第2部分(第4/6 页)

oing　longwave　radiation　does；　the　latter　is　absorbed　by　the　gaseous　pollutants　of　the　dome　and　reradiated　back　to　the　urban　surface。

Cities；　then；　are　warmer　than　the　surrounding　rural　areas；　and　together　they　produce　a　phenomenon　known　as　the　urban　heat　island。　Heat　islands　develop　best　under　particular　conditions　associated　with　light　winds；　but　they　can　form　almost　any　time。　The　precise　configuration　of　a　heat　island　depends　on　several　factors。　For　example；　the　wind　can　make　a　heat　island　stretch　in　the　direction　it　blows。　When　a　heat　island　is　well　developed；　variations　can　be　extreme；　in　winter；　busy　streets　in　cities　can　be　17℃　warmer　than　the　side　streets。　Areas　near　traffic　lights　can　be　similarly　warmer　than　the　areas　between　them　because　of　the　effect　of　cars　standing　in　traffic　instead　of　moving。　The　maximum　differences　in　temperature　between　neighboring　urban　and　rural　environments　is　called　the　heat…island　intensity　for　that　region。　In　general；　the　larger　the　city；　the　greater　its　heat…island　intensity。　The　actual　level　of　intensity　depends　on　such　factors　as　the　physical　layout；　population　density；　and　productive　activities　of　a　metropolis。

The　surface…atmosphere　relationships　inside　metropolitan　areas　produce　a　number　of　climatic　peculiarities。　For　one　thing；　the　presence　or　absence　of　moisture　is　affected　by　the　special　qualities　of　the　urban　surface。　With　much　of　the　built…up　landscape　impenetrable　by　water；　even　gentle　rain　runs　off　almost　immediately　from　rooftops；　streets；　and　parking　lots。　Thus；　city　surfaces；　as　well　as　the　air　above　them；　tend　to　be　drier　between　episodes　of　rain；　with　little　water　available　for　the　cooling　process　of　evaporation；　relative　humidities　are　usually　lower。　Wind　movements　are　also　modified　in　cities　becau