The heat island effect occurs when urban areas accumulate more solar heat than the surrounding countryside, producing temperatures that are up to 22℉ (12℃) warmer in the city than in its surrounding rural areas. According to the U.S. Environmental Protection Agency, a city with a population of at least 1 million people is, on average, 1.8-5.4℉ (1-3 ℃) warmer than nearby rural areas . The heat island effect in cities occurs because of large areas in cities (approximately 30-45% of the total land cover in cities) are covered in pavement structures and soak up a great deal of solar heat each day .
This heat island in cities increases the demand for energy to cool buildings, increases energy costs, increases pollution, increases the production of greenhouse gases, increases the incidences of heat-caused illnesses and death, and negatively impacts water quality .
Methods used to reduce the heat island effect in cities
The following are four primary methods that are used to reduce the heat island effect in cities. Using a combination of these techniques will increase the overall effectiveness of mitigating the heat island effect in a particular city.
- Increase green spaces and trees
The presence of trees and other vegetation reduces air and surface temperatures through shade and evapotranspiration (the releasing of water vapor through a plant’s leaves). It has been found that temperatures in shaded areas can be as much as 20-45℉ (11-25℃) cooler than temperatures in unshaded areas.
The shading of buildings by vegetation decreases the demand for energy used for air conditioning, reducing air pollution and producing fewer greenhouse gas emissions .
- Green roofs
Green roofs are vegetative layers that are grown on top of a roof. Green roofs can be used to grow different types of vegetation (including trees and shrubs), or they can be used to grow crops as rooftop gardens or farms.
Green roofs help to cool the air in cities by providing shade, removing the heat from the air as the vegetation undergoes evapotranspiration, and reducing the surface temperature of the roof itself. Green roof temperatures can be cooler than the surrounding air, compared to the surface temperature of a conventional roof, which can be as high as 90℉ (50℃) warmer than the surrounding air .
Green roofs serve as insulators for buildings, thereby reducing the energy that is needed to heat and cool them. This reduced energy use results in a reduction in air pollution and greenhouse gas emissions. Because the green roofs provides insulation for a building, its occupants will be more comfortable and less likely to experiencing heat stress in the summer .
- Cool roofs
“Cool” roofs reflect sunlight and heat away from buildings, thus reducing a building’s roof temperature and helping to mitigate the heat island effect. Cool roofs can remain 50-60℉ (28-33℃) cooler than conventional roofs in the summer.
Because cool roofs absorb less heat, the building below the roof stays cooler, reducing energy demand for air conditioning in the summer, lowering energy use and the associated air pollution and greenhouse gases from energy production, and maintaining greater comfort in the buildings during hot days .
- Cool pavements
During hot summer days, conventional concrete pavement can reach as high as 120-150℉ (48-67℃), heating the air above them and the stormwater that flows into waterways. Cool pavements are paving materials that reflect solar energy, enhance water evaporation, and are designed to remain cooler than conventional pavements .
Cool pavements can be made from asphalt and concrete and also by using special coatings and grass paving .