The relationship between maximum heat islandintensity and urban population, the sky-view factor(a measure of building height and density), and theimpermeable surface coverage of cities is shown inFigure 7.9. The heat island intensity increases with allthree indices (Nakagawa, 1996).In many older towns and cities in western Europe andNorth America, the process of ‘counter-urbanization’has in recent years led to a decline in population, andit is worth considering whether this is reflected in adecline in the intensity of urban heat islands. Oneattempt to do this, in the context of London (Lee, 1992),revealed the perplexing finding that the heat-islandintensity has decreased by day, but increased by night.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 nightbecause reduced nocturnal turbulent mixing keeps thewarmer air near the surface. In mid-latitude cities suchas London, urban heat island effects are generallystronger in summer than in winter because of higherlevels of solar radiation being absorbed by buildingmaterials during the day.
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