10 4Natural systems and greenspaces
movements, forms where streamline airflow is disrupted by abrupt boundaries,
such as buildings and cliffs. Vortices are strong helical winds with a vertical axis.
Particles arelifted off surfaces to become air-borne, most by vortices, much by
turbulence, and least by streamlines. Streamline airflow is common in cropland
areas of the urban-region ring. However, turbulent airflows generally predomi-
nate around urban areas where the upper surfaces of buildings and vegetation
are so uneven in height. Turbulence not only lifts particulate matter into the
air, it increases heat loss and desiccates plants, so many species in natural areas
are at a competitive disadvantage in turbulent wind conditions.
On windless nights in hilly or mountainous urban regions,cool air drainage,
thedownslope flow of cool air, pushes a city’s warm air and pollutants upward
and out. This serves as a free cleaning and ventilation system (e.g., Stuttgart,
Germany; Spirn1984,Christina von Haaren, personal communication). The pri-
mary requirement is to keep nearby hillslopes in unbuilt condition, preferably
wooded (Gross 2002) and secondarily to keep high-rise buildings out of the main
valley-bottom air-drainage channels.
Noise is a particular issue in urban areas where it affects people and ani-
mals (Formanet al.2003,Miller2005). At modest noise levels, some species and
people become habituated (accustomed) to noise so that it no longer disturbs
them. However, traffic noise from roads and highways with more than about
10 000 vehicles passing per day appears to significantly degrade bird commu-
nities nearby, with the width of the degradation zone increasing with traffic
volume (Reijnenet al.1995, 1996,Formanet al.2002). In addition to traffic vol-
ume and distance from road, the proportion of truck traffic is significant in
determining noise levels. Not surprisingly, busy commuter rail lines also have
significant, though less-known, ecological effects. Also artificial night lighting
has a range of ecological effects around cities (Rich and Longcore 2006).
Several types of air pollutants are of major ecological importance in urban
regions, and also cause human health problems (Smith 1981, Gilbert 1991 ,Craul
1999,Nowak1994,Santamouris2001,Formanet al.2003):(1) Ozone(O 3 ), which forms photochemical smog (by combining with other
chemicals in sunlight); both can damage many plants).
(2) Hydrocarbons,which are liberated in fossil fuel combustion often cover
and alter soils, and are also part of photochemical smog.
(3) Carbon monoxide(CO), which presumably can kill animals in local spots.
(4) Carbon dioxide(CO 2 )andmethane(CH 4 ), which are global greenhouse
gases leading to higher temperature, sea-level rise, and more extreme
weather events.