Urban Heat Islands (UHI)
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 (27° – 50°C) hotter than the air, 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 (1° – 3°C) warmer than its surroundings. On a clear, calm night, however, the temperature difference can be as much as 22°F (12°C).
Why Do We Care About Heat Islands?
Elevated temperature from urban heat islands, particularly during the summer, can affect a community’s environment and quality of life. While some heat island impacts seem positive, such as lengthening the plant-growing season, most impacts are negative and include:
What Can Be Done?
Communities can take a number of steps to reduce the heat island effect, using four main strategies:
Installing cool reflective roofs;
Creating green roofs (also called "rooftop gardens" or "eco-roofs");
Increasing tree and vegetative cover;
Using cool pavements
Typically heat island mitigation is part of a community’s energy, air quality, water, or sustainability effort. Activities to reduce heat islands range from voluntary initiatives, such as cool pavement demonstration projects, to policy actions, such as requiring cool roofs via building codes. Most mitigation activities have multiple benefits, including cleaner air, improved human health and comfort, reduced energy costs, and lower greenhouse gas emissions.