Heat Islands and Urban Forests

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What is an Urban Heat Island?

Urban areas typically have higher surface and land temperatures than the temperatures found in surrounding rural areas - sometimes up to 10 o F higher (EPA).

Heat islands form like a large bubble over the urban area as a result of the natural land cover being replaced by pavement, buildings, and other infrastructure.

In the natural state, trees and other vegetation naturally cool the air through shade and the evaporation of water from soil and leaves. However, dark surfaces such as pavement and roads retain heat, which is exacerbated by waste heat from vehicles and machinery such as air conditioners.

Elevated temperatures can increase fuel bills by increasing the need for air conditioning, also raising air pollution levels, and the risk of heat-related illnesses.


Excessive heat claims more lives in the United States each year than hurricanes, lightning, tornadoes, floods, and earthquakes combined. Between 1979-1998, the CDC estimates that 7,421 deaths resulted from exposure to excessive heat in the U.S . (Center for Disease Control and Prevention)

Ozone Formation

High temperatures can increase the rate of ground-level ozone (O 3 ) formation, or smog. Ground level ozone is formed when Volatile Organic Compounds (VOCs) from common household and commercial products and Nitrous Oxides (NOx) from vehicle exhausts react when exposed to sunlight and in the presence of excessive heat.

Ozone in the upper atmosphere helps to protect us from the more harmful radiation from the sun, however at ground level, ozone can have many harmful health effects - including damage to the respiratory system, decrease in lung function and inflammation of airways.

Studies have shown that cooling Los Angeles by 5-7° F can reduce smog formation by 20% - which is the equivalent of taking all cars out of LA's airshed for one day. ( Rosenfeld et al, 1997)

This pollution can also damage vegetation and ecosystems within and downwind of cities. For instance, ground-level ozone interferes with the ability of plants to grow and store food, and visibly damages tree foliage.  In addition, ozone transported downwind of cities can make crops more susceptible to disease, insects, and harsh weather.

Above left - New York City photographed by NASA to show the areas of heat as opposed to light.  The darkest areas are the coolest while the white areas are the hottest.  The image above right shows the areas of vegetation in the same area.  The most vegetated areas are shown in the darkest green.  It is apparent from these images that the localized urban areas with the least vegetation tend to be the hottest.

Built up areas can cause a Heat Island effect

How can heat islands be reduced or cooled?

In urban areas, replacing dark roofs by installing cool or vegetated green roofs can help reduce the effect of heat retention.  The City has recently completed a large green roof project on City Hall and the Police Building which will not only help to reduce fuel bills, but will also help reduce stormwater runoff.   For further information and for images of this project, please CLICK HERE .

Replacing dark paving with cool, light, reflective surfaces helps prevent heat storage. Planting trees for use as natural shade and vegetation can help cut fuel bills by reducing the need for air conditioning.

According to the EPA, dark surfaces can be up to 70°F (21°C) hotter than lighter surfaces. That excess heat is transferred to the building itself, creating an increased need for cooling. By switching to light colored roofs, buildings can use 40% less energy.

Urban Forests

Cities need areas of natural land to provide ecological services, such as air and water purification, and nutrient cycling for the soils.  Natural lands such as urban forest provide greenery, scenic beauty and space for recreation. 

Urban forests also provide habitat for a variety of wildlife, and perhaps on a more philosophical level, natural areas can make us feel less isolated from nature in the modern, commercialized urban center and suburban sprawl.

According to USDA Forest Service data, in 2001 there were 156,545,000 trees in the urban areas of Virginia alone.  That is enough trees to store an estimated 28,960,000 tons of carbon.

Trees can serve many functions in an urban setting.  They cool the area in summer by offering shade, helping to reduce the heat island effect and reducing residential fuel bills.  They curb the flow of stormwater runoff, in turn slowing erosion of stream beds and other areas, recharging groundwater supplies, and helping to prevent flooding.  Urban forests also improve air quality and reduce greenhouse gases, combating climate change.    

Charlottesville was declared a Tree City USA in 2007 by The Arbor Day Foundation.  The Tree City USA® program, sponsored by The Arbor Day Foundation in cooperation with the USDA Forest Service and the National Association of State Foresters, provides direction and national recognition for urban and community forestry programs throughout the USA.  For further information please CLICK HERE .  The City is managing our urban forest, whose canopy covers 47% of the city, through the development and implementation of an Urban Forest Management Plan

25% of the city tree canopy in the U.S. has vanished over the past 30 years.  (American Forests, DC)