5. URBAN EFFECTS ON WEATHER AND CLIMATE 



Urbanization and Weather 



For centuries, man has speculated 

 that major battles, incantations, large 

 fires, and, lately, atomic explosions 

 could affect weather, although he 

 made no serious scientific attempts 

 to modify weather until 25 years ago. 

 Except for a few localized projects 

 involving precipitation increases and 

 fog dissipation, however, man's in- 

 tentional efforts have yet to pro- 

 duce significant, recognized changes. 

 Rather, the major means whereby 

 man has affected weather have been 

 inadvertent — through his urban en- 

 vironment. 



Growing Awareness of the Problem 



As long ago as 700 years or more, 

 London had achieved a size great 

 enough to produce a recognizable ef- 

 fect on its local weather, at least in 

 terms of reduced visibility and in- 

 creased temperature. Since major ur- 

 ban areas became prevalent in Europe 

 following the Industrial Revolution, 

 Europeans have directed considerable 

 scientific attention to this problem 

 of urban-induced weather change. 

 Now that major urban-industrial com- 

 plexes exist in many countries, world- 

 wide attention has grown rapidly, 

 particularly in the United States, 

 where the growth of megalopolitan 

 areas during the past ten to thirty 

 years has brought with it increasing 

 public and scientific awareness of the 

 degree and, in some cases, the seri- 

 ousness of urban effects on weather. 

 Recent studies documenting signif- 

 icant urban-related precipitation in- 

 creases in and downwind of Chicago, 

 St. Louis, and industrial complexes in 

 the state of Washington have further 

 focused scientific and public attention 

 on the urban-weather topic and its 

 considerable potential. 



Certainly, even the casual observer 

 is aware that visibility is more fre- 

 quently restricted in a major urban 

 complex than in rural areas, and that 

 this has come from smoke, other con- 

 taminants, increased fog, and their 

 additive, smog. Most Americans are 

 now aware that the temperature with- 

 in a medium-to-large city is generally 

 higher at any given time of the day 

 or season than it is in rural areas. 

 This temperature effect has been rec- 

 ognized and measured for many 

 years, since its measurement, at least 

 at the surface, is relatively easy. "Heat 

 islands" for many cities of various 

 sizes have been well documented. 



Urban areas also act as an obstacle 

 to decrease winds near the surface, 

 to increase turbulence and vertical 

 motions above cities, and to create, 

 occasionally, a localized rural-urban 

 circulation pattern. There have been 

 enough descriptive studies, further- 

 more, to reveal that many other 

 weather conditions are also being 

 changed, often dramatically, by urban 

 complexes. Although available re- 

 sults indicate that urban-induced 

 weather changes are restricted to the 

 cities and their immediate downwind 

 areas and have little effect on macro- 

 scale weather conditions, the "urban 

 flood" and advent of the megalopolis 

 could conceivably lead to significant 

 weather changes over large down- 

 wind regions. 



Value Judgments — The question 

 of desirability of the weather changes 

 wrought by urbanization has only re- 

 cently been considered. The fact that 

 many of the urban-induced changes 

 have occurred gradually has not only 

 made them difficult to measure quan- 

 titatively within the natural variabil- 

 ity of weather, but has also made 



them less obvious and, therefore, un- 

 wittingly accepted by the urban 

 dweller. Now that urbanization is 

 nearly universal, American citizens 

 have suddenly become aware of 

 many of the urban-induced weather 

 changes. In general, such changes as 

 increased contaminants, higher warm- 

 season temperatures, lower winds, 

 added fog, increased thunder and hail, 

 added snowfall, and decreased visibil- 

 ity are considered undesirable. Cer- 

 tain urban-related weather changes 

 are desirable, however, including 

 warmer winters and additional rain- 

 fall to cleanse the air and to add water 

 in downwind agricultural areas. 



In summary, then, with respect to 

 their effects on weather, urban areas 

 sometimes act as volcanoes, deserts, 

 or irregular forests; as such, they pro- 

 duce a wide variety of weather 

 changes, at least on a local scale, and 

 these changes can be classed as bene- 

 ficial or detrimental depending on the 

 locale and the interests involved. 



Type and Amount of 

 Weather Change 



The changes in weather wrought 

 by urbanization include all major 

 surface weather conditions. The list 

 of elements or conditions affected in- 

 cludes the contaminants in the air, 

 solar radiation, temperature, visibil- 

 ity, humidity, wind speed and direc- 

 tion, cloudiness, precipitation, atmos- 

 pheric electricity, severe weather, and 

 certain mesoscale synoptic weather 

 features (e.g., it has been noted that 

 the forward motion of fronts is re- 

 tarded by urban areas). (See Figure 

 IV-11) 



The degree of urban effect on any 

 element will depend on the climate, 



113 



