The Urban Physical Environment: Temperature and Urban Heat Islands

Abstract

The term urban heat island (UHI) describes the phenomenon in which cities are generally warmer than adjacent rural areas. The UHI effect is strongest with skies free of clouds and with low wind speeds. In moist temperate climates, the UHI effect causes cities to be slightly warmer in midday than rural areas, whereas in dry climates, irrigation of vegetation in cites may cause slight midday cooling compared to rural areas. In most climates, maximum UHIs occur a few hours after sunset; maximum intensities increase with city size and may commonly reach 10°C, depending on the nature of the rural reference. Since the recognition of London’s UHI by Luke Howard in the early 1800s, UHIs of cities around the world have been studied to quantify the intensity of UHIs, to understand the physical processes that cause UHIs, to estimate the impacts of UHIs, to moderate UHI effects, and to separate UHI effects from general warming of Earth caused by accumulation of greenhouse gases in the upper atmosphere. This chapter reviews a portion of the literature on UHIs and their effects, literature that has expanded greatly in the last two decades spurred on by a series of successful international conferences. Despite considerable research, many questions about UHI effects remain unanswered. For example, it is still not clear what portion of the long-term trends of increasing temperatures at standard weather stations is caused by UHI effects and how much is contributed by greenhouse gas effects. Also not well quantified is the effect of increasing tree cover in residential areas on temperatures The process of urbanization alters natural surface and atmospheric conditions so as to create generally warmer temperatures (Landsberg, 1981). Oke (1997) suggested that urban atmospheres provide the strongest evidence we have of the potential for human activities to change climate. In the 20th century, rapid urbanization occurred worldwide, and today the majority of the world’s population lives in cities. Increased temperature in cities, termed the urban heat island (UHI) effect, is present all around the world and both contributes to global climate change and, in turn, is exacerbated by global climate change (Mills, 2007; Sanchez-Rodriguez et al., 2005). With increasing energy shortages, the importance of G.M. Heisler, USDA Forest Service, Northern Research Station, SUNY ESF, 5 Moon Library, Syracuse, NY 13210 (gheisler@fs.fed.us); A.J. Brazel, School of Geographical Sciences and Urban Planning, Arizona State University, Coor Hall, 975 S. Myrtle, Tempe, AZ 85287-5302 (abrazel@asu.edu). doi:10.2134/agronmonogr55.c2 Copyright © 2010. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, 5585 Guilford Road, Madison, WI 53711, USA. Agronomy Monograph 55. Urban Ecosystem Ecology. J. Aitkenhead-Peterson and A. Volder (ed.)