Academic journal article The Geographical Bulletin

Sky-View Factor Estimation: A Case Study of Athens, Georgia

Academic journal article The Geographical Bulletin

Sky-View Factor Estimation: A Case Study of Athens, Georgia

Article excerpt

INTRODUCTION

Urban surface geometry has a significant but complex influence on the urban atmo-sphere. The taller buildings characteristic of urbanized areas often act to restrict out going long-wave radiation, which can thwart nocturnal cooling and contribute to an urban heat island (UHI) effect (Oke 1981). The UHI phenomenon is important because it partially explains why cities can sometimes experience higher temperatures when com-pared with the surrounding countryside (Coutts et al. 2007). High levels of urban heating have been linked to poor urban air quality (Sarrat et al. 2006) as well as altered precipitation (Shepherd 2006) and snowmelt patterns (Todhunter et al. 1992; Semá deni- Davies & Bengtsson 1998). Furthermore, as the areal extent of cities expands (Lang & Knox 2009), the impact of urban landscapes and the subsequent UHI effect may no longer be confined to just the urban core, but spread to more suburban regions (Stone et al. 2010; Stone and Rodgers 2001).

Consequently, a better understanding of the UHI effect can be facilitated by accurately measuring urban geometry. One commonly utilized approach is to calculate the sky-view factor (SVF), which can be simply defined as "a measure of the degree to which the sky is obscured by the surroundings for a given point" (Grimmond et al. 2001, 903). SVF can be defined more technically as the ratio of radiation received (or emitted) by a planar surface compared to the radiation emitted (or received) by the entire hemispheric environment (Watson & Johnson 1987). SVF is a dimensionless parameter with values ranging from 0 to 1 (Chapman & Thornes 2004). A value of 0 means the sky is completely ob-structed, such as in a tunnel, while a value of 1 would occur in an entirely open and unobstructed space. Since the SVF provides a parameterization of the urban geometry's effect on the radiation balance, it is often considered a relevant variable when analyzing the formation of nocturnal heat islands (Oke 1981). Indeed, both Gal et al. (2009) and Svensson (2004) have found that a fairly strong negative relationship exists between SVF and UHI intensity, especially during calm, clear nights. These two studies used regression models to analyze the relationship between SVF values and the urban-rural temperature gradient, which yielded R2 values of 0.63 (Gal et al. 2009) and 0.58 (Svensson 2004) respectively.

The overall purpose of this study is to assess the spatial distribution of SVF values throughout various land use classes in Athens, GA. The chosen methodology is based on an assessment of the existing SVF literature as it relates to UHI effect. After evaluating the potential implications of the collected SVF values with regard to various land use patterns, I discuss the usefulness of SVF for evaluating the magnitude of the UHI effect in Athens.

Over the past few decades, various techniques for calculating SVF have been explored. These efforts began with the development of analytical methods, which used geometric equations based on the height and width of the urban canyon to calculate SVF. Oke (1981) and Johnson and Watson (1984) conducted some of the most influential re-search, although their methodologies were fairly simplistic and only incorporated building structure and form. These early equations frequently overlooked vegetation and other obstacles, which in some cases may produce inflated SVF values, because all obstructions are not considered.

In more current work, researchers have adopted optical and photographic methods, since they theoretically provide a more ac-curate measurement of SVF (Chapman & Thornes 2004). The photographic methodology uses a fish-eye lens to capture an image of the urban canyon (Grimmond et al. 2001). Fish-eye lenses purposely distort the edge of an image, which causes the slightly curved appearance, in order to achieve a wide-angle panoramic perspective. For SVF studies, the wide-angle enables researchers to view the entire hemisphere of an urban canyon at a given location. …

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