Thursday, January 22, 2009


Measurement of Heat Island

Several techniques are applied to measure heat islands. The importance of these techniques depends upon the nature of requirements. Micro-scale heat island measurements are done by the temperature sensors and some instrumentation which are fairly accurate and well-established. However, in the viewpoint of large-scale measurements such as a mega-city, these are not useful. Site observation with the help of sensors in a mobile source such as a car is one of the important tools to measure heat island effect but is labor intensive and the result difficult to validate due to varying weather conditions each time the observation is done.

Recently, remote sensing technology with the help of satellite images is commonly being used to get information on heat islands. Remote sensing techniques can be used to obtain the thermal images of the place in concern and provide information on land use. Loss of green surfaces, information on surface reflectivity of solar radiation and buildings can be obtained with the help of satellite images. The comparison between past and present date can show the trend of heat island along with land use information which are very important in identifying the degree of severity of heat island phenomenon in a particular place.

There are inherent problems though of remote sensing technology in the planning process. It can provide thermal images but there is difficulty in segregating the types of thermal sources such as from mobile sources or stationary sources. It provides snapshot of situation without any knowledge of the mechanisms that is going on in the urban system. The land use, building and transportation information could be obtained from remote sensing techniques but it is not possible to see their contribution and sensitiveness on the heat island phenomenon.

The information obtained from remote sensing need to be coupled with numerical climatic models in order to analyze the effect of various planning alternatives of land use and heat discharge to improve the urban heat environment. These models are able to study the physical climatic phenomenon in the urban system. In this sense, remote sensing data along with Geographic Information System (GIS) is a powerful tool in providing information to the numerical models which can study, simulate various planning alternatives and can predict the implications on heat environment. Numerical models are the powerful tools to understand the mechanisms of heat island. These models can be validated with site data measurements or from remote sensing techniques.

The following image is an aerial thermal image of a mall and surroundings located in Huntsville, Alabama. The image, courtesy of NASA, was taken approximately five hours after sunset. The dark shades correspond to cooler temperatures. The mall parking lot (lower left quadrant of the photo) has a temperature of 24.0 degrees Celsius, while a forest, located in the upper right quadrant has a temperature of 17.1 degrees Celsius.

Although satellite data are very useful for analysis of the urban heat island effect at a coarse level, they do not lend themselves to developing a better understanding of which surfaces across the city contribute to or drive the development of the urban heat island effect. Analysis of thermal energy responses for specific or discrete surfaces typical of the urban landscape (e.g. asphalt, building rooftops, vegetation) requires measurements at a very fine spatial scale (i.e., <15m)>

The explosion of new knowledge on the theoretical aspects of urban climate change is not well matched by practical applications. In particular, urban designers and planners are yet to utilize the current knowledge to develop architectural and urban design strategies for the mitigation of the negative effects of urban heat island. This is in part due to some weaknesses in current methods. For example, some of the problems associated with remote sensing techniques hinder the detection of air temperature heat island that directly affects human comfort as opposed to surface temperature heat island. These problems include, difficulties in "seeing" the vertical active surfaces, the not so well defined coupling of surface and air temperatures in urban areas and inhomogeniety of urban surfaces leading to a patch work of emissivity and albedo. The problem with urban-rural difference method in general is that it assumes weather over time remains constant. Furthermore, the intra-urban differences are ignored. It is pointed out that it is the intra-urban climatic difference that is of value for urban planners and designers interested in mitigating the negative effects of UHIs. In other methods, it is assumed that rural climate is somehow "natural" to the area. However, in the context of rapid global urbanization, there are very few rural areas remaining with their "natural" climates intact.


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