PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM 



The Influence of Urban Growth on Local and Mesoscale Weather 



The fact that large human settle- 

 ments change the atmospheric con- 

 ditions in their immediate vicinity 

 has been recognized for over a cen- 

 tury. Up to very recently, however, 

 it was considered that these influences 

 were strictly local in character. Anal- 

 ysis in depth has shown that this 

 may not be the case at all and that 

 urban influences on the atmosphere 

 may well reach considerably beyond 

 the urban confines. 



The causes for effects of towns on 

 weather and climate are easily traced. 

 First, human activities, especially 

 combustion processes, produce heat. 

 In some cities in northern latitudes 

 during the winter this added energy 

 may be a sizable fraction of the 

 solar energy impinging on the same 

 area. In recent years, airconditioning 

 has also been adding heat to the air 

 in summer by dumping the excessive 

 indoor heat into the surrounding at- 

 mosphere. 



The energy balance is further al- 

 tered because urban surfaces replace 

 vegetation of low heat capacity and 

 heat conductivity with stony surfaces 

 of high heat capacity and heat con- 

 ductivity. These same urban sur- 

 faces also alter the water balance. 

 Rain runs off rapidly, diminishing 

 the natural system of evaporation 

 and evapotranspiration, not only fur- 

 ther altering the energy balance by 

 reducing evaporative cooling but also 

 throwing great burdens on drainage 

 and runoff systems at times of intense 

 precipitation. 



Compact areas of buildings and 

 dwellings also alter the natural air 

 flow. They create considerable aero- 

 dynamic roughness. This may cause 

 changes in the low-level wind profiles 

 up to several thousand feet in the 

 atmosphere. 



Most important, probably, is the 

 effect of cities on atmospheric com- 



position, not only locally but even 

 for many dozens, if not hundreds, of 

 miles downwind. Literally hundreds 

 of different chemical compounds from 

 industrial and combustion processes 

 are blown into the atmosphere. The 

 blind faith of the past trusted that 

 friendly air currents would dilute and 

 dispose of them harmlessly. Yet 

 many of these admixtures have be- 

 come semi-permanent residents of the 

 atmosphere, where they undergo fur- 

 ther chemical change through the im- 

 pact of solar radiation and by inter- 

 action with the water vapor in the 

 atmosphere. 



Meteorological Changes and 

 their Consequences 



Many of the meteorological altera- 

 tions in urban areas have been quan- 

 titatively assessed. Most of them are 

 universally agreed to. In enumerat- 

 ing them we proceed from the sim- 

 pler to the more complex and, almost 

 in parallel, from the noncontrover- 

 sial to the controversial aspects of the 

 problem. 



The Water Balance — It is per- 

 fectly obvious that, by replacing the 

 naturally spongy vegetative surface 

 with impervious roofs, parking lots, 

 and streets, any falling rain will 

 quickly run off. Indeed, urban drain- 

 age systems are designed to carry 

 the waters rapidly into streams and 

 rivers. The consequence is that flood 

 waters may gather more rapidly and, 

 in case of excessive rainfalls, not only 

 increase crests but also cause rapid 

 flooding of low-lying districts in ur- 

 ban areas. The lag time of flood 

 runoff may be cut in half by the 

 impervious areas. 



Heat Islands — The excess energy 

 production of a city and its altered 

 heat balance, because of changes in 

 albedo and heat characteristics of 

 the man-made surface, creates one 

 of the most notable atmospheric 



changes in urban areas. It has been 

 given the very descriptive label "heat 

 island." This term designates a tem- 

 perature excess that covers the urban 

 area. It is most pronounced in the 

 sectors of highest building and popu- 

 lation concentrations; on calm, clear 

 nights it can reach or even exceed 

 10 Farenheit compared with rural 

 surroundings. (See Figure IV-12) Re- 

 cent experiments have shown that a 

 single block of buildings will produce 

 a measurable heat-island effect. At 

 the same time, the reduced evapora- 

 tion caused by rapid runoff and re- 

 duced vegetation as well as this tem- 

 perature increase reduces the relative 

 humidity at the surface. 



Wind Circulation — The previously 

 mentioned increase in surface rough- 

 ness causes decreased wind speed at 

 the surface. The heat island also 

 induces wind convergence toward the 

 urban area. In daytime, the highly 

 overheated roof and black-top sur- 

 faces create convective updrafts, es- 

 pecially in summer. The updrafts 

 induce a higher degree of cloudiness 

 over the city and contribute to the 

 release of showers over the city. 

 At night, inversions of temperature 

 form over the rural and suburban 

 areas while temperature-lapse con- 

 ditions continue in a shallow layer 

 over the city core. This temperature 

 distribution induces a closed circu- 

 lation system within a metropolitan 

 area, which in turn contributes to 

 concentrations rather than dispersion 

 of pollutants when the general wind 

 circulation is weak. 



Solar Radiation — Pollutants act in 

 an important way on the incoming 

 solar radiation. The aerosol absorbs 

 and scatters the solar radiation, af- 

 fecting principally the shorter wave- 

 lengths. This means that the long- 

 wave ultraviolet radiation is radically 

 weakened and its possible beneficial 

 effects as killer of germs and activa- 

 tor of vitamin D in the human skin 



116 



