Understanding and Analysing the Urban Heat Island (UHI) Effect in Micro-Scale

Understanding and Analysing the Urban Heat Island (UHI) Effect in Micro-Scale

Ali Soltani (University of South Australia, Adelaide, Australia & Shiraz University, Shiraz, Iran) and Ehsan Sharifi (School of Architecture and Built Environment, University of Adelaide, Adelaide, Australia)
DOI: 10.4018/IJSESD.2019040102

Abstract

The shortage of vegetation cover alongside urban structures and land hardscape in cities causes an artificial temperature increase in urban environments known as the urban heat island (UHI) effect. The artificial heat stress in cities has a particular threat for usability and health-safety of outdoor living in public space. Australia may face a likely 3.8°C increase in surface temperature by 2090. Such an increase in temperature will have a severe impact on regional and local climate systems, natural ecosystems, and human life in cities. This paper aims to determine the patterns of the UHI effect in micro-scale of Adelaide metropolitan area, South Australia. The urban near-surface temperature profile of Adelaide was measured along a linear east-west cross-section of the metropolitan area via mobile traverse method between 26 July 2013 and 15 August 2013. Results indicate that the while the maximum UHI effect occurs at midnight in the central business district (CBD) area in Adelaide, the afternoon urban warmth has more temperature variations (point-to-point variation), especially during the late afternoon when local air temperature is normally in its peak. Thus, critical measurement of heat-health consequences of the UHI effect need to be focused on the afternoon heat stress conditions in UHIs rather than the commonly known night time phenomenon. This mobile traverse urban heat study of Adelaide supports the hypothesis that the UHI effect varies in the built environment during daily cycles and within short distances. Classical UHI measurements are commonly performed during the night – when the urban-rural temperature differences are at their maximum. Thus, they fall short in addressing the issue of excess heat stress on human participants. However, having thermally comfortable urban microclimates is a fundamental characteristic of healthy and vibrant public spaces. Therefore, urban planning professionals and decision makers are required to consider diurnal heat stress alongside nocturnal urban heat islands in planning healthy cities. The results of this article show that the diurnal heat stress varies in the built environment during daily cycles and within short distances. This study confirms that the maximum urban heat stress occurs during late afternoon when both overall temperature and daily urban warmth are at their peak. Literature indicates that diurnal heat stress peaks in hard-landscapes urban settings while it may decrease in urban parklands and near water bodies. Therefore, urban greenery and surface water can assist achieving more liveable and healthy urban environments (generalisation requires further research). A better understanding of daily urban warmth variations in cities assists urban policy making and public life management in the context of climate change.
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1. Introduction

Australia may face a likely 3.8°C increase in surface temperature by 2090 (the worst scenario: A1F1) (CSIRO, 2007). Such an increase in temperature will have a severe impact on regional and local climate systems, natural ecosystems, and human life in cities. Heat stress can reach up to 10°C in urban settings compared to their peri-urban surroundings (Erell et al., 2011). Natural landscapes in and around cities are increasingly replaced by hard and impermeable surfaces during urban development projects (Girardet, 2008, Harden et al., 2014). Shortage of vegetation cover in cities, alongside urban structure and hard surfaces, cause an artificial temperature increase in urban environments (Gartland, 2008, Stone, 2012). The artificial heat stress in cities has a particular threat for usability and health-safety of outdoor living in public space (Nikolopoulou, 2004, Kovats and Hajat, 2008). In response to such substantial extra heat load in cities, people increasingly move into air-conditioned buildings to benefit from the indoor thermal comfort. However, anthropogenic heat – generated from indoor air-conditioning – causes an ever-increasing outdoor temperature.

In this context, this paper aims to determine the daily patterns of urban warmth in Adelaide metropolitan area via a mobile traverse method. A better understanding of daily urban warmth variations in cities assists urban policy making and public life management in the context of climate change.

1.1. Background

Large-scale changes in the natural landscape cause urban space inhabitants to suffer from the effect of an artificial heat stress in cities relative to their peri-urban vicinities, known as the urban heat island (UHI) effect. Background literature of the UHI effect indicates that such artificial increase of urban temperature occurs because of changes in energy and water budget in the built environment (Karatasou et al., 2006a, Oke, 2006b, Gartland, 2008, Erell et al., 2011 (Soltani, Mehraein, & Sharifi, 2012)). Howard’s study of urban warmth in London indicates that the mean annual temperature (20-years average) in London central area is 2.5°C higher than its countryside (1833). The peak temperature difference of 3°C is reported during February (mid-winter). Similar urban heat stress has been reported in Paris by the second half of the 19th century (Gartland, 2008, Stewart, 2011).

The UHI investigations are commonly document the UHI phenomenon and its behaviour. These large-scale urban climate studies contribute mainly to the understanding of the UHI effect mechanism via comparing city centres and their rural surroundings (Oke, 1987, Oke, 1988, Paterson and Apelt, 1989, Tapper, 1990). Numerous case studies strongly support the relatively higher temperature in highly developed urban areas including city centres (see Figure 1). However, the accuracy and applicability of many of these case studies are under criticism in more advanced urban climate research by highlighting instrumental and measurement variations (Oke, 2006b, Stewart, 2011).

Figure 1.

Reported magnitude of the UHI effect in 12 cities since the 1980s

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