Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 242

occur when winds are less than 2m s 1 and relative humidity is high. These
conditions mainly occur around midday. High temperatures reduce the relative
humidity, and higher wind speeds mix and disperse the pollutants. The overall
reduction in shortwave radiation has created lower mean annual maximum
temperatures in rural areas on the downwind side of the urban center, associated
with the weak transport of the pollution layer. These conditions are broadly
representative of many other cities around the globe, but in larger cities such as
Mexico City, especially in the most built-up areas, the UHI influence is domi-
nant over any cooling created by the air pollution layer.
Air pollutants may either enhance or diminish urban climate development and
impact (see Table 7.3 ). During the day, when fine particulates and ozone scatter
and absorb incoming shortwave radiation, its availability at ground level is
reduced. However, through absorption, particulates and ozone also increase
the radiative heating of the boundary layer (Arnfield 2000 ). This increases
stability and reduces sensible heat flux, but potentially also increases downward
longwave radiation toward the surface. At night, warm urban surfaces release
excess longwave radiation to the atmosphere, which can be absorbed by the
atmospheric pollutants, and then re-released back toward the surface, enhancing
the UHI.
7.8 Remote sensing and the UHI
The use of remote sensing to assess the spatial and temporal variations in UHI
and other urban characteristics has some major benefits in the study of urban
climates. Remotely sensed information has been used to define land use varia-
tions, to evaluate the strength of the UHI, and to help understand the process of
urban surface-atmospheric exchanges. The current state of remote sensing in
urban climate studies is extensively reviewed by Voogt and Oke ( 2003 ). There is
considerable potential for detailed UHI evaluation (Figure 7.6 ), but also some
major complications to overcome.
Infrared thermal imagery from various satellite sensors (ATLAS, LANDSAT,
AVHRR and so forth) is used for surface heat-release assessment, in wave-
lengths within the 8-12 mm band (the ''atmospheric window'' in the infrared,
where longwave radiation under clear skies escapes to space with minimal
gaseous absorption). The visual result, presented as a series of squares (or
pixels), can provide an attractive picture of the variation in surface temperatures
in an urban environment. An example is presented in Figure 7.11 (Plate 6 ) for
Phoenix, Arizona. The thermal pattern in Figure 7.11 suggests that there are
urban-rural temperature differences on the order of 5 8C, depending on the
surface.
Thus major advantages are that spatial distributions can be assessed, and areas
of ''hot'' or ''cold'' thermal emissions detected (Roth et al. 1989 ). A series of
satellite images taken over time can allow an assessment of how diurnal changes