The Urban Heat Island Phenomenon
On warm summer days with calm winds, the air in a city can be 2-8 degrees F hotter
than the surrounding countryside. Scientists call this phenomenon the "urban heat
island" effect. This occurs because in urban areas, there are fewer trees, and other
natural vegetation to shade buildings, block solar radiation and cool the air by
evapotranspiration-the evaporation of water from the surfaces of leaves and the soil.
In addition, roof and paving materials with low reflectivity absorb more of the sun's
rays, causing both surface temperature and overall ambient air temperature in an
urban area to rise.
Urbanization Has Dramatically Altered the Natural Landscape
Source: Environmental Protection Agency
Urban heat islands increase the demand for energy needed to cool homes and
buildings. As the temperature in an urban area rises, more cooling energy is needed to
maintain comfort levels in building structures. Currently, one-sixth of the electricity
consumed in the U.S. is used for cooling purposes, at an annual cost of $ 40 billion.
Reductions in urban air temperature by just a few degrees could save consumers
millions of dollars on their utility bills each year, while at the same time reducing
harmful air emissions, such as sulfur dioxides (SO2) and nitrogen oxides (NOx), which
are produced when fossil fuels are burned to generate electricity.
In addition, lower ambient air temperatures can decrease the amount of ground level
ozone, or smog. Ozone (O3) is a photochemical oxidant and the major component of
smog. While O3 in the upper atmosphere is beneficial to life by shielding the earth
from harmful ultraviolet radiation from the sun, high concentrations of O3 at ground
level are a major health and environmental concern.
O3 is not emitted directly into the air but is formed through complex chemical reactions
between precursor emissions of volatile organic compounds (VOC) and NOx in the
presence of sunlight.
Transportation and industrial sources emit both VOCs and NOx. VOCs are emitted from
sources as diverse as automobiles, chemical manufacturing, dry cleaners, paint shops
and other sources using solvents. Sunlight and temperature stimulate these reactions
so that peak O3 levels occur typically during the warmer times of the year.
The exact relationship between temperature and ozone formation varies among
geographical areas. However, recent simulations performed by Lawrence Berkeley
National Laboratory (LBNL) show that for the Los Angeles basin, a reduction in
summer temperatures by approximately 6 degrees F translates into an overall average
reduction in smog of about 12 percent.
Mitigation Strategies
A long-term strategy of planting shade trees and installing reflective materials for roofs
and pavements can mitigate the urban heat island effect and help reduce associated
economic, environmental, and health-related costs.
When the sun beats down on buildings covered with dark colored roofing materials,
most of the heat collected by the roof is transferred inside, increasing the demand for
air conditioning.
Installing highly reflective roofs will keep buildings cooler and reduce energy bills.
Research conducted in Florida and California indicates that buildings with highly
reflective roofs require up to 40 percent less energy for cooling than buildings covered
with darker, less reflective roofs.
Roads, parking lots, and driveways paved with dark, heat absorbing materials (e.g.,
asphalt) also contribute to the urban heat island effect. Increasing the albedo of these
surfaces through the use of reflective paving materials will help to cool down the
surrounding ambient air temperature.
Planting shade trees reduces the amount of heat absorbed by buildings by directly
shielding them from the sun's rays. In addition, trees, shrubs, and other plants help
reduce ambient air temperatures through a process known as "evapotranspiration."
This occurs when water absorbed by vegetation evaporates off of the leaves and
surrounding soil to naturally cool the surrounding air.
Within 10-15 years - the time it takes a tree to grow to a significant size - strategically
placed trees can reduce heating and cooling costs for a typical home or office by an
average of 10-20 percent. Additional benefits from trees include reductions in storm
water runoff, erosion, and urban noise, to name a few.
On warm summer days with calm winds, the air in a city can be 2-8 degrees F hotter
than the surrounding countryside. Scientists call this phenomenon the "urban heat
island" effect. This occurs because in urban areas, there are fewer trees, and other
natural vegetation to shade buildings, block solar radiation and cool the air by
evapotranspiration-the evaporation of water from the surfaces of leaves and the soil.
In addition, roof and paving materials with low reflectivity absorb more of the sun's
rays, causing both surface temperature and overall ambient air temperature in an
urban area to rise.
Urbanization Has Dramatically Altered the Natural Landscape
Source: Environmental Protection Agency
Urban heat islands increase the demand for energy needed to cool homes and
buildings. As the temperature in an urban area rises, more cooling energy is needed to
maintain comfort levels in building structures. Currently, one-sixth of the electricity
consumed in the U.S. is used for cooling purposes, at an annual cost of $ 40 billion.
Reductions in urban air temperature by just a few degrees could save consumers
millions of dollars on their utility bills each year, while at the same time reducing
harmful air emissions, such as sulfur dioxides (SO2) and nitrogen oxides (NOx), which
are produced when fossil fuels are burned to generate electricity.
In addition, lower ambient air temperatures can decrease the amount of ground level
ozone, or smog. Ozone (O3) is a photochemical oxidant and the major component of
smog. While O3 in the upper atmosphere is beneficial to life by shielding the earth
from harmful ultraviolet radiation from the sun, high concentrations of O3 at ground
level are a major health and environmental concern.
O3 is not emitted directly into the air but is formed through complex chemical reactions
between precursor emissions of volatile organic compounds (VOC) and NOx in the
presence of sunlight.
Transportation and industrial sources emit both VOCs and NOx. VOCs are emitted from
sources as diverse as automobiles, chemical manufacturing, dry cleaners, paint shops
and other sources using solvents. Sunlight and temperature stimulate these reactions
so that peak O3 levels occur typically during the warmer times of the year.
The exact relationship between temperature and ozone formation varies among
geographical areas. However, recent simulations performed by Lawrence Berkeley
National Laboratory (LBNL) show that for the Los Angeles basin, a reduction in
summer temperatures by approximately 6 degrees F translates into an overall average
reduction in smog of about 12 percent.
Mitigation Strategies
A long-term strategy of planting shade trees and installing reflective materials for roofs
and pavements can mitigate the urban heat island effect and help reduce associated
economic, environmental, and health-related costs.
When the sun beats down on buildings covered with dark colored roofing materials,
most of the heat collected by the roof is transferred inside, increasing the demand for
air conditioning.
Installing highly reflective roofs will keep buildings cooler and reduce energy bills.
Research conducted in Florida and California indicates that buildings with highly
reflective roofs require up to 40 percent less energy for cooling than buildings covered
with darker, less reflective roofs.
Roads, parking lots, and driveways paved with dark, heat absorbing materials (e.g.,
asphalt) also contribute to the urban heat island effect. Increasing the albedo of these
surfaces through the use of reflective paving materials will help to cool down the
surrounding ambient air temperature.
Planting shade trees reduces the amount of heat absorbed by buildings by directly
shielding them from the sun's rays. In addition, trees, shrubs, and other plants help
reduce ambient air temperatures through a process known as "evapotranspiration."
This occurs when water absorbed by vegetation evaporates off of the leaves and
surrounding soil to naturally cool the surrounding air.
Within 10-15 years - the time it takes a tree to grow to a significant size - strategically
placed trees can reduce heating and cooling costs for a typical home or office by an
average of 10-20 percent. Additional benefits from trees include reductions in storm
water runoff, erosion, and urban noise, to name a few.
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