Measuring Heat Islands

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Measuring Heat Islands
Heat Island Video Segments

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Heat islands may be identified by measuring surface or air temperatures.

Surface Measurements

Satellite Image of Atlanta's Urban Heat Island

Satellite (Lands at TM) image of multi-nodal heat island in Atlanta, GA. Darker tones denote higher temperatures.

The satellite image of Atlanta, GA is an example of a surface-based measurement, which records radiant emissions, or energy reflected and emitted from the land, including roofs, pavements, vegetation, bare ground, and water.

All surfaces give off thermal energy, or radiant emissions, that is emitted in wavelengths. These wavelengths can be identified and measured using remote sensing. Remote sensing is a process in which instruments are used to "capture" the relationship of objects and materials observable at a distance.

Using radiometers mounted on aircraft or a satellite allows researchers to easily collect a large number of surface observations. The National Aeronautics and Space Administration (NASA) conducted flyovers using an aircraft-mounted sensor in Baton Rouge, Sacramento, and Salt Lake City.

Several cities also use Landsat satellite data to classify land cover and identify heat islands. The Landsat 7 satellite, a U.S. satellite used to acquire remotely sensed images of the earth's land surface and surrounding coastal regions, provides information from which we can derive surface temperatures and evaluate heat islands.

Air Temperature Measurements

The urban heat island graphic illustrates a city's typical air temperature profile. Air temperatures are usually measured at about 5 feet (1.5 meters) above the ground, where standard weather observations are taken. Various sources can be Graphic depicting a typical rise in temperature from rural areas to an urban center. used to take these measurements including:

National Weather Service stations.
Military weather stations.
Urban weather station networks. While there is no uniform urban network, several areas have established ad hoc systems, often as part of a special research program. The Baltimore Ecosystem Study (BES) is one example.
Regional weather station networks – agricultural, air quality monitoring, and utility networks.
Field campaigns and transect studies , which involve using hand-held measurement devices or mounting measurement equipment on cars or aircraft.

These data sources are used to evaluate long-term and short-term trends in air temperature. Some stations have recorded temperature and other meteorological observations for over 100 years.

Disadvantages of Surface and Air Temperature Measurements

Surface measurements taken using remotely sensed imagery do not fully capture radiant emissions from vertical surfaces, such as a building's wall. This is because the equipment primarily observes emissions from horizontal surfaces like streets, rooftops, and treetops.
Remotely sensed data represent radiation that has traveled through the atmosphere twice, as wavelengths travel from the sun to the earth as well as from the earth to the atmosphere. Thus, the data need to be corrected to accurately estimate surface properties including solar reflectance and temperature.
Using air temperature measurements to evaluate a heat island is complicated and requires locating weather stations both within the city and in non-urban areas close to the city. Researchers need to consider changes in instrumentation, sampling, data recording methods, and station microclimate.

Further, because airports often are used as the source of "non-urban" data in urban-rural comparisons, these comparisons become less valid as areas around airports become more urbanized.
Air temperatures may not provide a reliable indicator of the thermal properties of various surfaces, depending on the height of the measurement. Because the atmosphere has fluid properties, the relationship between surface and air temperature decreases with altitude.

Despite these disadvantages, remotely sensed data provide a powerful representation of urban heat islands. Additionally, air temperatures are directly linked to many of the important impacts of heat islands, making air temperature a useful indicator of impact severity.

Removing the Heat Island Effect from the Global Temperature Records

Heat islands may skew long-term temperature records as urbanization encroaches on weather stations located near the outskirts of town. Consequently, researchers need to remove heat island effects from global temperature records to accurately account for climate change.

To minimize bias in the temperature trend observed at urban weather stations, researchers need a broad geographic sampling (both urban and rural) of data, as well as mid-atmospheric temperature observations.