What Are Urban Heat Islands?
Heat islands are regions in urban areas where the ambient air temperature is consistently higher than surrounding rural and suburban areas. As cities develop, increasing amounts of surfaces are paved over, the ability of the earth to shield, absorb, and dissipate heat is compromised. There are several specific mechanisms that contribute to this effect, but the end effect is the same: pockets of significantly higher temperature called “heat islands.”
Natural landscapes like forests provide shade and moisture that keep temperatures low even when the weather is hot and sunny. The process of evaporation absorbs heat, so bodies of water, even small ponds and creeks, help lower temperatures. Even random plants in meadows and fields transpire water from their leaves, which also consumes thermal energy. In urban areas, most surfaces are paved or covered with buildings, vastly limiting the ability of natural areas to moderate temperatures. In addition, the properties of construction materials in urban areas themselves significantly exacerbate the problems that start with loss of natural areas:
- Low solar reflectance is the most important contributor to the development of heat islands. The term refers to the fraction of solar energy that’s reflected by a surface. In practice, the more energy that’s reflected by a material, the less heat is absorbed. Traditional roofing materials have low reflectance values and typically absorb 85 to 95 percent of the energy reaching them.
- Thermal emissivity refers to how much heat a material will give up at a given temperature. Any material at all will absorb heat until it reaches thermal equilibrium, at which point it begins to give off heat in equal measure. Materials with high thermal emissivity reach equilibrium at a lower point, meaning they begin to give off heat at a much lower point. This is the mechanism behind the extremely high temperatures found on typical rooftops. The air over standard black asphalt roofs, for example, can be as much as 55 to 85°F hotter than the ambient air temperature.
- Heat capacity refers to the amount of energy needed to raise the temperature of a material. For example, liquid water can absorb a significant amount of heat before it starts to get hot. Conventional roof materials, in contrast, have much lower heat capacity.
The overall effect of these properties means that compared to natural landscapes, surfaces found in developed areas (roads, sidewalks, roofs, etc.) reflect very little solar energy and store relatively small amounts of heat, so they quickly return that solar energy to the air in the form of heat. In urban areas, these types of surfaces and structures are highly concentrated, so they produce pockets of increased heat called heat islands.
Many parts of the US are seeing patterns of rising temperatures and more frequent, dangerous heat waves. Unfortunately, these trends are intensifying the effect of heat islands. Coupled with growing urban populations and associated urban sprawl, natural land areas are decreasing, which in turn strengthen the effect of heat islands.
Contributing Factors
In heavily developed areas, such as the downtown areas of some cities, areas with many closely spaced buildings act as huge thermal masses that can’t release their heat. Add in tall buildings and normal air flow is impeded, meaning wind cannot disperse the air to bring the temperature down. Closely packed areas also mean high concentrations of “anthropogenic” heat producers like automobiles, air conditioners, and industrial facilities, which only add to the heat load.
Effects
Urban heat islands affect not only the quality of life of city residents and employees, they also produce effects that have serious consequences on the environment:
- Air pollution and greenhouse gases rise as utilities produce more electricity to meet ever-increasing demands for air conditioning.
- Ground-level ozone levels increase when sunlight and higher temperatures speed up chemical reactions.
- High daytime temperatures caused by heat islands inhibit the ability of the air to cool at night, resulting in higher average temperatures overall. These effects, coupled with high levels of air pollution, lead to poor air quality and present significant health issues for urban populations.
- Water quality beyond the specific urban area is impaired when stormwater, landing on hot urban surfaces, heats up and drains into storm sewers and is eventually discharged into rivers, streams, ponds and other surface waters. This kind of sudden rise in water temperature can cause stress and even death throughout an aquatic ecosystem that might be far removed from the city environment itself.
What Effective Solutions Exist?
Communities around the world have developed a menu of proven strategies to reduce the effect of heat islands:
- Promoting (even requiring) the widespread use of green roofs
- Use of cool roofs, which are composed of highly reflective materials that prevent solar rays from heating roofing materials and keep the air at significantly lower temperatures.
- Use of cool pavements, which may be reflective or permeable
- Support of green spaces that feature trees and vegetative cover.
- Adjusting urban growth through zoning and permitting, as well as fostering the implementation of proven solutions for new and established construction.
The natural environment has developed very effective mechanisms for mitigating the stress of high temperatures, including shade that prevents solar energy from reaching the ground, moisture retention in the soil to support evaporative cooling, and transpiration by plants that cools both the plants and the surrounding air. Green roofs take advantage of those mechanisms and have proven to be the most effective solution to the spectrum of problems that arise from urban heat islands.
Green Roof Advantages
Just as in nature, the simple act of growing a layer of vegetation on a roof reduces the temperature of the roof itself and the surrounding air. In fact, the surface of a vegetated roof can actually be cooler than the ambient air, compared to conventional rooftop surfaces, which can reach temperatures as much as 90°F higher than the ambient air temperature.
Green roofs are an attractive, flexible solution that can be installed on institutional, educational, industrial and government facilities along with office, commercial, and high density residential buildings. Green roofs are widely used in Europe on private residential structures, and they’re quickly gaining popularity in the US.
Green roofs offer so many advantages that it will take a separate article to cover them. A few examples of green roof benefits includes; an increased roof lifespan, improved R- values, higher market value and salability, and even reduced stormwater fees from the city.
