![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Frequent Questions
What are the effects of heat islands?
How can cities mitigate heat islands?
What are the benefits of cool roofs?
What are some examples of cool roofing applications?
What are the benefits of green roofs?
What are some examples of green roof applications?
What are some examples of cool pavement applications?
How does urban forestry mitigate the heat island effect?
What are the benefits of urban vegetation?
What are some examples of shade tree and vegetation applications?
How are heat islands, global warming, and climate change related?
What is a heat island?
Heat islands are characterized by urban air and surface temperatures that are higher than nearby rural areas. Many U.S. cities and suburbs have air temperatures up to 10° F (5.6° C) warmer than surrounding natural land cover.
[See
About Heat Islands for
more information.]
How do heat islands form?
Heat islands form as cities replace natural land cover with pavement, buildings, and other structures. Built areas absorb more of the sun's heat than do natural surfaces, causing surface and air temperatures to rise. The loss of trees and shrubs also eliminates the natural cooling effects of shading and evapotranspiration, a process that draws heat from the air to convert water contained in vegetation to water vapor.
[See
About Heat Islands for
more information.]
When do heat islands form?
Heat islands can occur at any time, regardless of season or time of day. They are often largest in the evening when urban areas release stored heat energy into the air from roads and other structures. This urban-rural cooling difference produces maximum heat island intensities three to five hours after sunset. Information presented in this web site pertains mainly to summertime heat islands.
[See
About Heat Islands for
more information.]
What are the effects of heat islands?
Heat islands can increase air conditioning demand, the incidence of heat-related illness and mortality, and power plant emissions of air pollution and greenhouse gases. In addition to this direct increase in power plant emissions, elevated ambient temperatures can speed up the heat-dependent reaction that forms ground-level ozone.
[See
Human Health and the
Environment for more information.]
What is ozone?
Ozone is an odorless, colorless gas that can be "good" or "bad" depending on where it is in the atmosphere. "Good" ozone occurs in the stratosphere, approximately 10 to 50 kilometers above the Earth's surface, where it forms a protective layer that shields us from harmful ultraviolet radiation. Stratospheric ozone forms what is known as the ozone layer.
"Bad" ozone, or ground-level ozone, exists closer to
the Earth's surface. Ground-level ozone is formed by a
chemical reaction between nitrogen oxides and volatile
organic compounds in the presence of heat and sunlight.
Ground-level ozone is a harmful pollutant and the main
constituent of smog.
[See Human Health and the Environment for more information.]
How can cities mitigate heat islands?
Cities can mitigate heat islands by installing cool roofs or green roofs; using cool pavements; and planting shade trees and vegetation throughout an urban area.
The extent to which communities can capture the
benefits of these heat island reduction strategies
depends on factors such as the types and effectiveness of
mitigation measures used, prevailing weather patterns,
climate, geography, current urban canopy cover, and
pollution transported from up-wind regions.
[See
Mitigation
Strategies for more information.]
What is albedo?
Albedo is the ability of a surface material to reflect incident solar (short wave) radiation. It is expressed on a scale of 0 to 1 where a value of 0.0 indicates that a surface absorbs all solar radiation, and an albedo value of 1.0 represents total reflectivity. Alternatively, albedo can be expressed as a percentage between 0% and 100%. Light-colored surfaces generally have higher albedos than dark-colored surfaces.
[See
Mitigation
Strategies for more information.]
What is emittance?
Emittance is the amount of thermal radiation – also known as infrared or long wave radiation – that a material releases or radiates away from its surface. Emittance is expressed as a number between 0.0 and 1.0, or 0% and 100%. With the exception of metals, most building materials have emittances above 85%.
[See
Cool Roofs for
more information.]
What is a cool roof?
Cool roofs have a high solar reflectance, or albedo, and may also have a high emittance. EPA's ENERGY STAR ® Roof Product Program has cool roofing specifications for both low-slope (primarily commercial) and sloped (primarily residential) roofs. ENERGY STAR low-slope roofs should have an average initial albedo of at least 0.65 while sloped roofs have an average initial albedo of at least 0.25. Although emittance is not a qualifying criterion for ENERGY STAR ®, a value of at least 0.85 further reduces roof temperatures and can decrease the amount of heat transferred to building interiors.
[See
Cool Roofs for
more information.]
What are the benefits of cool roofs?
On a hot, sunny summer day, traditional roofing materials can reach peak temperatures of 190°F (88°C). By comparison, cool roofs reach maximum temperatures of 120°F (49°C). In buildings with air conditioning (AC), cool roofs can save money on energy bills, lower peak energy demand, and reduce power plant emissions of air pollution and greenhouse gases. In buildings without AC, cool roofs can increase indoor occupant comfort by lowering top-floor temperatures. In both cases, cool roofs can contribute to urban heat island mitigation.
[See Cool Roofs for more information.]
What are some examples of cool roofing applications?
Low-slope cool roofing applications, which are typically used in the commercial building sector, include single-ply and cool coatings. Single-ply roofing is often constructed of white rubber or plastic material that is glued or fastened over the entire roof surface. Cool coatings have the consistency of thick paint, and contain cement particles or elastomeric polymers. They can be applied to a wide range of existing and new roof surfaces.
Sloped cool roofing applications include concrete or
clay tile with a white pigment added to the surface or
mixed in during manufacturing. These products are
typically used in the residential sector.
[See Cool Roofs for more information.]
What is a green roof?
Green roofs, or rooftop gardens, are planted over existing roof structures, including industrial facilities, residences, offices, and other commercial property. These "living" roofs consist of a waterproof, root-safe membrane that is covered by a drainage system, lightweight growing medium, and plants.
[See Green Roofs for more information.]
What are the benefits of green roofs?
Green roof benefits include:
- Reducing rooftop temperatures and heat transfer to
the surrounding air;
- Decreasing summertime indoor temperatures, which
reduces air conditioning demand and peak energy
consumption for cooling;
- Lessening pressure on sewer systems through the
absorption of rainwater;
- Filtering pollution – including heavy metals
and excess nutrients – through the processes of
bio- and phytoremediation; and
- Protecting underlying roof material, reducing
noise, providing a habitat for birds and other small
animals, and improving the quality of life for building
inhabitants.
Reducing the urban heat island effect by decreasing rooftop temperatures through evapotranspiration, which cools the surrounding air.
[See Green Roofs for more information.]
What are some examples of green roof applications?
Green roofs are generally classified either as intensive or extensive. Intensive green roofs require a minimum of one foot of soil. Trees and shrubs are usually planted, adding 80 to 150 pounds per square foot of load to the building. These roofs require complex irrigation and drainage systems, and significant maintenance. Intensive roofs are often accessible to the public.
Extensive green roofs require only 1 to 5 inches of
soil. Low lying plants and grasses are usually planted,
and 12 to 50 pounds per square foot of load may be added.
These roofs use simple irrigation and drainage systems,
and require little maintenance. Extensive green roofs
usually are not accessible to the public.
[See Green Roofs for more information.]
What is a cool pavement?
Although there is no official standard or labeling program to designate cool paving materials, the term "cool pavement" is often used to describe light-colored or permeable pavements. Pavements with high solar reflectance may absorb less heat than traditional products and contribute to heat island mitigation.
[See Cool Pavements for more information.]
What are some examples of cool pavement applications?
Asphalt concrete and portland cement concrete – commonly called "concrete" and "asphalt," respectively – are the most common paving materials for sidewalks and streets. Most new concrete has a solar reflectance, or albedo, of 35% to 40%; the solar reflectance of fresh asphalt is typically 5% to 10%.
Over time, the albedos of these pavements change. Asphalt lightens as the binder wears away to expose the underlying rock aggregate, while concrete darkens from the build-up of tire residue, dirt, and oil. To maximize the albedo of both types of pavement, lighter-colored aggregate can be used in the pavement mix.
Permeable, or porous, pavements allow water to percolate and evaporate, cooling the pavement surface and surrounding air. Permeable pavements can be constructed from a number of materials including concrete, asphalt, and plastic lattice structures filled with soil, gravel, and grass.
[See Cool Pavements for more information.]
What is urban forestry?
Urban forestry is the process of increasing and maintaining the vegetative land cover in an urban area. Many cities plant and maintain trees and other vegetation outside of commercial buildings, along highways, and on residential streets. Planting shade trees in urban parking lots is another common application.
In addition, trees and vegetation can be planted
strategically – next to homes and buildings –
to save energy. This blocks the sun's rays and reduces
heat transfer to interiors along with the need for air
conditioning.
[See Trees and Vegetation for more information.]
How does urban forestry mitigate the heat island effect?
Increasing the amount of urban vegetation decreases local ambient air temperatures through shading and evapotranspiration. The U.S. Department of Agriculture Forest Service estimates that maximum mid-day air temperature reductions are in the range of 0.07°F (0.04°C) to 0.36°F (0.2°C) for every one percent increase in the canopy cover.
Shading can play an important role in building-level cooling by preventing solar radiation from coming in contact with, and being absorbed by, building materials.
In addition, urban forestry provides cooling through evapotranspiration. A mature 40 foot tree with a crown of 30 feet can decrease air temperature by transpiring as much as 40 gallons of water per day.
[See Trees and Vegetation for more information.]
What are the benefits of urban vegetation?
Benefits of urban vegetation include:
- Decreasing air conditioning demand and peak energy
consumption for cooling by shading buildings from solar
radiation;
- Ambient cooling from evapotranspiration;
- Increasing property values through aesthetic
enhancement; and
- Enhancing air quality by removing particulate pollutants from the air and by decreasing the emissions associated with air conditioning energy demand.
[See Trees and Vegetation for more information.]
What are some examples of shade tree and vegetation applications?
Shade trees and vegetation can be used in parking lots to prevent storm water runoff, and around buildings to decrease interior temperatures. In most U.S. cities, trees should shade the east and especially west walls to maximize cooling savings. Planting trees directly to the south may provide little shade in the summertime and block desired sun in the wintertime. Effective tree planting efforts require detailed information on species selection, site selection and preparation, and ongoing maintenance.
[See Trees and Vegetation for more information.]
How are heat islands, global warming, and climate change related?
Heat islands describe local-scale temperature differences between urban and rural areas. In contrast, global warming refers to the gradual rise of worldwide average surface temperatures.
[See
About Heat Islands for
more information.]