Image Processing Reference
In-Depth Information
urban heat effect (Oke
1987
; Gallo et al.
1995
; Lo et al.
1997
; Owen et al.
1998
; Nichol
2003
; Voogt and Oke
2003
). In addition, the non-porous human-made materials
give rise to reduced evapotranspiration and more rapid
runoff of rainwater (Kim
1992
). Urban heat island has
been observed and documented for more than one and
half centuries (Howard
1833
; Oke
1987
). Depending on
the location, the urban heat island effect can be exacer-
bated by the absence or removal of vegetation, which
provides shade and evapotranspiration to cool the air on
warm days.
The dynamics of the heat island effect are a function of
the time, meteorological conditions, local and urban char-
acteristics, and consequently, could be unique for particular
urban areas. In general, parks, lakes and open areas, appear
relatively cooler compared to commercial, industrial or
dense buildings (Roth et al.
1989
). The urban heat intensity, DT(u − r), is the differ-
ence between the urban maximum temperature (u) and the non-urban low tempera-
ture (r), and is controlled by the unique characteristics of particular urban and suburban
areas. For example, in cities with tall buildings, the three-dimensional structures
alter the airflow that could reduce heat loss resulting in higher temperatures (Oke
1987
; Nichol
1996, 1998
).
Why should urban planners, environmentalists, and residents worry about urban
heat island? There are positive as well as negative effects from urban heat island.
Positive contributions include the potential for early budding and blooming of flowers
and trees in urban compared to non-urban areas, and the reduction in winter heating.
The adverse effects, which appear to outweigh any positive gains, include stress for
humans as the summer discomfort levels rise, higher demand for air-conditioning
in the summer, and the speeding up of the process of chemical weathering of building
of materials, especially in the tropical and other low latitude areas. Air conditioners
use energy in the form of fuel and electricity and result in increases in the emission
of SO
2
, CO, CO
2
and NO
2
, which contribute to global warming and climatic change
(Akbari
2002
). In temperate climates, the use of air-conditioning could result in the
formation of fog, which could raise the level of pollution in the air (Rosenfeld
et al.
1995
).
Traditionally, urban heat island effects are observed from
the measurement of air temperatures from fixed weather
observation stations and sensors mounted on moving vehicles.
The urban heat island effect is best developed in the night
under cloudless skies and calm winds. Any urban heat inten-
sity gradients are obliterated by strong winds. In ideal con-
ditions, a village or small town with a population of over
1,000 inhabitants exhibit a heat island effect (Oke
1987
).
Large cities may exhibit higher heat intensities than smaller
ones, but in general the morphology of a city ultimately
urban materials
such as construc-
tion material,
roofs, asphalt
absorb more heat
from the sun and
subsequently
release this
energy causing
urban areas to be
warmer compared
to the surrounding
non-urban areas
giving rise to the
urban heat effect
the dynamics of
the heat island
effect are a
function of
the time,
meteorological
conditions, local
and urban
characteristics
