Microclimate and air pollutants 103Foranenvironmental condition such as temperature, an individual organ-
ism has a somewhat narrow optimum range in which it can survive, grow, and
reproduce (Barbouret al.1987,Gilbert 1991, Odum and Barrett2005). Above and
below that optimum the individual grows, but does not successfully reproduce.
Above and below the growth ranges the organism survives, and beyond the sur-
vival ranges the organism dies.High temperature,for example associated with the
heat-island effect or global climate change, increases an organism’s metabolic
ratewhich in turn may exceed the organism’s ability to get resources. High tem-
perature also accelerates development, such as leaf-bud opening and flowering,
which consequently may be damaged by spring frosts. Still higher temperatures
desiccate plants, and ultimately inactivate enzymes causing death.
Species composition changes with higher temperature, so, for example, on
bird feeders in residential areas, cool-region species are gradually replaced by
warm-region species. Higher temperature may lead to more air pollution and
more severe effects on organisms. A major way to reduce air temperature is
to maintain or restore vegetation cover, particularly of trees (Zipperer and
Foresman 1997). Trees provide cool shade and also pump considerable water
totheair in evapotranspiration, a process that cools the surrounding air. For
example, in Berlin, a small greenspace of about 5--30 ha (12--75 acre) may cool the
air 0.5-- 2◦C (1--4◦F), a medium 30--300 ha greenspace some 2--3.5◦Candalarge
300 ha (750 acre) greenspace may reduce summer air temperature 3.5--5.5◦C
(6--10◦F) (von Stulpnagelet al.1990). Also a large greenspace such as Tiergarten
in Berlin, Mont Royal in Montreal, or Golden Gate Park in San Francisco, cools
summer temperatures in the city for several hundred meters or over a kilometer
downwind (Schmid1975,vonStulpnagelet al.1990).
Thecoastal effectof a major water body such as a large lake or the sea, often
cools air temperature inland for several kilometers in spring and early summer,
and similarly warms the air in autumn. The coastal effect also increases relative
humidity and fog or cloud conditions.
Air-borne particles (particulate matter) in urban regions typically serve as con-
densation nuclei around which water droplets form, producing aerosols. When
cooled, rain falls so that cities and areas downwind of cities often have some-
what elevated rainfall amounts. So, while high temperature tends to desiccate,
greenspaces, street trees, green roofs, and other urban vegetation help to main-
tain moist air.
Wind, another major urban microclimatic factor, comes in three forms
(Brandleet al.1988,Forman1995,Arnfield 2003).Streamline airf lowoccurs in
parallel layers over relatively smooth surfaces such as large fields and smooth
rounded hills.Turbulence, composed of eddies with (usually) up-and-down air