PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM 



Figure IV— 1 1 —WEATHER CHANGES RESULTING FROM URBANIZATION 



The table summarizes changes in surface weather conditions attributable to urban- 

 ization. Changes are expressed as percent of rural conditions. 



nearness to major water bodies, on 

 topographic features, and city size 

 and components of the industrial 

 complex. Furthermore, the amount of 

 effect on the weather at any given 

 time depends greatly on the season, 

 day of the week, and time of day. 

 Thus, urban solar radiation is de- 

 creased much more in winter than 

 summer; is decreased on weekdays; 

 and is decreased more in the morning 

 than in the afternoon. Temperature 

 increases resulting from the heating 

 of urban structures are much greater 

 in winter than in summer; hence, the 

 average urban air temperature in win- 

 ter is 10 percent higher than that in 

 rural areas, whereas in summer it is 

 only 2 percent higher. However, ur- 

 ban temperatures during certain sea- 

 sons and weather conditions can be 

 as much as 35 percent higher or 5 

 percent lower than nearby rural tem- 

 peratures. 



It should be emphasized that op- 

 posite types of changes in certain 

 weather conditions are produced at 

 different times. For example, fog is 

 generally increased by urbanization, 

 although certain types of fogs are ac- 

 tually dissipated in large cities. Wind 

 speeds are generally decreased, but 

 they increase in some light wind con- 



ditions. Snowfall is generally in- 

 creased by urban areas, but under 

 certain conditions the city heat actu- 

 ally melts the descending snow, trans- 

 forming it into rain. 



Current Scientific Status 



Most studies of urban effects on 

 weather have been descriptive and 

 based on surface climatic data. Fur- 

 thermore, only a few studies have at- 

 tempted to investigate the causative 

 factors and the physical processes in- 

 volved in urban-produced weather 

 changes. Without careful investiga- 

 tions of the processes whereby urban 

 conditions affect the weather, there is 

 little hope for developing an adequate 

 understanding and, hence, predictive 

 capabilities. 



Data Base — The present data base 

 is woefully inadequate for studies 

 of most urban-affected weather ele- 

 ments. Two-dimensional spatial de- 

 scriptions of urban effects on weather 

 elements are now adequate only for 

 temperature patterns. Data for 

 weather changes in the vertical are 

 totally inadequate for temperature as 

 well as for all other weather elements. 



Descriptive types of urban-weather 

 studies based on existing historical 

 records tend to be seriously limited in 

 their spatial information. For instance, 

 studies of urban-rural fog differences 

 have typically been based on surface 

 values from a point in the central city 

 and one at the airport; although these 

 may indicate a 30 percent difference, 

 they fail to describe the horizontal 

 distribution of fog over the urban 

 or rural environs. 



Unfortunately, adequate descrip- 

 tions of the surface weather changes 

 are not available for most metropoli- 

 tan areas of the United States. Study 

 of the urban-weather relationships in 

 the United States has been much 

 more limited than that in Europe be- 

 cause the surface weather-station net- 

 works in and around American cities 

 have been too sparse. Information 

 useful for such practical problems as 

 city planning can be developed for 

 major U.S. metropolitan centers only 

 on the basis of thorough comparative 

 studies of data from denser urban- 

 rural surface networks than currently 

 exist around most American cities. 



Instrumentation — Satisfactory 

 tools to perform needed monitoring 

 and study of urban-induced weather 

 changes are available. Major advances 

 in the development of airborne equip- 

 ment to measure meteorological vari- 

 ables and aerosols provide the poten- 

 tial for obtaining the vertical data 

 measurements needed to develop 

 time-dependent, three-dimen- 

 sional descriptions of the weather ele- 

 ments around cities. Field studies of 

 the airflow and vertical temperature 

 distributions at Cincinnati and Fort 

 Wayne, Indiana, have used these new 

 instruments and techniques in pio- 

 neering research. 



Theory and Modeling — The basic 

 theoretical knowledge and formulas 

 exist for understanding the atmos- 

 pheric chemistry and physics in- 

 volved in urban-weather relation- 

 ships. Ultimately, studies of the 

 urban factors that affect weather 

 elements will provide the inputs 



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