PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM 



ready probable, effects of city-pro- 

 duced pollutants on precipitation 

 processes. One of them can be 

 caused by the high emission rates 

 of minute lead particles from the 

 tetraethyl-lead additive to gasoline. 

 Some of these particles combine with 

 iodine brought into the atmosphere 

 primarily from oceanic sources to 

 form lead-iodide. This compound 

 has been shown to form very efficient 

 and active freezing nuclei, which can 

 trigger precipitation processes in only 

 slightly sub-cooled cloud droplets. 

 The lead particles are so small that 

 they will stay in suspension for long 

 distances and thus trigger precipita- 

 tion at places far removed from the 

 sources of the lead. Even more 

 ominous could be the swamping of 

 the atmosphere by condensation nu- 

 clei. These are produced in urban 

 areas in prodigious amounts in con- 

 centrations surely two orders of mag- 

 nitude higher than in uncontaminated 

 air. There are literally hundreds of 

 thousands of these nuclei in a cubic 

 centimeter, and even the most hygro- 

 scopic of them competes for the 

 available moisture in the air. The 

 more nuclei there are, the more likely 

 it is that the cloud droplets that form 

 will be very small because of the 

 large number of competing centers 

 around which condensation occurs. 

 Small cloud droplets have more dif- 

 ficulty in coalescing and forming rain 

 than large droplets. Hence it is quite 

 possible, although not proven beyond 

 doubt, that in some urban areas or 

 downwind from them a decrease in 

 rainfall could occur. This is one of 

 the effects requiring careful watch in 

 future research. 



Atmosplieric Stagnation — When 

 weather conditions favor slight winds 

 and surface temperature inversions, 



air layers in metropolitan areas be- 

 come veritable poison traps. These 

 can lead to the well-known health- 

 endangering pollution episodes. With 

 a number of metropolitan areas in 

 close proximity, a slight ventilation 

 will waft pollutants into the next 

 series of settlements within a few 

 hours or days and aggravate the situ- 

 ation there. This type of accumula- 

 tion has not been adequately investi- 

 gated either. But the whole area of 

 the United States east of the Ap- 

 palachians from northern Virginia to 

 southern Maine may be affected by 

 cumulative pollution effects. There 

 are also other megalopolitan areas in 

 the country that may need similar 

 attention. Computer simulation of 

 such atmospheric-stagnation periods 

 has made some progress but is still 

 severely restricted by the inadequacy 

 of the mathematical models and the 

 lack of sufficient actual observations. 



Many of the micrometeorological 

 alterations brought about by urbani- 

 zation have been well documented in 

 a number of cities. They have re- 

 cently been followed, stey by step, in 

 a rural area that is in the process of 

 becoming urbanized — the new town 

 of Columbia, Maryland, where popu- 

 lation density has increased from a 

 few hundred to a few thousand in- 

 habitants and will increase to a hun- 

 dred thousand in the current decade. 

 Many of the characteristic changes 

 in temperature, wind, humidity, and 

 runoff are already observable. This 

 continuing study in a planned, grow- 

 ing community may greatly further 

 our knowledge of the micrometeoro- 

 logical changes. 



Implications for Town Planning 



It is proper to ask whether we can 



turn this knowledge to use in future 

 town planning and redevelopment of 

 older cities. The answer is affirma- 

 tive. Natural environments charac- 

 teristically have a varied mosaic of 

 microclimatic conditions, most of 

 which are destroyed by urbanization. 

 The detrimental effects are primarily 

 introduced by compact construction 

 with few interruptions, creating an 

 essentially new surface of roofs at 

 which energy interactions take place. 

 In many urban areas, vegetation has 

 been sharply diminished or even com- 

 pletely eliminated. Reversal of this 

 trend will bring about a desirably 

 diversified pattern of microclimate. 

 Two tall buildings with large green 

 and park areas surrounding them are 

 far preferable to the typical row 

 house or walk-up slum configura- 

 tion. The open construction charac- 

 teristic of suburban areas has caused 

 little climatic deterioration of the 

 environment. 



Air pollution will remain a prob- 

 lem. There is some merit in using 

 tall stacks for the effluents from 

 stationary sources. Appropriate loca- 

 tion, predicated on the general re- 

 gional airflow patterns, is indicated 

 for industrial sources of pollutants. 

 There is little substantive knowledge 

 on possible amelioration of pollutants 

 from mobile sources through highway 

 routing, construction, elevation, or 

 other engineering techniques. Con- 

 trol at the source seems to offer the 

 only tenable solution over the long 

 run. 



Too little is yet known about the 

 sinks of pollutants in urban areas, 

 although shrubbery and insensitive 

 plants seem to offer some help by 

 intercepting particulates. 



Urban Effects on Weather — the Larger Scales 



The possibility that human activi- 

 ties might be modifying large-scale 

 weather patterns or even the global 



climate has received much publicity. 

 The present state of atmospheric sci- 

 ence does not allow either firm sup- 



port or confident refutation of any 

 of the effects which have been pos- 

 tulated. 



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