Clear, accessible definitions for common urban planning terms.

What Is the Urban Heat Island Effect?

4 minute read

Man-made surfaces and urban density contribute to higher temperatures, intensifying heat waves and posing a growing danger to public health.

View of buildings in Bangkok business district through extremely orange sunlight

TWStock / Bangkok, Thailand

The urban heat island effect refers to the accumulated impact of buildings, roads, and other human-built structures that absorb the sun’s heat more than natural surfaces such as grass, soil, and trees and raise the surrounding air temperature.

First identified in the early 1800s and named in 1929 by Albert Peppler, the effect has grown more powerful as urbanization claims more undeveloped land and surfaces such as asphalt become ubiquitous in human settlements around the world, making some cities significantly warmer than surrounding rural and natural areas. The effect can also have noticeably different impacts on neighborhoods within the same city based on tree cover, green space, urban geometry, and building materials. Man-made surfaces retain heat longer, so urban areas also stay warmer for longer even after the sun sets.

According to the Environmental Protection Agency (EPA), heat islands can form due to several factors: the reduction of natural landscapes, building materials that absorb more heat than natural surfaces, the size and shape of buildings (which can influence wind flow), heat generated by appliances, cars, and other human activities, and weather and geography. Tall buildings can create ‘urban canyons’ that trap warm air and reduce air flow.

The EPA defines two types of heat islands. Surface heat islands are created when surfaces like asphalt absorb and emit heat at higher rates than natural surfaces such as grass. Atmospheric heat islands are a broader phenomenon formed when air in one area is warmer than in surrounding areas, such as a city being warmer than surrounding countryside.

Impacts of Heat Islands

Heat islands impact cities in several ways. Higher temperatures lead to more demand for cooling, particularly at peak times of the day, leading to higher energy consumption and the risk of overloaded grids and subsequent blackouts. This higher demand for electricity also produces more greenhouse gas emissions, while higher air temperatures can stimulate excessive ozone formation.

Higher temperatures can also affect water quality. Warmer runoff that makes its way into rivers, lakes, and other bodies of water can alter the metabolism of aquatic plants and wildlife and render bodies of water unlivable.

Heat-related illnesses and deaths, particularly among vulnerable groups like older adults and young children, are another risk posed by heat islands. Additionally, low-income people and renters who lack access to cooling, workers who spend their time outdoors or in poorly ventilated workplaces, and people with chronic conditions face disproportionately high risks of heat-induced conditions. During the massive heat wave that lingered over the Pacific Northwest in the summer of 2021, more than one thousand people died of heat-related causes in the United States and Canada.

Heat Equity

Because extreme heat affects some communities more than others, advocates use the term ‘heat equity’ to refer to policies that acknowledge and work to redress the unequal distribution of resources and infrastructure. In many cities, low-income neighborhoods and communities of color tend to have less robust tree canopies and shade, fewer resources such as covered bus stops or heat-reflective surfaces, and reduced access to air conditioning and other cooling options, either because units are not permitted in older apartment buildings or because the cost of electricity is prohibitive. As mentioned above, extreme heat also disproportionately affects other vulnerable groups, such as the elderly, people experiencing homelessness, and workers in outdoor settings, warehouses, or other dangerous environments.

Strategies To Combat Extreme Heat

Cities can address heat inequity by promoting the use of heat mitigating materials, funding projects that increase green space and tree cover, ensuring that all residents have access to cooling centers and other resources when needed, and guiding smart growth that minimizes sprawl and protects natural spaces.

Ensuring that all neighborhoods have plenty of trees, other shade foliage, green spaces, and permeable surfaces can mitigate the effects of extreme heat and improve public health. For example, New York City’s Cool Neighborhoods NYC program funds tree plantings, cool roof projects, energy assistance for low-income households, and education programs.

Cities can also provide resources so residents can plant their own trees and gardens and grow them sustainably. The arid desert city of Tucson, Arizona began reimbursing residents for rainwater collection systems to help water trees and plants.

Like ground-level parks and gardens, green roofs or roof gardens add a layer of vegetation that can reduce surface and air temperatures, improve air and stormwater quality, provide gardening opportunities for residents, and add green space to dense urban areas. In lieu of green roofs, cool roofing materials can help bring down temperatures and reduce energy needs for a building. Other cool materials can be used as pavement for streets, parking lots, and sidewalks.

More broadly, cities can employ smart growth strategies that prioritize public health by limiting sprawl, preserving green and open spaces, requiring urban design that minimizes heat increases, and dedicating resources to retrofitting streets and buildings with cooling and reflective materials.

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