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the climatic conditions of a large modern city as created by the city itself.
The roofs and walls of urban structures and the artificial surfaces of streets are warmed to a greater degree than are natural surfaces and thus increase the air temperature within a city. Industrial enterprises, heating systems, and motor vehicles heat the air over a city and pollute it with haze and the gaseous products of combustion, thereby increasing the number of condensation nuclei. As a result, there have been instances in which the duration of solar radiation in cities has decreased by 25–30 minutes per day (London), the amount of solar radiation has decreased (an average of 15 percent in the large cities of the USA), and the air temperature has risen, especially during the nights and winters. The average air temperature of a large city is l°-2° C higher than that of nearby areas, and the maximum temperature differential between them reaches 5°-8° C and more. Resulting air flows from the suburbs toward city centers (urban breezes) have been observed, and there has been an increase in the upward air movement over cities, a phenomenon accompanied by the formation of clouds. Cities have fewer days with snow and longer frostless periods and growing seasons for gardens. The relative humidity of the cities’ air has decreased an average of 6 percent while overcasts and annual rainfall have increased 10–15 percent.
The increased amount of condensation nuclei in city air along with lowered wind velocity (an average of 25 percent) is causing an increased incidence of fog (a 30-percent increase during summer and a 100 percent increase during winter in the USA) and an increase in its intensity. Smoky fog (smog) in a number of large cities is leading to an increase in the sickness and death rates, particularly from pulmonary illnesses and cardiovascular disorders. The diffusion of temperatures, air pollution, and wind direction and velocity depend on the arrangement of streets, parks, and green zones. The general principles of urban climates must be taken into consideration when planning new cities and city regions.
REFERENCESSheleikhovskii, G. V. Zadymlenie gorodov. Moscow-Leningrad, 1949.
Kratser, P. A. Klimat goroda. Moscow, 1958. (Translated from German.)
S. P. KHROMOV