URBAN EFFECTS ON WEATHER AND CLIMATE 



There is no doubt that cities modify 

 their own weather by the local pro- 

 duction of heat and addition of ma- 

 terial to the atmosphere. "Material" 

 includes water vapor (H_0) and car- 

 bon dioxide (COu) as well as the 

 gases and particulates commonly 

 classed as pollutants. City tempera- 

 tures exceed those of similarly ex- 

 posed rural areas, particularly at 

 night, but the most noticeable change 

 is in the solar radiation reaching the 

 ground, which is typically about 10 

 percent below that of upwind sur- 

 roundings. In considering the extent 

 to which effects on weather may 

 overstep the city boundaries, it is 

 convenient to look at three scales — 

 local, regional, and global. "Local" 

 refers to effects downwind of the 

 city at distances up to about 100 

 miles; "regional" to subcontinental 

 areas of the order of 1,000 square 

 miles; and "global" to the whole 

 world. 



Local Effects 



Local effects include deterioration 

 of visibility and reduction of solar 

 radiation, which are not in ques- 

 tion. At 100 miles distance, in New 

 England, one knows when the wind 

 is blowing from New York City. This 

 does not, in general, have repercus- 

 sions on the other weather factors 

 that are large enough to be estab- 

 lished by examining weather records. 

 If there are such effects they are 

 small and probably lost in the general 

 variance, although no very sophisti- 

 cated search has been made — for 

 example, among satellite cloud pic- 

 tures — to verify that speculation. 



In two or three instances, it has 

 been claimed that an increase of 

 precipitation downwind of cities has 

 been established. The best known 

 example is at La Porte, Indiana, where 

 an apparent considerable excess of 

 precipitation over surrounding areas 

 has been associated with industrial 

 activity (particularly steel mills) in 

 the Chicago and Gary, Indiana, areas. 

 This seemed to be a clear-cut case, 



but the skill and/or objectivity of 

 the one observer whose record estab- 

 lished the effect has recently been 

 questioned (with supporting evidence) 

 by other climatologists. In the other 

 cases that have been discussed, in- 

 cluding recent claims of an increase 

 of shower activity downwind of pulp 

 plants in Washington state, the statis- 

 tical evidence offered in support of 

 the hypothesis of modification is less 

 convincing than that for La Porte. 

 Physically, there is doubt whether 

 any precipitation increase that might 

 occur would be an effect of cloud 

 seeding by particulate pollutants or 

 of the increased triggering of convec- 

 tion by the heat and moisture sources 

 of the city. The latter explanation is 

 gaining favor. 



Regional Effects 



On the regional scale there is gen- 

 eral agreement that atmospheric tur- 

 bidity — a measure of the extinction 

 of solar radiation — has increased 

 over the past fifty years in western 

 Europe and eastern North America, 

 even in locations as remote from 

 cities as can be found in these areas. 

 Again, there is no indication that 

 the reduction in solar radiation reach- 

 ing the ground has had any effect 

 on other weather elements. Such 

 connections are extremely difficult to 

 establish, for reasons which will be 

 discussed later when we consider 

 global effects. 



There is, however, one possible 

 regional effect of pollution that is 

 causing international concern, though 

 it would not traditionally be con- 

 sidered a "weather" phenomenon. 

 This is the deposition in precipitation 

 of pollutants transported hundreds 

 of miles from their source, perhaps 

 across international boundaries. The 

 best-known case is the occurrence in 

 Scandinavia of rainfall with an un- 

 usual degree of acidity which has 

 been attributed to the transport of 

 pollutants emitted in Britain and Ger- 

 many. A similar geographical and 



meteorological situation exists in the 

 northeastern United States, where the 

 situation might repay investigation. 

 Persistently acidic rain or snow might 

 have long-term effects on forest ecol- 

 ogy and lead to reduced productivity 

 in forest industries. The connection 

 between the observation and its pre- 

 sumed cause is simply the fact that 

 no other explanation has been con- 

 ceived. Statistical or physical links 

 have not been demonstrated — in- 

 deed, our current ignorance in the 

 fields of atmospheric chemistry and 

 microphysics precludes a convincing 

 physical link. This is potentially one 

 of the most serious of the currently 

 unsolved scientific environmental 

 problems. 



Global Effects 



The possible modification of cli- 

 mate by industrial effluents has been 

 under serious scientific discussion for 

 more than thirty years and the ex- 

 tent, nature, and intractability of the 

 underlying problems is now becom- 

 ing evident. It was postulated in the 

 1930's, and it is now clearly estab- 

 lished, that the atmospheric COn con- 

 tent is increasing as a result of com- 

 bustion of fossil fuel. Radiative 

 transfer calculations indicate that if 

 the CO- content increases, and noth- 

 ing else changes, temperature at the 

 earth's surface will increase. No one 

 ever seriously suggested that "noth- 

 ing else changes," but it was noted 

 that during the first forty years of 

 this century recorded surface tem- 

 peratures did increase. The connec- 

 tion with CO:; increase was noted and 

 extrapolated. There were prophecies 

 of deluge following melting of the 

 polar ice. However, by 1960 it was 

 clear that surface temperatures were 

 falling, and at the same time the 

 continent-wide increase in turbidity 

 was noted. (A global increase cannot 

 be established because a network of 

 suitable observations does not exist.) 

 The obvious connection, on the hy- 

 pothesis that solar radiation at the 

 surface had decreased and nothing 



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