Geoscience Reference
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sensor technology have created major improvements in resolution and detail of
the infrared images, but not yet to the level of small-scale variations (roof
geometry for example). There has been better use of multi-directional emissions
to improve the accuracy of thermal results, but these are hampered by poor
emissivity information from various surfaces. Progress on finding a relationship
between SUHI and AUHI suggests that if meso-scale advection is reduced,
especially at night, the two values correlate better. Much more research needs
to be completed in this area. Voogt and Oke ( 2003 ) expect better resolution
thermal scanners in the future, higher quality and less expensive surface repre-
sentations, and the development of a portable high-resolution thermal scanner
that can be used at the surface to supplement the satellite data.
7.9 Mitigation of the UHI
Mitigation measures designed to reduce UHI and its impacts have been focused
on the use of trees and green space, and reducing absorption of solar radiation by
increasing surface albedos (Bonan 2002 ; McKendry 2003 ). The research com-
pleted in G¨ teborg, Sweden, clearly established the benefits of urban parks in
reducing the overall UHI (Upmanis et al. 1998 ). In general, the larger the park,
the stronger the temperature difference (up to 6 8C) with the urban center. The
thermal boundary between the park and the city was occasionally sharp (within
200m) but more often the transition occurred over several hundred meters
(0.3 to 0.4 8C per 100m). Park influences on the temperature of the built-up
area can extend to over 1 km, depending on size, but even small parks can have
an influence (to a few tens of meters from the park boundary). Within the larger
parks in particular, cooling rates at night can vary spatially, probably associated
with variations in Ys and evaporation rates.
Trees and green spaces in cities help shade buildings, reducing surface
temperatures and the need for air conditioning. They provide a surface for
interception and the soil allows infiltration of rainfall. Trees can reduce pollution
by physically cleaning the atmosphere, and they act as buffers to wind. Through
transpiration they increase urban evaporative cooling. Trees and parks also
provide a very pleasant social and cultural environment for the people living
and working in the city.
There are some disadvantages, however, which must be accepted. Trees and
parks take up room, potentially limiting development in parts of the city. They
require surface preparation and regular maintenance, which adds costs to local
government and individual budgets. Some trees release natural hydrocarbons,
which can add to the mix of anthropogenic pollutants that create photochemical
smog. If UHI temperatures, and wind speeds, are reduced, then UBL inversion
levels over the city could be reduced, potentially creating more frequent air pollu-
tion episodes. For cities in the northern parts of the Northern Hemisphere, the UHI
may have some important benefits during winter, and reduction may not be desired.
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