As urban areas develop, changes occur in their landscape. Buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry. These changes cause urban regions to become warmer than their rural surroundings, forming an “island” of higher temperatures in the landscape.
Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50–90°F hotter than the air1, while shaded or moist surfaces—often in more rural surroundings—remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining.
In contrast, atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F warmer than its surroundings. On a clear, calm night, however, the temperature difference can be as much as 22°F.
The temperatures described above do not represent absolute temperature values or any one particular measured heat island. Temperatures will fluctuate based on factors such as seasons, weather conditions, sun intensity, and ground cover. Surface and atmospheric temperatures vary over different land use areas. Surface temperatures vary more than air temperatures during the day, but they both are fairly similar at night.
Heat Island Cooling Strategies
Many communities are taking action to reduce urban heat islands using five main strategies: 1) increasing tree and vegetative cover, 2) installing green roofs, 3) installing cool—mainly reflective—roofs, 4) using cool pavements (either reflective or permeable), and 5) utilizing smart growth practices. The links below lead to detailed information on each of these strategies and examples of the activities that governments and communities are implementing.
• Trees and Vegetation: Increasing tree and vegetation cover lowers surface and air temperatures by providing shade and cooling through evapotranspiration. Trees and vegetation can also reduce stormwater runoff and protect against erosion.
• Green Roofs: Growing a vegetative layer (plants, shrubs, grasses, and/or trees) on a rooftop reduces temperatures of the roof surface and the surrounding air and improves stormwater management. Also called “rooftop gardens” or “eco-roofs,” green roofs achieve these benefits by providing shade and removing heat from the air through evapotranspiration.
• Cool Roof: Installing a cool roof – one made of materials or coatings that significantly reflect sunlight and heat away from a building – reduces roof temperatures, increases the comfort of occupants, and lowers energy demand.
• Cool Pavements: Using paving materials on sidewalks, parking lots, and streets that remain cooler than conventional pavements (by reflecting more solar energy and enhancing water evaporation) not only cools the pavement surface and surrounding air, but can also reduce stormwater runoff and improve nighttime visibility.
• Smart Growth: These practices cover a range of development and conservation strategies that help protect the natural environment and at the same time make our communities more attractive, economically stronger, and more livable.
For Further Reading:
• “Urban heat island”, article at Wikipedia: https://en.wikipedia.org/wiki/Urban_heat_island
• “Urban Heat Islands”, article at UCAR Center for Science Education: https://scied.ucar.edu/longcontent/urban-heat-islands
• “6 Causes of Urban Heat Islands and 4 Ways to Offset Them”, article at Buildings and Smarter Facility Management magazine: https://www.buildings.com/news/industry-news/articleid/22060/title/6-things-create-urban-heat-islands