URBAN EFFECTS ON WEATHER AND CLIMATE 



Figure IV-12 — HEAT ISLAND EFFECT 



(Illustration Courtesy of the American Meteorological Society) 



The figure shows the isotherm pattern for 2320 PST on 4 April 1952 superimposed 

 on an aerial photograph of San Francisco. The relation between the air tempera- 

 ture measured 2 meters above the surface and urban development is evident. A 

 temperature difference of 20°F. was observed on that calm, clear night between the 

 densely built-up business district (foreground) and Golden Gate Park (left rear). 



are reduced or eliminated. At the 

 same time, these actinic rays cause 

 a large number of photochemical 

 reactions in the welter of pollutants. 

 Many of them lead to obnoxious sec- 

 ondary products such as ozone, which 

 irritates mucous membranes, and 

 other equally undesirable products. 

 They cause notable reduction in vis- 

 ibility, which is not only aesthetically 

 objectionable but often detrimental 

 to aviation. Increased haze and fog 

 frequency, compared with the natural 

 environment, is a man-made effect, 

 a fact that becomes impressive be- 

 cause it is demonstrably reversible. 

 In some cities (e.g., London) where 

 the number of foggy days had grad- 

 ually increased over the decades, a 



determined clean-up of domestic fuels 

 and improved heating practices led 

 to immediate reduction in the fog 

 frequency. 



Precipitation — Much less certainty 

 exists about both the local and more 

 distant effects of city-created pol- 

 lutants on precipitation. The already 

 mentioned increased shower activity 

 in summer has probably little or 

 nothing to do with the pollutants. 

 It is primarily a heat effect, with 

 water-vapor release from combustion 

 processes perhaps also playing a role. 

 But we do have a few well-docu- 

 mented wintertime cases when iso- 

 lated snowfalls over major cities were 

 obviously induced by active freezing 



nuclei, presumably produced by some 

 industrial process. There is no in- 

 contestable evidence that over-all 

 winter precipitation over urban areas 

 has increased, but most analyses 

 agree that total precipitation over 

 cities is about 5 percent to, at most, 

 10 percent greater than over rural 

 environs, even if all possible oro- 

 graphic effects are excluded. More 

 spectacular increases observed in the 

 neighborhood of some major indus- 

 trial-pollution sources are probably 

 the effect of sampling errors inherent 

 in the common, inadequate rain- 

 gauge measuring techniques. 



Even so, there is major concern 

 about the very possible, if not al- 



117 



