Geoscience Reference
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Figure 12.23 Configurations of urban
pollution. (A) Urban pollution dome.
(B) Urban pollution plume in a stable
situation (i.e. early morning following
a clear night). Fanning is indicative
of vertical atmospheric stability. (C)
Pollution plume northeast of St Louis,
Missouri, on 18 July 1975.
Sources : (B) After Oke (1978); (C) After
White et al . (1976) and Oke (1978).
observed from high-flying aircraft to extend almost to
Washington, DC, 950 km away.
The impacts of air pollution include: direct mete-
orological effects (on radiative transfer, sunshine,
visibility, fog and cloud development), greenhouse gas
production (by release of CO 2 , CH 4 , NO x , CFCs and
HFCs), photochemical effects (tropospheric ozone
formation), acidification (processes involving SO 2 , NO x
and NH 3 ), and societal nuisance (dust, odour, smog)
affecting health and the quality of life especially in
urban areas.
resulting from energy consumption by combustion,
which in some cities may even exceed R n during the
winter. Although R n may not be greatly different from
that in nearby rural areas (except during times of sig-
nificant pollution) heat storage by surfaces is greater (20
to 30 per cent of R n by day), leading to greater nocturnal
values of H ; LE is much less in city centres. After long,
dry periods, evapotranspiration may be zero in city
centres, except for certain industrial operations, and in
the case of irrigated parks and gardens, where LE may
exceed R n . This lack of LE means that by day 70 to 80
per cent of R n may be transferred to the atmosphere as
sensible heat ( H ). Beneath the urban canopy, the effects
of elevation and aspect on the energy balance, which
may vary strikingly even within one street, determine
the microclimates of the streets and 'urban canyons'.
2 Modification of the heat budget
The energy balance of the built surface is similar to soil
surfaces described above, except for the heat production
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