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Applying the principles the radiation and heat balances at the interface of the atmosphere and the solid ground surface, we can anticipate the impact of man as his cities spread, replacing a richly vegetated countryside with blacktop and concrete. Not only do the thermal properties of the surface change, but also hydrologic factors of evaporation and transpiration.
In the urban environment, the absorption of solar radiation causes higher ground temperatures for two reasons. First, foliage of plants is absent, so that the full quantity of solar energy falls upon the bare ground. Absence of foliage also means absence of transpiration, which elsewhere produces a cooling of the lower air layer.
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A second factor is that roofs and pavements of concrete and asphalt hold no moisture, so that evaporative cooling cannot occur as it would from a moist soil The thermal effect is that of converting the city into a hot desert. The summer temperature cycle close to the pavement of a city may be almost as extreme as that of the desert floor. This surface heat is conducted into the ground and stored there. The thermal effects within a city are actually more intense than on a sandy desert floor, because the capacity of solid concrete, stone, or asphalt to conduct and hold heat is greater than that of loose, sandy soil. Because of more rapid conductivity,
the solid materials absorb heat to a greater depth than loose, dry soil in a given period of heating. An additional thermal factor is that vertical masonry surfaces absorb insolation or reflect it to the ground and to other vertical surfaces. The absorbed heat is then radiated back into the air between buildings.
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As a result of these changes in the radiation and heat balance, the central region of a city typically shows summer air temperatures several degrees higher than for surrounding suburbs and countryside. This phenomena leads to the formation of heat island over the city.
