KEEPING YOUR COOL
How Communities Can Reduce the Heat Island Effect
Millions of people living in and around cities experience summertime temperatures that are higher than those in surrounding
natural areas. These urban "heat islands" increase energy demand for air conditioning, raise air pollution levels, and can
cause heat-related illness and death. As temperatures continue to rise due to climate change, urban areas are more likely to
experience more frequent, more intense, and longer heat waves. Fortunately, there are proven actions that communities
can take today to keep cool in the future.
The Basics: What Is a Heat Island?
Heat islands are zones of relative warmth created by urban air and surface temperatures that are higher than those of
nearby rural areas. Air temperatures in a large city can be 2-22° F (1-12° C) higher than its rural surroundings.
The sketch below shows a hypothetical city's heat island profile at two times during a 24-hour period, demonstrating how
temperatures typically rise during both the day and night as you move from rural areas toward dense downtown areas.
Typical Heat Island Temperature Differences Between Urban and Rural Areas
Surface Temperature (Day)
Air Temperature (Day)
Surface Temperature (Night)
Air Temperature (Night)
3
4->
(0
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Keeping Your Cool: How Communities Can Reduce the Heat Island Effect
What Causes Heat Islands?
Heat islands form when cities replace natural land cover—trees, grass, wetlands—with
pavement and buildings. These changes lead to higher urban temperatures because:
• Removing trees and vegetation eliminates the natural cooling effects of shade and
evaporation of water from soil and leaves.
• Pavement, rooftops, and other non-reflective surfaces absorb heat during the day and
release it at night, inflating overnight temperatures.
• Tall buildings and narrow streets reduce wind flow and heat air that is trapped between
them.
• Waste heat from vehicles, factories, and air conditioners add warmth to the air, further
increasing the heat island effect.
Heat islands are also influenced by a city's geography and prevailing weather conditions. For
example, strong winds and rain can flush out hot, stagnant air from city centers, while sunny,
windless conditions can intensify heat islands.
How Do Heat Islands Affect Us?
Higher temperatures affect people's health, air and water quality, and the amount of energy
that we use for summertime cooling.
People's Health: Heat islands can intensify extreme hot weather, which can cause breathing
problems, heat cramps, and heat stroke, and may lead to illness or even death—especially in
vulnerable populations such as the elderly.
Air Quality: Heat islands raise energy demand to power air conditioning, which in turn can
increase utility bills and increase power plant emissions of carbon pollution that causes climate
change. Higher temperatures also accelerate the chemical reaction that produces ground-level
ozone, or smog.
Water Quality: Hot pavements heat up stormwater runoff, which can hurt aquatic life in local waterways.
Energy Use: Heat islands are responsible for 5-10 percent of summertime electricity demand, leading to higher electricity
bills, pressure on the electricity grid, and brownouts and blackouts.
How Can Communities Cool Down?
Communities that want to cool down have options. They include installing reflective cool roofs; planting trees and
vegetation, including "green" roofs; and using cool paving materials for roads, sidewalks, and parking lots.
"Urban canyon" photo courtesy of Frederick V. Slocum, Minnesota State University
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Keeping Your Cool: How Communities Can Reduce the Heat Island Effect
Four Strategies to Reduce Heat Islands
Green Roofs replace heat-absorbing
traditional roofing materials surfaces with
plants, shrubs, and small trees.
BENEFITS:
• Lower energy bills and energy demand
• Less air pollution
• Better stormwater management and
water quality
• Aesthetic and habitat benefits
I Cool Pavements store less heat and reflect
more solar radiation than conventional
pavements. They may be permeable or porous to
allow water to percolate and evaporate, cooling
the pavement surface and surrounding air.
BENEFITS:
* Reflective pavements can improve nighttime
visibility
• Permeable pavements can reduce
stormwater runoff and tire noise, and can
improve safety during rainstorms
2 ) Cool Roofs reflect or release the sun's solar
energy rather than retain it, are typically 50-60°
F (28-33° C) cooler than traditional roofs.
BENEFITS:
& Lower energy bills and energy demand
» Less air pollution
• Increased comfort of occupants
4) Trees and Vegetation provide shade and
cool the air through evapotranspiration.
BENEFITS:
• More green space
• Better stormwater management
* Less air pollution and erosion
• Lower energy bills and energy demand
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Keeping Your Cool: How Communities Can Reduce the Heat Island Effect
What Are the Benefits of Cooling Down?
Implementing cooling strategies across a community has many
benefits, including reducing summertime temperatures, lowering
energy use and costs, curbing air pollution, and alleviating heat-
related health issues.
By adding these strategies to long-term local and regional
planning, communities can also prepare for or adjust to higher
temperatures and changing conditions that come with climate
change. For instance, planting trees helps prevent flooding while
also lowering local temperatures.
Reducing Urban Heat Islands:
Compendium of Strategies
Urban Heat Island Basics
Learn more about these strategies in
EPA's Reducing Urban Heat Islands:
Compendium of Strategies, available
at: http://epa.gov/heatisland/
resources/compendium.htm.
Photo courtesy of American Society of Landscape Architects,
ENERGY STAR partner, 2014
About EPA's Heat Island Reduction Program
Through its Heat Island Reduction Program, EPA works with local officials, community groups, researchers, and other
stakeholders to identify opportunities to implement heat island reduction programs and policies that create comfortable
and sustainable communities.
HEAT ISLAND
REDUCTION
PROGRAM
Learn more at:
epa.gov/heatisland
November, 2014
Publication Number: 430F14041
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