The replacement of croplands, forests, and open grassy fields
with roads, buildings, and other impervious surfaces and structures
changes the relationship between incoming solar radiation and outgoing
terrestrial radiation within watershed areas. The conversion of pervious
surfaces to impervious surfaces alters local energy balances through
changes in:
- the albedos of surfaces,
- the specific heat capacities and thermal conductivities of surfaces,
- the ratio of sensible heat to latent heat flowing from the surface
into the atmosphere
Albedo - The percentage of incoming solar radiation
reflected by a surface. This reflected energy is unavailable
for sensible and latent heating, therefore the albedos
of watershed surfaces determines their relative rates
of heating. Urbanization decreases the overall albedo
of watershed surfaces by approximately 10%.
Sensible Heat - Heat energy which is felt and
can be measured with a thermometer. Heat energy not utilized
in evapotranspiration is released to the atmosphere as
sensible heat. The more energy that enters the atmosphere
as sensible heat, the higher the relative air temperatures
over watershed surfaces.
Latent Heat - Heat energy stored in
water vapor; it cannot be felt or measured with a thermometer.
It is not available for sensible heating. Latent heat
enters the atmosphere when water is evaporated from the
surface. Since evaporation removes heat , it is a cooling
process.
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The relationship between sensible heat and latent heat
is described by the Bowen ratio and the Sensible Heat Index. The summer
sensible heat indices for deserts, impervious urban surfaces, and
deciduous forests are 95%, 80%, and 25% respectively.
Bowen Ratio - Available sensible heat energy
divided by available latent heat energy. This ratio (B)
is used to calculate the sensible heat index.
Sensible Heat Index - Sensible heating divided
by total heating (sensible + latent). The index is formulated
as B/(B+1). Multiplying this ratio by 100 yields the percentage
of total heat energy at the surface used to raise the temperature
of air above it.
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Source: CGIS
Source: CGIS
Surface materials respond differently when exposed to
the same amounts of solar radiation. Some heat rapidly whereas others
heat slowly. This property is called specific heat.
Specific heat is the amount of energy required
to raise the temperature of one gram of a substance 1 °C.
For example, it takes five times more energy to raise the
temperature of water than it does to raise the temperature
of concrete; therefore water has the higher specific heat
capacity.
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Impervious surface materials such as asphalt, concrete,
and brick heat rapidly after absorbing relatively small
amounts of energy, whereas considerably more energy is needed
to raise the temperatures of wood, wet mud, and water. Also,
impervious surfaces quickly release heat to the atmosphere.
These phenomena, coupled with other factors, results in
higher daytime temperatures for urban areas compared to
surrounding rural areas.
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Impervious surfaces are more thermally conductive than
dry pervious surface materials. The thermal conductivity
of materials is usually expressed in watts divided by meter
multiplied by the Kelvin temperature (W/mK).
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Source: CGIS
The lower albedos, lower specific heat capacities, and
higher thermal conductivities of impervious surfaces, coupled
with reduced evaporative cooling, serve to increase daytime
temperatures over urban areas, especially during the summer.
These factors contribute to the phenomenon known as the
urban heat island effect. Maximum summer temperatures for
urban areas are between 1.0 to 3.0 °C (1.8 - 5.4 °F)
warmer than surrounding rural areas. The increasing imperviousness
of watersheds due to sprawling development does not bode
well for human health and comfort given projected increases
in Mid-Atlantic heat waves brought on by global warming.
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Curious about Urban Heat Islands and Global Warming?
Looking to BEAT THE HEAT? Check these cool sites:
Cooling Our Cities (US Department of Energy)
http://www.eere.energy.gov/
American Forests
www.americanforests.org/
Urban Climatology and Air Quality
wwwghcc.msfc.nasa.gov/urban/
Global Hydrology and Climate Center
wwwghcc.msfc.nasa.gov/ghcc_home.html
EPA Global Warming: Find out howl global warming will
effect the environment, the economy, and your state.
www.epa.gov/globalwarming/
Mid-Atlantic Regional Assessment Team - MARA. Find out
about the potential consequences of global warming for the Chesapeake
Bay Watershed and adjacent areas.
www.essc.psu.edu/mara
