The explanation that has been tentatively advancedto explain this is that there has been a decrease in thereceipt of daytime solar radiation as a result of vehicularatmosphere pollution, whereas at night the presenceof such pollution absorbs and re-emits significantamounts of outgoing terrestrial radiation, maintaininghigher urban nocturnal minimum temperatures. Theurban heat island may also be more marked at night because reduced nocturnal turbulent mixing keeps the warmer air near the surface. In mid-latitude cities such as London, urban heat island effects are generally stronger in summer than in winter because of higher levels of solar radiation being absorbed by building materials during the day .In winter the urban – rural contrast is weaker because solar energy absorption is lower and hence there is less energy to radiate, despite higher levels of urban space heating
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