Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 219

Cities and their inhabitants also have a broader impact, influencing directly
and indirectly climates beyond their boundaries. The ''footprint'' of urban
areas extends downwind as a consequence of the transport of pollutants, and
downriver as a consequence of storm water and sewerage flows; regionally as
a consequence of the construction of dams to provide water, and intensive
agriculture to provide food to sustain city inhabitants; and globally through
enhancement of greenhouse gases and aerosols, significant fractions of which
are emitted in urban environments.
In this essay, attention is directed primarily to the climatic processes and
effects within and near cities. It is important to recognize that there is no one
urban climate or effect. Urban areas are not always hotter, drier, or windier
than the pre-existing or surrounding landscape, a consequence of the vari-
ability of cities in terms of their surface cover and roughness, energy use and
emissions, and regional setting (cities in arid settings have different apparent
effects on the pre-existing or regional climate to those constructed in tropical
rainforests or coastal wetlands). This essay will consider some of the critical
issues that must be considered when documenting urban climates and identify
key factors that influence the magnitude of effects.
7.3.2 The variability of cities and the importance of scale
Globally, cities are highly variable, not only in terms of size and population,
but in terms of the key properties of the surface that influence energy, mass
and momentum exchanges and thus climate. Both within and between cities,
roughness (the size, shape, and separation of buildings and vegetation) and
surface cover (the radiative, thermal, and moisture properties of all facets of
the urban fabric and their spatial arrangement) vary. Commercial, industrial,
suburban, downtown, areas of different cities are not the same (see by way of
example Figure 7.3 ). Contrast, for example, central areas of European and
North American cities, or downtown residential neighborhoods with suburbs
on the fringes of cities, the products of urban sprawl.
Key to interpreting and/or modeling urban effects on atmospheric pro-
cesses is a conceptual understanding of the spatial scales at which urban
geometry and surface materials vary, and thus the appropriate scales at which
to study climatic effects. Based on Oke ( 1984 ), three spatial scales, micro,
local, and meso are commonly recognized based on the natural range of
scales in urban morphology (Figure 7.4 ). At the micro scale (10 1 -10 2 m),
where issues of dispersion around buildings or within a canyon are relevant,
important spatial differences in processes occur in response to variability in
building dimensions or canyon height-to-width ratios and orientations. At the
local scale (10 2 -10 4 m), processes represent the integrated response of an
array of buildings, vegetation, and paved surfaces. At this scale, spatial