Cities and local climate

Anyone who lives in suburban areas and works in the city has surely experienced the differences in the climate of large urban conglomerations compared to the climate in zones that are far from the city. Cities are generally much warmer than the surrounding areas, besides being more polluted, and in the winter months the conditions for thermal inversion are often present. In other words, large cities seem to modify local climatic conditions. What is this phenomenon due to?
The progressive replacement of land and vegetation by tar, asphalt and cement is the first cause. These materials absorb large quantities of heat, that they then release slowly, behaving as “heat wells”.   In this manner, the temperatures in the cities are sensibly higher than in the surrounding areas. Secondly the fact that the buildings that are grouped together, close to one another, hinders air circulation, which also favours the heating process. The progressive increase of cement surfaces prevents infiltration of water in the ground, therefore the land that is built up with cement and covered with asphalt is less humid than natural soil. The air in the cities, therefore is generally dryer. Since the evaporation of atmospheric humidity contributes to cooling the air itself, in this case this characteristic favours heating of the air and also a slower cooling after sunset. For this reason the inhabitants in the cities do not experience the night’s cooling effect on warm summer nights.
In summer, most of the heat released by the large cities favours convectional phenomena and the formation of storms. Apparently the presence of very high buildings seems to favour the formation of cumulonimbus clouds. Also the greater emission of polluting substances in the form of gases and dusts, that are typical of industrialized areas with a high population density, contribute to modifying the characteristics of the atmosphere, in particular the capacity to absorb and radiate heat. Statistical studies show that the increase in the temperatures in the cities is proportional to the density of the population.

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