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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