Whereas an acre of forest or cropland receives as much sunlight as an acre of buildings, it is well known that the green space will be cooler because of transpiration (water evaporation) and shading of the ground. Typical urban surfaces, such as concrete and asphalt, get much hotter than vegetated surfaces during the day. They store the energy and release it at night, thus creating a dome of warmer air over the city. Thus, the "urban heat island" causes increased air conditioning usage, as well as being directly related to increased ozone formation, a major pollutant in our cities.
Rapid population growth in the last 25 years (27% between 1970-80, and
from 33% in 1980-90) has made Atlanta one of the fastest growing
metropolitan areas in the United States. This rapid growth has translated
into an approximately 17% decline in forest land in the Atlanta
metropolitan region between 1973 and 1992. Atlanta is also a model for
what is in store for other cities, especially in developing nations where
the industrial revolution is taking off and threatens large areas of
forest or jungle. This enormous transition of land from forest and
agriculture to an urban landscape results in subsequent changes in the
land-atmosphere energy balance relationships.
To
understand the relationship of land use patterns to heat production and
its effect on the lowest layers in the atmosphere, the GHCC conducted the
Atlanta Urban Heat Island Experiment in May 1997. As the NASA Learjet
carried the Advanced Thermal and Land Applications Sensor (ATLAS) 5 km
above the city on May 11-12, 1997, the GHCC asked Atlanta-area school
children to measure temperatures around their schools to provide
"ground truth" data (as well as a science lesson for the
students). Using the ATLAS data as a baseline for land cover/land use,
GHCC scientists and their partners are performing "prospective"
modeling on how meteorological conditions and air quality will change,
predicated on the Atlanta Regional Commission's (ARC) 20-year plan. They
hope to provide the ARC and other planning or decision-making bodies with
model output that can be used to modify or revise growth plans for the
Atlanta metropolitan area, and to help mitigate or ameliorate the
expansion of the urban heat island effect or possible further
deterioration in air quality.
Other research performed by GHCC scientists has been in association with
the Southern Oxidant Study (SOS), a consortium of universities and
government laboratories. In 1995, a large-scale observation program in the
Nashville area included several major research aircraft, and was probably
the biggest concerted air pollution field campaign ever conducted in the
U.S. Ozone data collected by aircraft dramatically illustrated the effect
of urbanization upon local air chemistry. Analysis of data from this
project, including its use in the validation and development of more
sophisticated models, continued in 1997.
Building on the Atlanta and SOS research, an EPA-funded GHCC project has been launched to examine urban land use and its effect on thermal characteristics and processes that drive the development of the urban heat island phenomenon for nine cities across the United States. Important information will be gained that can be used to augment the Cool Communities program by having a better understanding of how the size, geographic location, and land cover/land use composition of different U.S. cities affects heating patterns. Data collected using aircraft and/or satellite sensors will be used as input to a sophisticated EPA model used by the Lawrence Berkeley National Laboratory to determine the effects of land use urban heating on the generation of ozone. By combining the aircraft data with satellite data in some of the cities, knowledge will be gained that will allow the application of satellite data alone for other cities.
Information provided by: http://wwwghcc.msfc.nasa.gov