第2部分(第3/6 页)

r；　food；　and　energy　of　various　kinds　is　matched　by　an　output　of　sewage；　solid　waste；　air　pollutants；　energy；　and　materials　that　have　been　transformed　in　some　way。　The　quantities　involved　are　enormous。　Many　aspects　of　this　energy　use　affect　the　atmosphere　of　a　city；　particularly　in　the　production　of　heat。

In　winter　the　heat　produced　by　a　city　can　equal　or　surpass　the　amount　of　heat　available　from　the　Sun。　All　the　heat　that　warms　a　building　eventually　transfers　to　the　surrounding　air；　a　process　that　is　quickest　where　houses　are　poorly　insulated。　But　an　automobile　produces　enough　heat　to　warm　an　average　house　in　winter；　and　if　a　house　were　perfectly　insulated；　one　adult　could　also　produce　more　than　enough　heat　to　warm　it。　Therefore；　even　without　any　industrial　production　of　heat；　an　urban　area　tends　to　be　warmer　than　the　countryside　that　surrounds　it。

The　burning　of　fuel；　such　as　by　cars；　is　not　the　only　source　of　this　increased　heat。　Two　other　factors　contribute　to　the　higher　overall　temperature　in　cities。　The　first　is　the　heat　capacity　of　the　materials　that　constitute　the　city；　which　is　typically　dominated　by　concrete　and　asphalt。　During　the　day；　heat　from　the　Sun　can　be　conducted　into　these　materials　and　stored　—　to　be　released　at　night。　But　in　the　countryside　materials　have　a　significantly　lower　heat　capacity　because　a　vegetative　blanket　prevents　heat　from　easily　flowing　into　and　out　of　the　ground。　The　second　factor　is　that　radiant　heat　ing　into　the　city　from　the　Sun　is　trapped　in　two　ways：　by　a　continuing　series　of　reflections　among　the　numerous　vertical　surfaces　that　buildings　present　and　by　the　dust　dome　the　cloudlike　layer　of　polluted　air　that　most　cities　produce。　Shortwave　radiation　from　the　Sun　passes　through　the　pollution　dome　more　easily　than　outg