meteorology-第5章

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less。　This　is　a　thing　which　we　can　often　observe　in　other　places



too。　Wells；　for　instance；　give　off　more　vapour　in　a　north　than　in　a



south　wind。　Only　the　north　winds　quench　the　heat　before　any



considerable　quantity　of　vapour　has　gathered；　while　in　a　south　wind



the　evaporation　is　allowed　to　accumulate。



　　Water；　once　formed；　does　not　freeze　on　the　surface　of　the　earth；



in　the　way　that　it　does　in　the　region　of　the　clouds。







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　　From　the　latter　there　fall　three　bodies　condensed　by　cold；　namely



rain；　snow；　hail。　Two　of　these　correspond　to　the　phenomena　on　the



lower　level　and　are　due　to　the　same　causes；　differing　from　them　only



in　degree　and　quantity。



　　Snow　and　hoar…frost　are　one　and　the　same　thing；　and　so　are　rain



and　dew：　only　there　is　a　great　deal　of　the　former　and　little　of　the



latter。　For　rain　is　due　to　the　cooling　of　a　great　amount　of　vapour；



for　the　region　from　which　and　the　time　during　which　the　vapour　is



collected　are　considerable。　But　of　dew　there　is　little：　for　the　vapour



collects　for　it　in　a　single　day　and　from　a　small　area；　as　its　quick



formation　and　scanty　quantity　show。



　　The　relation　of　hoar…frost　and　snow　is　the　same：　when　cloud



freezes　there　is　snow；　when　vapour　freezes　there　is　hoar…frost。



Hence　snow　is　a　sign　of　a　cold　season　or　country。　For　a　great　deal



of　heat　is　still　present　and　unless　the　cold　were　overpowering　it



the　cloud　would　not　freeze。　For　there　still　survives　in　it　a　great



deal　of　the　heat　which　caused　the　moisture　to　rise　as　vapour　from



the　earth。



　　Hail　on　the　other　hand　is　found　in　the　upper　region；　but　the



corresponding　phenomenon　in　the　vaporous　region　near　the　earth　is



lacking。　For；　as　we　said；　to　snow　in　the　upper　region　corresponds



hoar…frost　in　the　lower；　and　to　rain　in　the　upper　region；　dew　in　the



lower。　But　there　is　nothing　here　to　correspond　to　hail　in　the　upper



region。　Why　this　is　so　will　be　clear　when　we　have　explained　the　nature



of　hail。







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　　But　we　must　go　on　to　collect　the　facts　bearing　on　the　origin　of



it；　both　those　which　raise　no　difficulties　and　those　which　seem



paradoxical。



　　Hail　is　ice；　and　water　freezes　in　winter；　yet　hailstorms　occur



chiefly　in　spring　and　autumn　and　less　often　in　the　late　summer；　but



rarely　in　winter　and　then　only　when　the　cold　is　less　intense。　And　in



general　hailstorms　occur　in　warmer；　and　snow　in　colder　places。



Again；　there　is　a　difficulty　about　water　freezing　in　the　upper　region。



It　cannot　have　frozen　before　becoming　water：　and　water　cannot　remain



suspended　in　the　air　for　any　space　of　time。　Nor　can　we　say　that　the



case　is　like　that　of　particles　of　moisture　which　are　carried　up



owing　to　their　small　size　and　rest　on　the　iar　（the　water　swimming　on



the　air　just　as　small　particles　of　earth　and　gold　often　swim　on



water）。　In　that　case　large　drops　are　formed　by　the　union　of　many



small；　and　so　fall　down。　This　cannot　take　place　in　the　case　of　hail；



since　solid　bodies　cannot　coalesce　like　liquid　ones。　Clearly　then



drops　of　that　size　were　suspended　in　the　air　or　else　they　could　not



have　been　so　large　when　frozen。



　　Some　think　that　the　cause　and　origin　of　hail　is　this。　The　cloud　is



thrust　up　into　the　upper　atmosphere；　which　is　colder　because　the



reflection　of　the　sun's　rays　from　the　earth　ceases　there；　and　upon　its



arrival　there　the　water　freezes。　They　think　that　this　explains　why



hailstorms　are　commoner　in　summer　and　in　warm　countries；　the　heat　is



greater　and　it　thrusts　the　clouds　further　up　from　the　earth。　But　the



fact　is　that　hail　does　not　occur　at　all　at　a　great　height：　yet　it



ought　to　do　so；　on　their　theory；　just　as　we　see　that　snow　falls　most



on　high　mountains。　Again　clouds　have　often　been　observed　moving　with　a



great　noise　close　to　the　earth；　terrifying　those　who　heard　and　saw



them　as　portents　of　some　catastrophe。　Sometimes；　too；　when　such　clouds



have　been　seen；　without　any　noise；　there　follows　a　violent



hailstorm；　and　the　stones　are　of　incredible　size；　and　angular　in



shape。　This　shows　that　they　have　not　been　falling　for　long　and　that



they　were　frozen　near　to　the　earth；　and　not　as　that　theory　would



have　it。　Moreover；　where　the　hailstones　are　large；　the　cause　of



their　freezing　must　be　present　in　the　highest　degree：　for　hail　is



ice　as　every　one　can　see。　Now　those　hailstones　are　large　which　are



angular　in　shape。　And　this　shows　that　they　froze　close　to　the　earth；



for　those　that　fall　far　are　worn　away　by　the　length　of　their　fall



and　become　round　and　smaller　in　size。



　　It　clearly　follows　that　the　congelation　does　not　take　place



because　the　cloud　is　thrust　up　into　the　cold　upper　region。



　　Now　we　see　that　warm　and　cold　react　upon　one　another　by　recoil。



Hence　in　warm　weather　the　lower　parts　of　the　earth　are　cold　and　in　a



frost　they　are　warm。　The　same　thing；　we　must　suppose；　happens　in　the



air；　so　that　in　the　warmer　seasons　the　cold　is　concentrated　by　the



surrounding　heat　and　causes　the　cloud　to　go　over　into　water



suddenly。　（For　this　reason　rain…drops　are　much　larger　on　warm　days



than　in　winter；　and　showers　more　violent。　A　shower　is　said　to　be



more　violent　in　proportion　as　the　water　comes　down　in　a　body；　and　this



happens　when　the　condensation　takes　place　quickly；…though　this　is　just



the　opposite　of　what　Anaxagoras　says。　He　says　that　this　happens　when



the　cloud　has　risen　into　the　cold　air；　whereas　we　say　that　it



happens　when　the　cloud　has　descended　into　the　warm　air；　and　that　the



more　the　further　the　cloud　has　descended）。　But　when　the　cold　has



been　concentrated　within　still　more　by　the　outer　heat；　it　freezes



the　water　it　has　formed　and　there　is　hail。　We　get　hail　when　the



process　of　freezing　is　quicker　than　the　descent　of　the　water。　For　if



the　water　falls　in　a　certain　time　and　the　cold　is　sufficient　to　freeze



it　in　less；　there　is　no　difficulty　about　its　having　frozen　in　the　air；



provided　that　the　freezing　takes　place　in　a　shorter　time　than　its



fall。　The　nearer　to　the　earth；　and　the　more　suddenly；　this　process



takes　place；　the　more　violent　is　the　rain　that　results　and　the



larger　the　raindrops　and　the　hailstones　because　of　the　shortness　of



their　fall。　For　the　same　reason　large　raindrops　do　not　fall　thickly。



Hail　is　rarer　in　summer　than　in　spring　and　autumn；　though　commoner



than　in　winter；　because　the　air　is　drier　in　summer；　whereas　in



spring　it　is　still　moist；　and　in　autumn　it　is　beginning　to　grow　moist。



It　is　for　the　same　reason　that　hailstorms　sometimes　occur　in　the



late　summer　as　we　have　said。



　　The　fact　that　the　water　has　previously　been　warmed　contributes　to



its　freezing　quickly：　for　so　it　cools　sooner。　Hence　many　people；



when　they　want　to　cool　hot　water　quickly；　begin　by　putting　it　in　the



sun。　So　the　inhabitants　of　Pontus　when　they　encamp　on　the　ice　to



fish　（they　cut　a　hole　in　the　ice　and　then　fish）　pour　warm　water



round　their　reeds　that　it　may　freeze　the　quicker；　for　they　use　the　ice



like　lead　to　fix　the　reeds。　Now　it　is　in　hot　countries　and　seasons



that　the　water　which　forms　soon　grows　warm。



　　It　is　for　the　same　reason　that　rain　falls　in　summer　and　not　in



winter　in　Arabia　and　Ethiopia　too；　and　that　in　torrents　and　repeatedly



on　the　same　day。　For　the　concentration　or　recoil　due　to　the　extreme



heat　of　the　country　cools　the　clouds　quickly。



　　So　much　for　an　account　of　the　nature　and　causes　of　rain；　dew；



snow；　hoar…frost；　and　hail。







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　　Let　us　explain　the　nature　of　winds；　and　all　windy　vapours；　also　of



rivers　and　of　the　sea。　But　here；　too；　we　must　first　discuss　the



difficulties　involved：　for；　as　in　other　matters；　so　in　this　no



theory　has　been　handed　down　to　us　that　the　most　ordinary　man　could　not



have　thought　of。



　　Some　say　that　what　is　called　air；　when　it　is　in　motion　and　flows；　is



wind；　and　that　this　same　air　when　it　condenses　again　becomes　cloud　and



water；　implying　that　the　nature　of　wind　and　water　is　the　same。　So　they



define　wind　as　a　motion　of　the　air。　Hence　some；　wishing　to　say　a



clever　thing；　assert　that　all　the　winds　are　one　wind；　because　the



air　that　moves　is　in　fact　all　of　it　one　and　the　same；　they　maintain



that　the　winds　appear　to　differ　owing　to　the　region　from　which　the　air



may　happen　to　flow　on　each　occasion；　but　really　do　not　differ　at



all。　This　is　just　like　thinking　that　all　rivers　are　one　and　the　same



river；　and　the　ordinary　unscientific　view　is　better　than　a



scientific　theory　like　this。　If　all　rivers　flow　from　one　source；　and



the　same　is　true　in　the　case　of　the　winds；　there　might　be　some　truth



in　this　theory；　but　if　it　is　no　more　true　in　the　one　case　than　in



the　other；　this　ingenious　idea　is　plainly　false。　What　requires



investigation　is　this：　the　nature　of　wind　and　how　it　originates；　its



efficient　cause　and　whence　they　derive　their　source；　whether　one　ought



to　think　of　the　wind　as　issuing　from　a　sort　of　vessel　and　flowing



until　the　vessel　is　empty；　as　if　let　out　of　a　wineskin；　or；　as



painters　represent　the　winds；　as　drawing　their　source　from　themselves。



　　We　find　analogous　views　about　the　origin　of　rivers。　It　is　thought



that　the　water　is　raised　by　the　sun　and　descends　in　rain　and　gathers



below　the　earth　and　so　flows　from　a　great　reservoir；　all　the　rivers



from　one；　or　each　from　a　different　one。　No　water　at　all　is



generated；　but　the　volume　of　the　rivers　consists　of　the　water　that



is　gathered　into　such　reservoirs　in　winter。　Hence　rivers　are　always



fuller　in　winter　than　in　summer；　and　some　are　perennial；　others　not。



Rivers　are　perennial　where　the　reservoir　is　large　and　so　enough



water　has　collected　in　it　to　last　out　and　not　be　used　up　before　the



winter　rain　returns。　Where　the　reservoirs　are　smaller　there　is　less



water　in　the　rivers；　and　they　are　dried　up　and　their　vessel　empty



before　the　fresh　rain　comes　on。



　　But　if　any　one　will　picture　to　himself　a　reservoir　adequate　to　the



water　that　is　continuously　flowing　day　by　day；　and　consider　the　amount



of　the　water；　it　is　obvious　that　a　receptacle　that　is　to　contain　all



the　water　that　flows　in　the　year　would　be　larger　than　the　earth；　or；



at　any　rate；　not　much　smaller。



　　Though　it　is　evident　that　many　reservoirs　of　this　kind　do　exist　in



many　parts　of　the　earth；　yet　it　is　unreasonable　for　any　one　to



refuse　to　admit　that　air　becomes　water　in　the　earth　for　the　same



reason　as　it　does　above　it。　If　the　cold　causes　the　vaporous　air　to



condense　into　water　above　the　earth　we　must　suppose　the　cold　in　the



earth　to　produce　this　same　effect；　and　recognize　that　there　not　only



exists　in　it　and　flows　out　of　it　actually　formed　water；　but　that　water



is　continually　forming　in　it　too。



　　Again；　even　in　the