RESEARCH STARTER

Urban heat island (UHI)

An urban heat island (UHI) is an area within a city that experiences significantly higher air and surface temperatures compared to its surrounding suburban and rural regions. This phenomenon has been observed since the early 19th century, when meteorologist Luke Howard noted warmer nighttime temperatures in London than in the countryside, attributing this to urban activities and fuel combustion. The UHI effect results from various factors, including urban materials like concrete and asphalt that absorb and retain heat, as well as the reduction of natural landscapes and water bodies that typically moderate temperatures.

Additionally, human activities in cities, such as increased energy consumption and vehicle emissions, contribute to higher local temperatures and can exacerbate air pollution, creating a feedback loop that further intensifies the heat. Research indicates that urban areas are experiencing more hot days than nearby rural locations, a trend that is expected to worsen as urban populations grow. Effective strategies to mitigate UHI effects include enhancing urban vegetation and green spaces, which can help cool the environment. Understanding UHI is crucial for accurately assessing historical temperature records and addressing climate change, as urban growth can distort temperature data comparisons over time.

Full Article

Definition

An urban heat island is a metropolitan area that is significantly warmer in surface and air temperature than its suburban and rural surroundings. English amateur meteorologist Luke Howard, in his book The Climate of London (1818), described temperature differences between London and its surrounding countryside. He noted that it was warmer at night in London than in the country, and he speculated that the cause might be the burning of fuels. Research over the subsequent two centuries has charted a variety of differences between cities and their surroundings, as distinctive urban landscapes, domestic and industrial structures, and the behavior of urban dwellers affect the ways in which solar and other heat enter and exit the area. Urbanization causes changes to the preexisting natural landscape, as original materials (soil, vegetation, rock, water, and so on) are gradually replaced or modified with materials (concrete, tile, and many others) that, in conjunction with the activities and life patterns of a city’s inhabitants, result in greater heat retention. This heat retention can, in turn, affect weather patterns.

Significance for Climate Change

Scientists have identified many interconnected causes for the urban heat island effect. During the day, the sun warms buildings and roadways. Pollution from automotive traffic and industrial processes contributes to the formation of clouds and smog, which help trap heat, and tall buildings limit the ability of winds to disperse such formations (the “canyon effect”). The predominance of land structures and the paucity of bodies of water lessen the influence of evaporation, which would use some of the heat energy for the formation of water vapor. Instead, that energy raises the ambient temperature. The concentration of human bodies in cities also contributes to their heat level. The increase in city heat in turn encourages activities, such as the use of air conditioning, that further contribute to localized heat increases, since air conditioners make interiors cooler by making the exterior city warmer. Moreover, increased power plant emissions resulting from the additional consumption of electricity contribute to global warming.

To the extent that urban areas encourage structures or behaviors that stimulate additional activities, increasing emissions of greenhouse gases (GHGs), urban heat islands may be viewed as contributing indirectly to global warming. (If cities are hotter than rural areas, then those who move to cities will be more likely to use energy to cool themselves than they otherwise would be.) Into the twenty-first century, according to an article in Scientific American, cities have had more searing hot days each year. In fact, in the decade between 2004 and 2014, twelve US cities averaged at least twenty more days a year above 90 degrees than nearby rural areas. In two-thirds of sixty cities analyzed, urbanization and climate change appeared to be combining to increase summer heat faster than climate change alone was raising regional temperatures. This trend continued into the 2020s. In 2022, cities across the US set records for heat waves of 90-plus degrees, with streaks of 90-plus degree days stretching to nearly seventy in some areas. By 2030, about 89 percent of North America's population was expected to live in urban areas, according to 2021 data from the University of Michigan's Center for Sustainable Systems. As large metropolitan areas fuse into megalopolises or megacities, the problems only become exacerbated. Accordingly, students of the urban heat island effect have seen the need for various degrees of urban redesign as a key element in attempts to mitigate the noxious consequences of the effect. Chief among these is an increase in, and optimal distribution of, urban vegetation and green space, including the planting and sustaining of suitable trees.

The urban heat island effect must be taken into account in any attempt to read the historical record of Earth’s temperature. To compare contemporary temperatures with those of a century ago requires comparability, and that can be affected by urban growth. Places where temperatures were measured in the past often have since experienced significant urban growth, with the associated increased temperature measurements. This growth makes direct comparisons of present and past measurements difficult. In light of this problem, climatologists must adjust the data on the basis of their best guess about distorting factors.


Bibliography

Gartland, Lisa. Heat Islands: Understanding and Mitigating Heat in Urban Areas. Earthscan/James and James, 2008.

“Heat Island Effect.” Environmental Protection Agency, 26 June 2025, www.epa.gov/heatislands. Accessed 29 Aug. 2025.

"'Heat Islands' Cook U.S. Cities Faster Than Ever." Scientific American, 22 Aug. 2014, www.scientificamerican.com/article/heat-islands-cook-u-s-cities-faster-than-ever. Accessed 29 Aug. 2025.

Heikkinen, Niina. "How People Make Summer Hotter." Scientific American, 25 Nov. 2014, www.scientificamerican.com/article/how-people-make-summer-hotter. Accessed 29 Aug. 2025.

Lee, Derek O. “Urban Climates.” Progress in Physical Geography, vol. 8, no. 1, 1984, pp. 1–31.

McKendry, Ian G. “Progress Reports: Applied Climatology.” Progress in Physical Geography, vol. 27, no. 4, 2003, pp. 597–606.

Maslin, Mark. Global Warming: A Very Short Introduction. Oxford UP, 2004.

Parker, David E. “A Demonstration That Large-Scale Warming Is Not Urban.” Journal of Climate, vol. 19, 2005, pp. 2882–2895.

Peterson, Thomas C. “Assessment of Urban Versus Rural In Situ Surface Temperatures in the Contiguous United States: No Difference Found.” Journal of Climate, vol. 16, no. 18, 2003, pp. 2941–2959.

Pine, Joshua, et al. "Urban Heat Island Effect Solutions and Funding." National League of Cities, 13 Feb. 2023, www.nlc.org/article/2023/02/13/urban-heat-island-effect-solutions-and-funding/. Accessed 29 Aug. 2025.

Souch, Catherine, and Sue Grimmond. “Applied Climatology: Urban Climates.” Progress in Physical Geography, vol. 30, no. 2, 2006, pp. 270–279.

Spencer, Roy W. Climate Confusion: How Global Warming Hysteria Leads to Bad Science, Pandering Politicians, and Misguided Policies That Hurt the Poor. Encounter Books, 2008.

Sturman, Andrew P. “Applied Climatology.” Progress in Physical Geography, vol. 22, no. 4, 1998, pp. 558–565.

"U.S. Cities Factsheet." Center for Sustainable Systems, 2024, css.umich.edu/publications/factsheets/built-environment/us-cities-factsheet. Accessed 29 Aug. 2025.

Waldrop, M. Mitchell. "Cities Are Getting So Hot It's Deadly—Can We Fix It?" Inverse, 20 Feb. 2024, www.inverse.com/science/urban-heat-wave-survival-tips. Accessed 29 Aug. 2025.

"What Are Heat Islands?" Environmental Protection Agency, 29 Aug. 2025, www.epa.gov/heatislands/learn-about-heat-islands. Accessed 29 Aug. 2025.

Full Article

Definition

An urban heat island is a metropolitan area that is significantly warmer in surface and air temperature than its suburban and rural surroundings. English amateur meteorologist Luke Howard, in his book The Climate of London (1818), described temperature differences between London and its surrounding countryside. He noted that it was warmer at night in London than in the country, and he speculated that the cause might be the burning of fuels. Research over the subsequent two centuries has charted a variety of differences between cities and their surroundings, as distinctive urban landscapes, domestic and industrial structures, and the behavior of urban dwellers affect the ways in which solar and other heat enter and exit the area. Urbanization causes changes to the preexisting natural landscape, as original materials (soil, vegetation, rock, water, and so on) are gradually replaced or modified with materials (concrete, tile, and many others) that, in conjunction with the activities and life patterns of a city’s inhabitants, result in greater heat retention. This heat retention can, in turn, affect weather patterns.

Significance for Climate Change

Scientists have identified many interconnected causes for the urban heat island effect. During the day, the sun warms buildings and roadways. Pollution from automotive traffic and industrial processes contributes to the formation of clouds and smog, which help trap heat, and tall buildings limit the ability of winds to disperse such formations (the “canyon effect”). The predominance of land structures and the paucity of bodies of water lessen the influence of evaporation, which would use some of the heat energy for the formation of water vapor. Instead, that energy raises the ambient temperature. The concentration of human bodies in cities also contributes to their heat level. The increase in city heat in turn encourages activities, such as the use of air conditioning, that further contribute to localized heat increases, since air conditioners make interiors cooler by making the exterior city warmer. Moreover, increased power plant emissions resulting from the additional consumption of electricity contribute to global warming.

To the extent that urban areas encourage structures or behaviors that stimulate additional activities, increasing emissions of greenhouse gases (GHGs), urban heat islands may be viewed as contributing indirectly to global warming. (If cities are hotter than rural areas, then those who move to cities will be more likely to use energy to cool themselves than they otherwise would be.) Into the twenty-first century, according to an article in Scientific American, cities have had more searing hot days each year. In fact, in the decade between 2004 and 2014, twelve US cities averaged at least twenty more days a year above 90 degrees than nearby rural areas. In two-thirds of sixty cities analyzed, urbanization and climate change appeared to be combining to increase summer heat faster than climate change alone was raising regional temperatures. This trend continued into the 2020s. In 2022, cities across the US set records for heat waves of 90-plus degrees, with streaks of 90-plus degree days stretching to nearly seventy in some areas. By 2030, about 89 percent of North America's population was expected to live in urban areas, according to 2021 data from the University of Michigan's Center for Sustainable Systems. As large metropolitan areas fuse into megalopolises or megacities, the problems only become exacerbated. Accordingly, students of the urban heat island effect have seen the need for various degrees of urban redesign as a key element in attempts to mitigate the noxious consequences of the effect. Chief among these is an increase in, and optimal distribution of, urban vegetation and green space, including the planting and sustaining of suitable trees.

The urban heat island effect must be taken into account in any attempt to read the historical record of Earth’s temperature. To compare contemporary temperatures with those of a century ago requires comparability, and that can be affected by urban growth. Places where temperatures were measured in the past often have since experienced significant urban growth, with the associated increased temperature measurements. This growth makes direct comparisons of present and past measurements difficult. In light of this problem, climatologists must adjust the data on the basis of their best guess about distorting factors.


Bibliography

Gartland, Lisa. Heat Islands: Understanding and Mitigating Heat in Urban Areas. Earthscan/James and James, 2008.

“Heat Island Effect.” Environmental Protection Agency, 26 June 2025, www.epa.gov/heatislands. Accessed 29 Aug. 2025.

"'Heat Islands' Cook U.S. Cities Faster Than Ever." Scientific American, 22 Aug. 2014, www.scientificamerican.com/article/heat-islands-cook-u-s-cities-faster-than-ever. Accessed 29 Aug. 2025.

Heikkinen, Niina. "How People Make Summer Hotter." Scientific American, 25 Nov. 2014, www.scientificamerican.com/article/how-people-make-summer-hotter. Accessed 29 Aug. 2025.

Lee, Derek O. “Urban Climates.” Progress in Physical Geography, vol. 8, no. 1, 1984, pp. 1–31.

McKendry, Ian G. “Progress Reports: Applied Climatology.” Progress in Physical Geography, vol. 27, no. 4, 2003, pp. 597–606.

Maslin, Mark. Global Warming: A Very Short Introduction. Oxford UP, 2004.

Parker, David E. “A Demonstration That Large-Scale Warming Is Not Urban.” Journal of Climate, vol. 19, 2005, pp. 2882–2895.

Peterson, Thomas C. “Assessment of Urban Versus Rural In Situ Surface Temperatures in the Contiguous United States: No Difference Found.” Journal of Climate, vol. 16, no. 18, 2003, pp. 2941–2959.

Pine, Joshua, et al. "Urban Heat Island Effect Solutions and Funding." National League of Cities, 13 Feb. 2023, www.nlc.org/article/2023/02/13/urban-heat-island-effect-solutions-and-funding/. Accessed 29 Aug. 2025.

Souch, Catherine, and Sue Grimmond. “Applied Climatology: Urban Climates.” Progress in Physical Geography, vol. 30, no. 2, 2006, pp. 270–279.

Spencer, Roy W. Climate Confusion: How Global Warming Hysteria Leads to Bad Science, Pandering Politicians, and Misguided Policies That Hurt the Poor. Encounter Books, 2008.

Sturman, Andrew P. “Applied Climatology.” Progress in Physical Geography, vol. 22, no. 4, 1998, pp. 558–565.

"U.S. Cities Factsheet." Center for Sustainable Systems, 2024, css.umich.edu/publications/factsheets/built-environment/us-cities-factsheet. Accessed 29 Aug. 2025.

Waldrop, M. Mitchell. "Cities Are Getting So Hot It's Deadly—Can We Fix It?" Inverse, 20 Feb. 2024, www.inverse.com/science/urban-heat-wave-survival-tips. Accessed 29 Aug. 2025.

"What Are Heat Islands?" Environmental Protection Agency, 29 Aug. 2025, www.epa.gov/heatislands/learn-about-heat-islands. Accessed 29 Aug. 2025.

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