Urban Heat Islands

by

Donald Yow

• LAST MODIFIED: 25 July 2023
• DOI: 10.1093/obo/9780190922481-0070

Introduction

Urban climates have existed ever since the first human settlements altered the natural environment thousands of years ago. During the first century CE, Pliny the Elder recorded anecdotal accounts about the differences between the air in ancient Rome and the air in rural surroundings. Urban development accompanied global population growth over the following centuries. Today, well over half of the world’s population lives in cities and that proportion is increasing. All urban inhabitants experience a local climate that is somewhat different from that in surrounding rural areas. That, coupled with increasing challenges presented by large-scale climate change, necessitates research to understand how the unique climates caused by urbanization behave, how they impact the residents who live there, and what can and should be done to address ill-effects equitably. The first formal scientific assessment of urban climate was published by Luke Howard almost two hundred years ago in 1833. Over time, urban climatology and meteorology have blossomed into rich fields encompassing studies of a range of modified climate effects including energy balance, roughness length, turbulence and wind, atmospheric moisture, and air quality. The focus of this review is the urban heat island (UHI) effect. The UHI is generally defined as higher temperatures in urban areas compared to rural areas, which is a common feature in most cities worldwide. The first section of this review lists a select number of publications and websites that provide a sound foundation to help those new to urban climate studies become acclimated to the field. The next section delves deeper into the definition of exactly what the UHI is and how it is observed. In particular, a distinction is made between UHIs in the canopy layer (usually measured 2m above the surface) and surface UHIs (measured using thermal imagery). The second section explores the impacts on human and natural systems caused by the UHI. This includes subsections about urban effects on long-term temperature records and human comfort, health, and mortality. The next section takes a look at the various methods used to limit the adverse effects of the UHI. Ideas ranging from green and blue strategies to advanced building materials are discussed. The last part of this review concerns environmental justice. Many of the people who face the direst of unintended consequences caused by urbanization are those who are least well equipped to handle the additional pressures.

Foundational Resources

Much has been written about urban climate and urban heat islands over the past two hundred years. The volume of the literature can be overwhelming. A Google search on “urban heat island” in summer 2022 yielded about 119,000,000 results in less than half a second. This section provides a short list of resources that are useful for those new to the field as well as more experienced researchers. In a monumental effort, Oke, et al. 2017 synthesizes what has been learned from the voluminous and diverse research about urban climate science. Its comprehensive content, excellent quality, and broad scope is accessible and meaningful to novices and experts alike. For those interested in conducting their own observational UHI study, Stewart and Mills 2021 provides detailed instructions guiding readers from planning stages to communicating the results. Recent developments in the field of urban climatology can be tracked on the International Association for Urban Climate (IAUC) website. From its inception, the IAUC has called for representation from the full range of the urban climate community. Arguably, no organization has a firmer grasp on the current pulse of urban climate science.

• International Association for Urban Climate.

  The IAUC website contains abundant resources including pdfs of three classic texts—Luke Howard’s The Climate of London Volume 1 (London: Phillips, 1820); Albert Kratzer’s The Climate of Cities [Stadtklima] (Braunschweig, Germany: Vieweg, 1956); and Tony Chandler’s The Climate of London (London: Hutchinson, 1965)—a list of useful links, teaching resources, and an extensive urban climate bibliography. The quarterly newsletter of the IAUC, The Urban Climate News, highlights recent urban climate news, and also contains feature articles, summaries of recent projects, and special reports. Each newsletter provides a bibliography of recent urban climate publications and a list of upcoming conferences. A full archive of the newsletter is available.

• Oke, T. R., G. Mills, A. Christen, and J. A. Voogt. Urban Climates. Cambridge, UK: Cambridge University Press, 2017.

  DOI: 10.1017/9781139016476

  An extremely valuable reference book for students, researchers, and practitioners interested in urban environments and urban climate. This important work guides readers to a comprehensive understanding of urban climate science from concepts and processes to adaptation and mitigation measures. An entire chapter is dedicated to the UHI. A brief history of urban climatology and extensive bibliography are also provided.

• Stewart, I. D., and G. Mills. The Urban Heat Island: A Guidebook. Amsterdam: Elsevier, 2021.

  An entire book dedicated to the UHI primarily written for those new to studying urban climate and the UHI specifically. The most studied types of UHIs—canopy-layer (CUHI) and surface (SUHI)—are the focus. The first part of the text describes the physical processes responsible for the UHI and actions used to manage it. The second part provides guidelines to help plan and conduct a CUHI study and a SUHI study.