefbrC,mostof thestationsmThailandshoweitherPhraeorPhisanuloktypefOg、

Amongthedifferentmeteorologicalelements，annualmeanminimumtemperatures maybethemostcloselylelatedtothetrendsinthｅｎｕｍｂｅｒｏｆｆＯｇｇｙｄａｙｓ、Dulingthe

periodofl951-1995,thetemperatureincreasedremarkablymNorthernThailand,andthere welCcleardifferencesbｅｔｗｅｅｎＭａｅＳａｒｉａｎｇａｎｄＭａｅＨｏｎＳｏｎａｎdothera１℃as,asshownin

FigulC､７(a).TheratesofincreaseatPhrae,ChiangMaiandPhitsanulokreached0.04°Ｃ/y，

0.03.C/yandOO2oqyfOrtheperiod,respectively・Ontheotherhand,theratesatMae

SariangandMaeHongSonshowedzeroorevennegatlvevalues(－００１°Ｃ/yatMaeHong

Son)．

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Figure7ChangesofAnnualMeanMeteoroIogicalElementsatSelectedStationsinNolthernThailand

Itisalsoworthytonotethedifferencesinthechangesofmeteorologicalelementsbe- tweenPhitsanulokandChiangMaiwithotherareas,asindicatedinFigu1℃s７(b)ａｎｄ(c)．

ContrarytothereportmRungdilokroajnandNimma(1990),whichstatedthattheannual meantemperaturefOrtheperiodl951-1989increasedremarkablyalloverThailand,afUr‐

theranalysisoftemperamreduringtheperiodofl951-1995mthissmdyshowsthatatthe sametimeitfellmNOrthemThailand・Ｅｖｅｎｓｏ,thetemperatu妃satPhitsanulokandChiang Maihavefallenstrikingly,asshowninFigure7(b)．Figure7(c)showschangesinannual meanrelativehumidity・Inaccordancewiththechangesofannualmeantemperature,the humidityhasincreased,withtheexceptionoftwostations,whichalaofcourse,Phitsanu-

lokandChiangMai・Humidityinthecitieshasshownaremarkabledecrease,ａｔａｒａｔｅｏｆ

Ｏ１８％/y・Thechangesinannualmeanvaporpressurealsoshowalargenegativetrendin

bothcities(Figure7(｡))．

1９ (a)Minimumtempe｢ature

．Ⅸ bC/S。

と卜！

^{｡￣‐。。}

(c)Relativehumidity

ｉｆｌＫ:rｌ

wAE戦2Ａ風上ムコリト

ｂ＝￣

■｡｡■■hb

:8吋wwVpaw満濁:雷国鞘虜Q等匡

DecreasesintheNumberofFoggyDaysinThaiIandandJapan1andPossibleCauses

ChiangMai,withapopulationofl82,O00inl996,isthelargestcityinNorthemThai- land;Phitsanulok,withapopulationof78,000,isthesecondlargesLTheriseintemperature theremaybeevidenceoftheprogressofurbanizationUrbanizationmayafTecttheurban heatandhumidity,whichaffectthenumberoffbggydayaAlthoughacitywithapopula- tionofl80,OOOdoesn，tnormaUyreceiverecognitionasbeｍｇｂｉｇ,thetrendsinthenumber offOggydaysinChiangMaiandPhitsanulokaresigniiicantlysimilartothoseinTbkyo andOsaka,OsakabeingthesecondlalgestcityinJapan(NomotoandTnkeyama,1998)．

mthetropics,thephenomenonofurbanizationhasagleatermHuenceontheairnearthe groundthaninotherclimaticregions,especiallyinthedryseason・Itisthoughtthatthe recentdecreaｓｅｓｉｎｔｈｅｎｕｍｂｅｒｏｆｆＯｇｇｙｄａｙｓｍａｙｂｅthelesultnotonlyofdefbrestation，

butalsoofurbanizationTbeminimuｍｔemperatulchasalsobeenrisingsharply,withthe vaporp妃ssuredecreasinginotherparts．

2.4DefbrestationandMonsoonRainfalIinNorthernThailand

Zhang(1986),studyingtheinHuenceofdefb1℃stationonlocalclimatesinXishuang‐

banna,pointedoutthatrainfalltherehaddec妃asedrema1kablyinthelaterainyseason,and suggestedthatitwasaresultoftheleductionofvaporpressureduetothedefbrestation、

Recently,totalannualrainfallhasdeceasedalloverThailand(RungdilokroajnandNimma，

1990).However,asshowninFigure8,whichindicatesthedecade-averagedtotalamount ofmonthlyrainfallatselectedstationsinNorthemThailand,mostofthestationstherehave observedincreasingrainfallintheearlyhalfoftherainyseasonThisincreasemayhave beentheresultofanincreasingBowenratioandinc1℃asingconvectiveinstabilitydueto defOrestationOntheotherhand,thetotalamountofmonthlyrainfalldecreasedsignallyin thelatterhalfoftheramyseason，ａｔｌｈｅｔｉｍｅｏｆａｂａｔｅｍｅｎｔｏｆｔｈｅＳＷｍｏｎsoon,andmay havebeencausedbydecreasingevapotranspirationfromthedecreasedfOIests・Kanae,ｅｔａＬ

５００ ４５０

肩４００

－

烏350￣

壱３００

－

二２５０．

：200

四１５０_Ｃ ト１ＣＯ ５０

０ 300

(b)ChiangRai (a)Phrae

０００００ ５０５０５ ２２１１ 一一句｝Ｅ四一．｝官。。Ｅ呵一里。』

０

ＪＡＮＦＥＢＭＡＲＡＰＲＭＡＹＪＵＮＪＵＬＡＵＧＳＥＰＯＣＴＮＯＶＤＥＣＪＡＮＦＥＢＭＡＲＡＰＲＭＡＹＪＵＮＪＵＬＡＵＧＳＥＰＯＣＴＮＯＶＤＥＣ

300 300

(｡)ＭａｅＨｏｎｇＳｏｎ (c)MaeSariang ｜司一■一四一．芒．。Ｅ呵一呵一Ｃ」 ^{２２１１ ５０５０５ ０００００}

釦印印加釦２２１１ －－ｍ』Ｅ一四一．芒口。Ｅ⑱｜⑪←。』

蝋

0 ０

ＪＡＮＦＥＢＭＡＲＡＰＲＭＡＹＪＵＮＪＵＬＡＵＧＳＥＰＯＣＴＮＯＶＤＥＣＪＡＮＦＥＢＭＡＲＡＰＲＭＡＹＪＵＮＪＵＬＡＵＧＳＥＰＯＣＴＮＯＶＤＥＣ

Ｆｉｇｕｒｅ８Ｄｅｃａｄｅ－ＡｙeragedTotalMonthlyRainfallalSelectedStationsinNorthernThaila､。

2０

SeikiNomoto

(2001)studiedtheHuctuationofrainfallinandaroundThailandinAugustandSeptember usinganumericalsimulation,andweleabletoexplainthedec1℃aseofrainfallinSeptember byanalterationofthevegetationdistribution

Monsoonrainfall,ｅｖｅｎｉｎＭａｅＳａｒｉａｎｇａｎｄＭａｅＨｏｎＳｏｎ,clearlydemonstratesthe

effectsofdefOrestationlnthenearfnture,thepattemoffbggydaysinbothstationscould

changetoaPhraetype．

3．ChangesintheNumberofFoggyDaysinJapanandReductionofPaddy

FieldＡｒｅａ

3.1FoggyRegionsandFbgSeasons

Figure9showsadistributionofaverageannualfbggydays(1961-1990)inJapanThe numbersa1℃highintheinlandmountainousTegｉｏｎａｎｄｉｎａｌ巴asalongtheseas,especially

thosefacingthePacificOceanThehighest（141days)isrecordedatKaruizawainthe

mountainousregionofcentralJapan，whereslopefbgstypicallyoccur・AdvectionfOgs

occurwiththemostfiPequencyalongthePacificcoastofHokkaido・Numberstherereach

ll2daysatKushｉｒｏａｎｄｌｌ３ｄａｙｓａｔＮｅｍｕｒＱＩｔｉｓｗｅｌｌｋnownthatradiationfOgistypically seenintheinlandmountainousbasins､ＴｈｅｎumbersofannualfOggydaysexceedlOOdays

atTbyookaandHitoyoshi．

FigureDDistributionofAveragedAnnualFoggyDaysinJapan

ThefOgseasonsinJapanaredependentonthediffe1℃ntwaysmwhichcoolingaffects fbgfOrmationFigulClOshowstimedistributionsoffOggydays・Ｔｈｅｍａｘｉｍｕｍｎｕｍｂｅｒｏｆ

２１

DeclEasesinthcNumberofFbggyDaysinThailandandJapan,andPossibIcCauses

７ 1２

(b）KYOTO (a)TAKAYAMA

1０ ６

５４３２ ⑩メロで罰団但」。」ｏＱＥコヱ

８ ６ ４

⑩メロロ訟覇ど」。」③ＰＥコヱ

２ １

0

ザQdpJd‘』。‘ず鉾緋oか‘ず 沙くぶ』。‘鞍獣少少教oけざ砂

０８６ ２１１ ８６４ １ １ １

(c）

KＵＳＨＩＲＯ (｡)KARUIZAWA

４２０８６ １１１ の奇で議団巳」ｏ』ｏＱＥコヱ ２０８６ １１ ⑭１：両面２」。』のＣＥコヱ ４２０

４２０

ザ２６゜j，砂野設寸絆叙｡ひざ砂 FigurelOTimeDistributi〔

ザくぷＪｄ歓破沙夕捗叔｡J乎絨 TimeDistributionofFOggyDaysalSelectedStations

averagemonthlyfbggydays（1961-1990)inTnkayama,amountainousbasin,wasseenin NovembeEbecauseradiativecoolingoccursatnightduetothedryandstableairtypically seenduringthismonth(FigurelO(a))．AlthoughfOgoccursfl巴quentlythelcinthecold season,itisimportanttonotethefactthatthewarmseasonalsoseesasmallincreaseinthe numberoffbggydays,whichwillbementionedlate凪

FoggydaysinKyoto,ａｂｉｇｃｉｔｙｗｉｔｈａｐｏｐｕｌａｔｉｏｎｏｆＬ３９ｍｉllion,showsaremarkable contrastbetweenthecoldandwarmseasons(Figu1℃１０(b)）FbgfTequentlyappearsinthe

coldseason，

ＷｈｅｎｗａｒｍａｌｒｐａｓｓｅｓｏｖｅｒａｃｏｌｄｓｅａSurface,advectionfOgisfOrmed，AtKushiro，

ｗｈｉｃｈｌｉｅｓｏｎｔｈｅＰａｃｉｆｉｃｃｏａｓｔｏｆＨｏkkaido，fOggydaysinthewarmseasonoutnumber thoseinthecoldseason(FigurelO(c)).Thenumbersinmountainousregionsalsoshowa remarkableinclmseinthewarmseason,whenwannandmoistsub-tropicalairadvectsinto Japan(FigurelO(｡))．

22

SeikiNommo

3.2RecentChangesintheNumberofFoggyDays

Figulc11showschangesinannualfbggydaysatselectedstationssincel95LFoghas beendecreasingrapidlyintwomajorcities(KyotoandOsaka)since1960,andtherehas beenalmostnofbgtheIesincel970(Figurell(a)).Assumingthatthetrendsofitsperiod arelinear,theratesofdecreasereach290daysperyearatOsakaandL68days/yatKyoto，

respectivelySeveralfactors，suchastheimpermeabilityofcertainarcasandheatisland phenomena,mayaffecttheurbanhumidityfield，whichcancontrolthenumberoffOggｙ days．

印、叩印釦、０１１ 切扣、、印加⑩ｍ０^１１１１

町奇で財団２」。』賢一Ｅ。Ｚ

甑

巨函

(日)Ｃｉｔ^{ａ）ＵＩ[ｙ} (b）Basin

呂句で目■２」。」ＢＥコヱ

ｄ－ＦＳ－５Ｄ＝｡

uＤ－ＵＤ－ＧｊＤ－Ｌｎ＿［別

０００００００００００ ０８６４２０８６４２ ２１１１１１ ⑩百℃訟已２も」和囹Ｅコヱ

帥扣如、即印扣、０１１１１

⑪胃で勧司ど」。」恥己Ｅコヱ ＝1【雨UE7NW7n(｡）Mountain

RPoFPbp9･p9dWaPciPd

Linesin(b)showIhetrendofthechanges・

FigurellChangesofAnnuaIFoggyDaysalSelectedStationssincel951

ltisalsosulprisingthatthenumberoffOggydaysdecreasedconstantlyintheinland mountainousbasinsthroughouttheperiod(Figul巴11(b))．Theratesofdecreasereached 2.18days/yatHitainKyushu,L46days/yatTHkayama,CentralJapanandL79days/yat Asahikawa,Hokkaido,respectively・IfthistrendcontinuesintothefUmlathenumberofan- nualfOggydayswillhitzeroatHitaandTnkayamabytheyear2013・AlthoughAsahikawa cityhadapopulationof362,OOOinl997,T1akayamaandHitaarerelativelysmallcitieswith populationsof62,ＯＯＯａｎｄ６３,000.1tcanbesaidthatthethreestationsshowedespecially largeratesofdec1℃ase,andasimilartendencycouldbeseenatmoststationsinmountainous basinsalloverthecountry、

Ｏｎｔｈｅｏtherhand,coastal,islandandmountainstationsdidnotshowthecontinuous decreaseinannualfOggydays(Figuresll(c),(｡))．Theyappearedtofluctuatedepending onthegeneralatmosphericCirculation・

MonthlyfOggydaysmaybeagoodmdicatorofannualchangesinfOggydays・Figure l2indicatesthedecade-averagedmonthlyfOggydays,ｗｉｔｈｔｈｅ４０ｙｅａｒｓ（1951-1990)。i‐

videdintofOurtenns,atselectedstationsinthemountainousbasinsandbigcities､Hitaand T1akayamasawadrasticchangebetweenthesecondandthirddecades,butonlyinthewalm seasons,afactthatcannotbefOundoutthroughananalysisofannualfbggydays(Figules l2(a),(b)).ＴｈｅｎumberoffOggydaysduringthethimdecadewasfewerthanhalfofthose

2３

DccreasesintheNumbcrofFbggyDaysmThailandandJapanandPossib]eCauscs

０８６４２０８６４２０ ２１１１１１ 昌呂議副ど」。」の。Ｅコヱ ２０８６４２０ １１ ⑩易句で易凹凶〔単」。」の口Ｐ』コヱ(a)ＨITA

i麺 Illnlll

識《｡'jdvj駛載タマJ'夢｡β鰔討 ずく伊ｊｄｖ４ｉ(さ‘識:'>vs9,夢砂蚕絨

４２０８６４２０ １１１ ⑩訂で菌凶＆」。』①。Ｅゴヱ ６４２０８６４２０ －１１１ 呪訂で猷団。｝」。』の。Ｅコヱ(。)ASAHIKAWA (｡)KYOTO

３s<６．駁餃‘ダダ獣叙皎急PＶ鱗DP駮?９Ｗダムj`夢c'軒｡′

Figllrel2Decade-AvemgedMonthlyFoggyDaysaIScIectedStiltionsinMountainousBasinsand Bigcities

oftheseconddecadeinthewarmseasonOntheotherhand,suchchangescannotbeseen inthecoldseasonThedrasticdecreaseinthethirddecadethroughoutthewalmseasonsis alsoobservedatmanystationsintheinlandmountainousbasinsofJapan

Asahikawa,whichisshowninFigurel2(c),isoneexceptiontotheabove-mentioned observation、ThecaseofthismountainousbasininnorthemJapan，wherethenumberof fOggydaysdeclcasedgraduallytowardthefOurthdecadeineachmonth,isdifferentfrom thoseofHitaandTnkayama・Becauseofthecoarsenessofthisobservationalnetwork,ｗe cannotsafelyjudgewhetherthecaseofAsahikawaisexceptionalorwhetherthiswasa commonchangeinthemlandmountainousbasinsinNorthernJapan，

AsshowninFigu１℃１２(｡),thebigcityhadaldativelysmallnumberoffOggydays

inandafterthethirddecade,ｅｖｅｎｉｎｔｈｅｃｏｌｄｓｅａｓｏｎｌｔｃａｎｂｅｓａｉｄthattheenvnonments fOrfOgfOrmationmayhavechangeddrasticallyinthethirddecade，ａｎｄｔｈｉｓｓｈｏｕｌｄｂｅ ｗｉｍｅｓｓｅｄｂｙｓｏｍｅｍeteorologicalelements、

Figurel3showsthechangesofmonthlyfOggydays,alongwithsomemeteorological elements,inKyotoduringNovember,DecemberandJanuary・ThesemonthsalcinthefOg seasoni、Kyoto、ThenumberofmonthlyfOggydaysinc1℃asesatlirst,andthendeclCases rapidly(Figurel3(a)).Asmentionedabove,fOgcanbefblmedwhentheairneartheground iscooledsufficiently,orthroughtheadditionofenoughwatervaportotheainHowever，

therearenodetectablemeteorologicalelementsthatcanexplainthechangeinthenumberof monthlyfOggydays・Themonthlymeanminimumairtemperaturesroseslightly,whilethe diumalrangesofairtemperaturedecreasedduringtheperiod(Figurel3(b)).Themonthly meanvaporpressureinKyotodecreasedgraduaIlyduringtheperiod(Figurel3(c))．The numberofrainydaysandtotalamoumofrainfallinJanuaryalsodec1℃ased,althoughthey showedextensiveHuctuationthroughouttheperiod(Figulel3(d)).Itistruethatthechanges seeninthemeteorologicaldatapredictfewerfOggydays,althoughthefactsdonotfitinwith thepredictionslnthecaseofcityfOg,weshouldtakeintoconsiderationnotonlytheurban

2４

SeikiNomoto

3０ 』一１１１ ４２０８６４２０２４

垂

(a）

５０５０５ ２２１１

呪訂で』団凶＆」。』⑪ＱＥコヱ ⑩」。當世⑩ＱＥ⑩」

0

‘鐵愈鰔鐵鐵鰊鰊獣獣鰊鐵鐵絨 歓鐡鹸鐵鰊鰔鰔‘ず鐡鐡鰔鐵‘

４２０８６４２０

１１Ｉ （ロユニ）２コ⑪⑩。ａ』。Ｑ■ン

2５ 6０

唖亟

(｡） １１１８．０４２０ ４④』０００００【Ｕｎ》ｎｕ （ＥＥ）一一唱巨肩』」ｏ芒弓ｏＥ■｜ロ］ロト

０５０５ ２１ｌ ⑭一百℃要』洞』」。」２］Ｅコヱ

０

‘鐡鐵鐵鐵鰔鰔.SSO獣獣鐵鐵總絨鐡鐵叡鰔絨鐵鐵鋏籔絨鐡鰔鐡鐵

（a)NumbeToffbggydays,(b)MinimumIempcratuleanddiumalrangeofaiTtemperatura(c)Vaporpres‐

sureand〔｡)NumberofrainydaysandtotalamountofrainfalL

Figurel3ChangesofMonthlyFoggyDaysandMeteorologicalEIementsmKyotointheColdSeason

humidityandheatislandphenomena,butalsootherfactorssuchasairpollution．

3.3BasinFOgSandtheReductionofPaddyFieIdArea

Itcanalsobeseeninthemountainousbasinsthatchangesinthemonthlymeanmmi- mumairtemperamresincreasedslightly,whereasthevaporpressure,numberofrainydays andtotalamountofrainfallineachmonthdecreasedduringtheperiod、Furlhennore，a possiblecausefOrthedecreaseinthenumberoffOggydaysinthemountainousbasinscan

befbund．

Figurel4showsthechangesintotal、umberoffOggydaysinthewarmseaso、(April- September),ｃｏｌｄseason(Octobe炉Ｍａ1℃h)andofpaddyiieldal己ainselectedbasins・A reductionofricepaddyfieldaleahasoccurredalloverJapansincel969,fOllowingthe Japanesegovemment，sfOodpolicy，Thechangesinpaddyfieldalcacorrespondwellto thoseinthenumberoffOggydaysinthewarmseason,ｗｉｔｈtheexceptionofthecaseof

Asahikawa(Figurel4(｡)）ItissulprisingthatthecolTelationcoefficientsbetweenpaddy

lieldareaandfbggydaysinthewarmseasonreachO92fbrTnkayama,O86fbrHitaand O84fbrUeno・Durmgtheperiod,thepaddyiieldalcaintheTnkayamabasinfirstinc1℃ased (towardstheearlyl960s)andthendecreased(sincethemiddleofthel960s).Thechanges inpaddyfieldareastronglyaffectedthenumberoffOggydaysinthewalmseason,which increasedtowardstheearlyl960sandhavedecreasedsincethemiddleofthel960s・

ThefactthatthepaddyfieldareatherefellfiPoml,400hainthefirsthalfofthel960s to800hainl990indicatesthelossofavastamountofwatervaporsupplytothebasinailZ

2５

JＷ

^上

(c）

￣ Ｖ--．￣￣･ｖＹＷ

国

Dec疋asesintheNumberofFOggyDaysinTmailandandJapan,andPossibIeCauses

加師、加釦佃釦犯仰０

2500 ^{８７６５４３２１ ０００００００００} ５４ 叩珈

!'iii:jiiiil;ｌｌ１`!i,:’ ^{…ｗｌＩ１型。}

^』し．

⑭尹勾Ｕ詞凶凶。←」ｏ」③ロニヒコヱ 堅呵で団四皇」。」図Ｅコヱ（２）のｐ』⑤二・車壱七口Ｑ血卸血的２１Ｉ－ｂ （２）何口』ロ冥豊ぞご旬匹鋤鋤柳趣椰岬岬”。

ｉｉｉ!:;jii:iii駒(1ijim(！

;ii二ijiil:iiii蝋 _{箪逗朕戸日 箪逗朕戸日 箪逗朕戸日}

０

鐡鍜.『，銭｡FPC§

００００００００ ７６５４３２１ 釦扣釦印側釦如沁０

（呵二）⑤□』口里①喀津石で⑤Ｑ阿仙剛仙血噸姻醐０ ₁₄₀₀₀

12000

ｸﾞー Cp

10000ら

8000； ■ で

6Coo，2

40噸：_△

２０００ ０

(c)TAKAYAMA (d）ASAHIKAWA

呂丹団鞄巳」ｏ」ｏＱＥゴヱ の奇で旬帥どち』○ｐＥ。Ｚ

EE斑

四ワーＵｎ－－ｑ品マーＧＥ

BoIdlinesshowthetrmdinthenumbeToffbggydaysinlhewaImseasona､。[hinIines,ｔｈｏｓｅｉｎｔｈｅｃｏ１ｄ

ｓｅａｓｏｎ，

Figurel4ChangesinTbtalAmountofFoggyDaysintheWarTnSeason(April-September)andin theColdSeason(October-March)andofPaddyFieldAreas

4．Conclusion

Fromthisstudy,wecandrawthefOllowingconclusionsonrecentchangesinthenum- beroffOggydaysinThailandandJapan

lnThailand,thenumberoffOggydaysbeginstoincreaseattheendoftherainyseason，

reachingamaximuminthemiddleoftｈｅｄｒｙｓｅａｓｏｎＴｈｅｄｌｙｓｅａｓｏｎｉｓａｌｓｏｃonsideredthe fOgseason．

Duringtheperiodofl951-1995,thenumberofannualfOggydaysdecreasedthrough- outThailand・Theratesofdecreasereached2・O1daysperyearatPhraeand2D4days/y atSuratThanLInNorthemThailand,thetlcndsoffOggydayscanbeclassifiedintothree types・IntheMaeSariangtype,thedeclcaseinfOggydayscouldbeconiirmedonlyatthe monthsofthestartortheendofthefOgseason・Ontheotherhand,inthePhraetype,itwas ObservedineachmonththroughoutthefOgseasonMostofthestationsinThailandshowed eitherPhraeorPhitsanuloktypefbg

Amongsomemeteorologicalelements,theminimumtemperatureandthevaporpres- surcmaygiveusmfbrmationconcerningthedecreaseoffbggydays､TheminimumtempeP aturehasbeenincreasingdrasticallyalloverThailandduringtheperiodanditstendencies haverelationshipwiththemagnitudeofdecreasingfbggydays・Theauthorcannotsafely judgewhytheminimumtemperaturehasbeenincreasingrapidlyandthevaporpressure decreasingalloverthecountry・InNorthemThailand，thehighincreaseoftheminimum temperatureandthehighdecreaseofthevaporpressureareobservedinbigcities,which maybecausedbyurbanizatiolL

Althoughfbgisalocalphenomenon,dataonthefbrcstarearate(byprefecm１℃）is infOrmativeofthetendencyofthenumberoffbggydays・ManyplCfecturesinNorthem

2６

SeikiNomolo

ThailandhadrichfOrestsinl973，ｂｕｔｓａｗｔｈｅａｌｃａｈａｌｖｅｂｙｌ９９５，Ｅｖｅｎｓｏ，MaeHong SonPrefecturehasretainedhighcoveragetodate・MaeSariangandMaeHongSon,which arelocatedintheprefectuTe，didnotshowacontinuousdecreaseoffOggydays・Further 妃searchonthelocalfOrestareaandlandusewillaidinunderstandingthedec1℃asein fOggydaysmThailand

InJapan,thenumberofannualfOggydayshasalsofaUenstrikinglyinthemountainous basinsandbigcitiesinrecentyears・Thedecreaseratereached290daysperyearatOsaka and2」８days/yatHita・

Atmanystationsinthemountainousbasins，ｔhenumberoffOggydaysdecreasedin thewam1seasonOntheotherhand,thoseinbigcitiesdecreasedthroughouttheyear,with fOgbeingraresincel970

A1℃ductionofpaddyHeldareamayaffecｔｔｈｅｎｕｍｂｅｒｏｆｆＯｇｇｙｄａｙｓｉｎｔｈｅｗａｎｎ season・CorrelationcoefficientsbetweenthechangesofpaddyfieldareaandfOggydaysin thewarmseasonreachｅｄＯ・g2atTklkayama,０．８６ａｔＨｉｔａａｎｄＯ８４ａｔＵｅｎｏ､InordertocalTy thisworkfurther,itisessentialtotakeintoconsiderationdetailedobservationsofthebasins andtoconductnumericalsimulations．

AcknowledgmemS

TheauthorwouldliketogratefUllyacknowledgeProfS・VanpenSurarerks，Poonsap Tiyayon,CharoonSukkasemandPuangpetchDhanasin，ChiangMaiUniversity,Ｍｓ・Ｓｕ‐

dapomNimma,MeteorologicalDepartmentandM凪VichaiKijmeaChiangMaiRegional ForestOflicefOrtheirhelpincollectingdataandgivingsuggestions．

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