Green Cities: An A-to-Z Guide Heat Island Effect Contributors: Nevin Cohen & Paul Robbins Print Pub. Date: 2011 Online Pub. Date: May 04, 2010 Print ISBN: 9781412996822 Online ISBN: 9781412973816 DOI: 10.4135/9781412973816 Print pages: 263-265 This PDF has been generated from. Please note that the pagination of the online version will vary from the pagination of the print book.
10.4135/9781412973816.n82 The New SchoolUniversity of Arizona Urban heat islands (UHIs) are regions of characteristic warmth associated with cities. They are perhaps the clearest expressions of unintentional climate modification by humans. Weather patterns, geographic setting, and urban configuration cause UHIs to vary in space and time; the heat island effect is typically most pronounced in city centers where building and population densities are highest, and during calm, cloudless evenings that promote maximum thermal differences between the city and surrounding countryside. UHIs have several economic, ecological, and social effects, notably on energy consumption, dispersal of air pollutants, and human health and comfort. Climate-sensitive urban design demonstrates some potential for mitigation of these effects, where appropriate. The UHI effect traditionally denotes a near-surface air temperature difference between the urban canopy-layer (the layer of atmosphere below the building tops) and a nearby rural area. Pedestrians directly experience this canopy-layer UHI, which ranges from a few degrees Celsius in moderately sized cities to approximately 10 12 degrees Celsius in the downtown cores of the largest cities. Daytime urban rural temperature differences are smaller and some portions of urban areas may even exhibit cool islands. Heat islands vary across cities depending on the local degree of urbanization; urban parks are relatively cool, for example, whereas greater nighttime warmth tends to occur at nodes of more intense development. Nighttime temperature increases rapidly near the urban rural boundary, and then rises more slowly toward the city center. Hence, UHIs are so named because their contours of equal temperature resemble those of elevation on a relief map. Urban rural temperature differences have been observed worldwide for centuries. The English chemist and amateur meteorologist Luke Howard was the first person to conduct formal observations of urban and rural thermal climates, as documented in his 1833 book The Climate of London. In distinguishing city and country temperatures around London, Howard observed that the city partakes much of an artificial warmth, induced by its structure, by a crowded population, and the consumption of great quantities of fuel. Page 2 of 5
[p. 263 ] Howard is generally considered to be the founder of urban climatology; modern pioneers include T. J. Chandler, Helmut E. Landsberg, and Timothy R. Oke. Published literature on observational canopy-layer UHIs is vast, most of it descriptive and most of it originating from large, midlatitude European and North American cities. Although UHIs were initially linked to city size or population, the main physical processes causing heat islands were clarified in the latter part of the 20th century. The genesis of nighttime canopy-layer UHIs can be traced to the unique materials and structure, or form, of cities. Urban materials such as asphalt, concrete, and brick are typically better than natural materials at storing daytime heat and releasing it at night; furthermore, vertical surfaces (building walls) in the city provide a greater surface area for this heat storage and release. Building walls also obstruct the sky and prevent nighttime radiant heat losses from streets. These processes slow the cooling of streets and the canopy-layer air during the late afternoon and evening, leading to a positive urban rural temperature difference that persists through the night. Waste heat released from transportation, heating/cooling, and industrial processes also plays a role in UHI formation, especially in cities with cold climates, high densities, or major industrial facilities. Additional factors related to wind sheltering, the removal of green space, and increased radiant heating from the polluted atmosphere above the city may contribute to UHI formation. The relative importance of these factors varies with city form, function, and location. Heat island magnitude, as measured by an urban rural temperature difference, depends on season, latitude, weather, and the surface character and state of the surrounding countryside. Clouds and wind in particular decrease heat island strength by reducing differences in heating and cooling rates between city and countryside. Seasonal variation of rural properties, such as soil wetness, foliage amount, and snow cover, significantly affects rural air temperatures and therefore UHI magnitudes. Corresponding seasonal variations in cities are blunted by the predominance of paved surfaces, the scarcity of vegetation, and the removal of snow. Canopy-layer UHIs are typically maximized in the summer season in midlatitudes; however, they may also be significant in winter in climates with high space heating requirements. In tropical latitudes, UHI intensity tends to be smaller than in midlatitudes and is greatest during the dry season. Page 3 of 5
In addition to the canopy-layer UHI, two other heat islands are commonly distinguished: The term boundary-layer UHIs refers to differences in air temperature in the lowest 0.1 1.0 kilometers (km) of atmosphere above the city and are smaller than canopy-layer UHIs (usually 1 2 degrees Celsius), and the term surface UHIs refers to differences in temperature between urban surfaces, such as buildings and roads, and rural surfaces, typically soil and vegetation. Surface UHIs are maximized during the day and vary dramatically in magnitude. Although linked, each of these heat island types involves different physical processes and different human consequences. Surface and canopy-layer UHIs affect building energy use for heating and cooling warm urban temperatures reduce energy requirements for space heating during the cool season and increase demand for air conditioning in the warm season. Heat islands influence the thermal comfort of pedestrians and building occupants, intensify heat waves, increase heat mortality rates, extend the growing season for urban gardeners, and reduce the likelihood of road ice and snow accumulation. Local weather patterns are also affected by heat islands, notably in the genesis of urban rural breezes and the enhancement of cloud and precipitation. Boundary-layer UHIs enhance smog formation by speeding up [p. 264 ] photochemical reactions. Overall, the UHI has negative effects in warmer climates but may present a mix of negative and positive effects in colder climates. As understanding of heat islands and urban climates continues to mature, effective communication of the associated impacts and potential solutions to planners and other decision makers remains an ongoing challenge. One hurdle is the reconciliation of climate-derived objectives with other urban planning goals, principles, and realities (e.g., cost, aesthetics, transportation, etc.). A further challenge, given the diversity of urban meteorological and geographical contexts worldwide, is to distill a practical and widely applicable set of design recommendations from a growing urban-climate knowledge base. Generic solutions may exist; for example, the addition of reflective surfaces and vegetation on streets and roofs (e.g., green roofs) for daytime urban air temperature reduction shows promise in some settings. Other features of cities, such as their physical structure, are not easily modified. The availability of urban climate resources and guidelines to planners is inadequate at present, which may largely explain the deficiency of climate principles, and UHI mitigation strategies more specifically, in urban planning and design. Page 4 of 5
E. Scott Krayenhoff, Iain D. Stewart University of British Columbia 10.4135/9781412973816.n82 See Also: Further Readings Landsberg H. E. The Urban Climate. New York: Academic Press, 1981. Oke T. R. The Energetic Basis of the Urban Heat Island. Quarterly Journal of the Royal Meteorological Society vol. 108/455 (1982). Voogt J. A. Urban Heat Island. In Causes and Consequences of Global Environmental Change, I. Douglas, ed. Encyclopedia of Global Environmental Change, Vol. vol. 3, R. E. Munn, ed. Chichester, UK: John Wiley & Sons, 2002. Page 5 of 5