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3.1 Introduction
The characteristics of the urban microclimate are of critical importance with regard
to inhabitants’ health and well-being (thermal comfort, heat stress, mortality rates)
as well as energy and environmental issues (Akbari 2005; Harlan and Ruddell
2011). In the last few years, the general awareness concerning the urban microclimate has been steadily rising. However, given the fact that world-wide an increasing
number of people live in cities, further research and planning efforts are needed to
better understand and address the effects of urban microclimate, its variance, and its
development. Given the complexity of the urban fabric, it is widely recognized that
heat storage in urban areas will be higher when compared to unbuilt areas (Grimmond
and Oke 1999; Piringer et al. 2002). Generally speaking, the undesired thermal
circumstances in the urban environment are caused in part by certain properties of
the materials used for construction of buildings, pavements, and roads, the urban
layout and structure including topography, morphology, density, and open space
configuration, as well as processes and activities such as transportation and industry
(Unger 2004; Grimmond 2007; Alexandri 2007; Kleerekoper et al. 2012; Shishegar
2013). These factors can affect, amongst other things, the way solar radiation is
absorbed by urban surfaces and the way air masses flow through the urban fabric.
Empirical observations in many cities around the world point to significantly higher
urban temperatures than the surrounding rural environment. This circumstance is
referred to as the urban heat island (UHI) phenomenon (see, for example, Voogt
2002; Arnfeld 2003; Blazejczyk et al. 2006; Oke 1981; Gaffin et al. 2008). Together
with climate change, this phenomenon can be crucial to the way we view urban
areas as living environments.
Recently, a number of research efforts have been initiated to better understand
the very specifics of the UHI phenomenon (see, for example, Arnfeld 2003;
Blazejczyk et al. 2006). Some of related foci of these efforts are to describe the
characteristics and patterns of UHI (Voogt 2002; Hart and Sailor 2007). Empirical
observations have shown that the UHI phenomenon shows different characteristics
during different seasons (Gaffin et al. 2008) and that it is pronounced differently
during the night and the day (Oke 1981). Furthermore, the intensity of urban heat
islands is believed to rise proportionally to the size and population of the urban area
(Oke 1972). More recently, Gaffin et al. (2008) performed a detailed spatial study of
New York City’s current UHI and concluded that summer and fall periods were
generally the strongest UHI seasons, consistent with seasonal wind speed changes
in the area. A simple quantitative indicator of urban heat island phenomenon is the
UHI intensity. The UHI intensity is defined as the difference between urban and
rural air temperature (Oke 1972).
Generally, heat island intensities are quantified in the range of 1–3 K, but under
certain atmospheric and surface conditions can be as high as 12 K (Voogt 2002).
Material properties of urban surfaces (Grimmond et al. 1991; Akbari et al. 2001) as
well as evapotranspiration, and anthropogenic heat emission (Taha 1997) can result
in higher urban temperatures. To address the implications of the UHI phenomenon,
A. Mahdavi et al.
3.1 Introduction
The characteristics of the urban microclimate are of critical importance with regard
to inhabitants’ health and well-being (thermal comfort, heat stress, mortality rates)
as well as energy and environmental issues (Akbari 2005; Harlan and Ruddell
2011). In the last few years, the general awareness concerning the urban microclimate has been steadily rising. However, given the fact that world-wide an increasing
number of people live in cities, further research and planning efforts are needed to
better understand and address the effects of urban microclimate, its variance, and its
development. Given the complexity of the urban fabric, it is widely recognized that
heat storage in urban areas will be higher when compared to unbuilt areas (Grimmond
and Oke 1999; Piringer et al. 2002). Generally speaking, the undesired thermal
circumstances in the urban environment are caused in part by certain properties of
the materials used for construction of buildings, pavements, and roads, the urban
layout and structure including topography, morphology, density, and open space
configuration, as well as processes and activities such as transportation and industry
(Unger 2004; Grimmond 2007; Alexandri 2007; Kleerekoper et al. 2012; Shishegar
2013). These factors can affect, amongst other things, the way solar radiation is
absorbed by urban surfaces and the way air masses flow through the urban fabric.
Empirical observations in many cities around the world point to significantly higher
urban temperatures than the surrounding rural environment. This circumstance is
referred to as the urban heat island (UHI) phenomenon (see, for example, Voogt
2002; Arnfeld 2003; Blazejczyk et al. 2006; Oke 1981; Gaffin et al. 2008). Together
with climate change, this phenomenon can be crucial to the way we view urban
areas as living environments.
Recently, a number of research efforts have been initiated to better understand
the very specifics of the UHI phenomenon (see, for example, Arnfeld 2003;
Blazejczyk et al. 2006). Some of related foci of these efforts are to describe the
characteristics and patterns of UHI (Voogt 2002; Hart and Sailor 2007). Empirical
observations have shown that the UHI phenomenon shows different characteristics
during different seasons (Gaffin et al. 2008) and that it is pronounced differently
during the night and the day (Oke 1981). Furthermore, the intensity of urban heat
islands is believed to rise proportionally to the size and population of the urban area
(Oke 1972). More recently, Gaffin et al. (2008) performed a detailed spatial study of
New York City’s current UHI and concluded that summer and fall periods were
generally the strongest UHI seasons, consistent with seasonal wind speed changes
in the area. A simple quantitative indicator of urban heat island phenomenon is the
UHI intensity. The UHI intensity is defined as the difference between urban and
rural air temperature (Oke 1972).
Generally, heat island intensities are quantified in the range of 1–3 K, but under
certain atmospheric and surface conditions can be as high as 12 K (Voogt 2002).
Material properties of urban surfaces (Grimmond et al. 1991; Akbari et al. 2001) as
well as evapotranspiration, and anthropogenic heat emission (Taha 1997) can result
in higher urban temperatures. To address the implications of the UHI phenomenon,
A. Mahdavi et al.