meteorology-第6章

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the　earth　remains　but　the　moisture　surrounding　it　is　made　to　evaporate



by　the　sun's　rays　and　the　other　heat　from　above；　and　rises。　But　when



the　heat　which　was　raising　it　leaves　it；　in　part　dispersing　to　the



higher　region；　in　part　quenched　through　rising　so　far　into　the　upper



air；　then　the　vapour　cools　because　its　heat　is　gone　and　because　the



place　is　cold；　and　condenses　again　and　turns　from　air　into　water。



And　after　the　water　has　formed　it　falls　down　again　to　the　earth。



　　The　exhalation　of　water　is　vapour：　air　condensing　into　water　is



cloud。　Mist　is　what　is　left　over　when　a　cloud　condenses　into　water；



and　is　therefore　rather　a　sign　of　fine　weather　than　of　rain；　for



mist　might　be　called　a　barren　cloud。　So　we　get　a　circular　process　that



follows　the　course　of　the　sun。　For　according　as　the　sun　moves　to



this　side　or　that；　the　moisture　in　this　process　rises　or　falls。　We



must　think　of　it　as　a　river　flowing　up　and　down　in　a　circle　and　made



up　partly　of　air；　partly　of　water。　When　the　sun　is　near；　the　stream　of



vapour　flows　upwards；　when　it　recedes；　the　stream　of　water　flows　down：



and　the　order　of　sequence；　at　all　events；　in　this　process　always



remains　the　same。　So　if　'Oceanus'　had　some　secret　meaning　in　early



writers；　perhaps　they　may　have　meant　this　river　that　flows　in　a　circle



about　the　earth。



　　So　the　moisture　is　always　raised　by　the　heat　and　descends　to　the



earth　again　when　it　gets　cold。　These　processes　and；　in　some　cases；



their　varieties　are　distinguished　by　special　names。　When　the　water



falls　in　small　drops　it　is　called　a　drizzle；　when　the　drops　are　larger



it　is　rain。







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　　Some　of　the　vapour　that　is　formed　by　day　does　not　rise　high



because　the　ratio　of　the　fire　that　is　raising　it　to　the　water　that



is　being　raised　is　small。　When　this　cools　and　descends　at　night　it



is　called　dew　and　hoar…frost。　When　the　vapour　is　frozen　before　it



has　condensed　to　water　again　it　is　hoar…frost；　and　this　appears　in



winter　and　is　commoner　in　cold　places。　It　is　dew　when　the　vapour　has



condensed　into　water　and　the　heat　is　not　so　great　as　to　dry　up　the



moisture　that　has　been　raised　nor　the　cold　sufficient　（owing　to　the



warmth　of　the　climate　or　season）　for　the　vapour　itself　to　freeze。



For　dew　is　more　commonly　found　when　the　season　or　the　place　is　warm；



whereas　the　opposite；　as　has　been　said；　is　the　case　with　hoar…frost。



For　obviously　vapour　is　warmer　than　water；　having　still　the　fire



that　raised　it：　consequently　more　cold　is　needed　to　freeze　it。



　　Both　dew　and　hoar…frost　are　found　when　the　sky　is　clear　and　there　is



no　wind。　For　the　vapour　could　not　be　raised　unless　the　sky　were　clear；



and　if　a　wind　were　blowing　it　could　not　condense。



　　The　fact　that　hoar…frost　is　not　found　on　mountains　contributes　to



prove　that　these　phenomena　occur　because　the　vapour　does　not　rise



high。　One　reason　for　this　is　that　it　rises　from　hollow　and　watery



places；　so　that　the　heat　that　is　raising　it；　bearing　as　it　were　too



heavy　a　burden　cannot　lift　it　to　a　great　height　but　soon　lets　it



fall　again。　A　second　reason　is　that　the　motion　of　the　air　is　more



pronounced　at　a　height；　and　this　dissolves　a　gathering　of　this　kind。



　　Everywhere；　except　in　Pontus；　dew　is　found　with　south　winds　and



not　with　north　winds。　There　the　opposite　is　the　case　and　it　is　found



with　north　winds　and　not　with　south。　The　reason　is　the　same　as　that



which　explains　why　dew　is　found　in　warm　weather　and　not　in　cold。　For



the　south　wind　brings　warm；　and　the　north；　wintry　weather。　For　the



north　wind　is　cold　and　so　quenches　the　heat　of　the　evaporation。　But　in



Pontus　the　south　wind　does　not　bring　warmth　enough　to　cause



evaporation；　whereas　the　coldness　of　the　north　wind　concentrates　the



heat　by　a　sort　of　recoil；　so　that　there　is　more　evaporation　and　not



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　fol