One of the fundamental components that sets a
city apart from its rural surroundings is the climate that prevails over
urban environments. In urban areas, buildings and paved surfaces have gradually
replaced preexisting natural landscapes. As a result, solar energy is absorbed
into roads and rooftops, causing the surface temperature of urban structures
to become 50 - 70 °F higher than the ambient air temperatures. (Taha,
Akbari & Sailor 1992). The image below shows albedo values for various
urban surfaces, the albedo is a measure of the amount of solar energy reflected
by the surface. As such, low albedo implies higher surface temperatures
since the larger amounts of energy are absorbed. As surfaces throughout
an entire community or city become hotter, overall ambient air temperature
increases. This phenomenon, known as an "urban heat island," can
raise air temperature in a city by 2 - 8 °F. (Oke 1987 and World Meteorological
Organization 1984).
 The resulting higher temperature caused by the urban
heat island has the effect of increasing the demand for cooling energy in
commercial and residential buildings. Increased demand for energy can cost
consumers and municipalities thousands of additional dollars in air conditioning
bills in order to maintain comfort levels. In addition, increased electricity
generation by power plants leads to higher emissions of sulfur dioxide,
carbon monoxide, nitrous oxides, and suspended particulates, as well as
carbon dioxide, a greenhouse gas known to contribute to global warming and
climate change. Finally, summer heat islands often accelerate the formation
of harmful smog, as ozone precursors such as nitrous oxides (NOx) and volatile
organic compounds (VOCs) combine photochemically to produce ground level
ozone. (SOS 1995).
On a larger scale, modification of the landscape through urbanization
alters the natural channeling of energy through the atmospheric, land and
water systems. Although large-scale atmospheric and climatic phenomena are
global in scope, urban areas cannot be viewed in isolation because the local
environment modifies the conditions in the thin air stratum above the ground,
generally referred to as the atmospheric boundary layer. As humans alter
the character of the natural landscape in the city-building process, the
local energy exchanges that take place within the boundary layer are affected.
Therefore, modification of the landscape influences the local (microscale),
mesoscale, and even the macroscale climate.
Return to Urban Studies
Responsible Official: Dr. Steven J. Goodman (steven.goodman@nasa.gov)
Page Curator: Diane Samuelson (diane.samuelson@msfc.nasa.gov)
Last Updated: August 5, 1999
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