Image Processing Reference
In-Depth Information
Reduction of vegetation cover impacts both of these
environmental conditions. Removal of vegetation perturbs
both mass and energy fluxes through the urban environment.
The net results are generally higher nighttime temperatures
and higher ozone and particulate concentrations in the air.
Higher nighttime temperatures have a well-documented
effect on heat stress during heat events (Curriero et al.
2002 ). Greater absorption of solar radiation by solid surfaces results in time delayed
re-radiation of the energy as heat. This, in turn, results in increased nighttime tem-
peratures. High nighttime temperatures are believed to contribute to the physical
stress of high daytime temperatures by forcing the human body to devote energy to
cooling for sustained periods of time. Heat-induced illnesses and deaths in large cit-
ies have been noted at least since the early part of this century (Gover 1938 ).
The presence and abundance of vegetation in urban areas may also influence air
quality and human health (Wagrowski and Hites 1997 ). Air pollutants, such as ozone
and fine (~2.5 mm) particulate matter (PM 2.5 ), have measurable adverse public health
impacts (Kinney 1999 ). The primary sources of these pollutants are motor vehicles,
power generation plants and heavy industry. Meteorology also influences the produc-
tion of some pollutants, with higher temperatures promoting the atmospheric reactions
which form ozone and secondary fine particles. Because vegetation provides abundant
surface area in contact with the atmosphere, the leaves serve as sinks for these surface
reactant pollutants. Through leaf stomata, trees can directly remove pollutant gases
such as CO, NO x , O 3 and SO 2 (Smith 1984 ; Fowler 1985 ; Nowak 1994 ; Taha et al.
1997 ). Urban trees play contributing roles in sequestering CO 2 and consequently
reduce global warming (Akbari 2002 ). In this sense, urban vegetation serves as in situ
air filters. On the other hand, urban vegetation experiences both short- and long-term
phenological changes and may itself be sensitive to subtle changes in environmental
conditions. While changes in the built component of the urban environment are often
documented at some level of detail, phenological changes in urban vegetation are not
under direct human control and are not generally monitored.
higher nighttime
temperatures may
cause heat-
induced illnesses
and deaths in
large cities
Remote Sensing and Urban Environmental Conditions
Information derived from airborne and/or satellite remote sensing, which is
instantaneous and does not require time synchronization, could be valuable to
city planners in establishing effective landscape policies, zoning, and greeni-
fication. Thermal infrared remote sensing offers an unparalleled technique to
measure radiant temperature, which is essential for the understanding of all
physical, biological, and chemical systems on the Earth, including urban
climate (Norman et al. 1995 ). The use of thermal observations have been
limited to some extent due to the difficulty in obtaining emissivity values
for the various surface materials as well as the incorporation of other vari-
ables that affect thermal radiance and energy partitioning at the surface in
thermal models. Nevertheless, several useful studies on temperature and
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