Urban Heat Island Effect

By Author: Henry Ford
Total Articles: 189
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Heat island is defined as an area which is urbanized and hotter than the surrounding rural areas. It is normally experienced when temperatures of an urban area rise beyond the temperatures of the suburban regions as well as the rural regions. A research study showed that in summer, the temperature of the surrounding air in the urban areas would rise between 6 oF to about 8 oF beyond the temperatures of the surrounding rural regions. Most of the activities taking place in the urban areas are the influential factors that affect the climatic conditions of those cities. The causes of urban heat effects include the destruction of natural vegetation due to construction of buildings together with other infrastructures in the urban areas. Waste heat which are released from the vehicles together with heat being released from factories, other industrial plants and air conditioners. To get broader perspective, contributing factors are discussed below.

Factor contributing to Urban Heat Effects
Anthropogenic sources of heat and pollutions come from the Cities. These cities are characterized by large percentage of dry asphalt ...
... and concrete and water proof surface which contain albedoes and capacities of heat which can convert sun radiations in to sensible heat (Stull 609). Urban heat effects are normally steered by three basic mechanisms which include increased heat absorption and storage potential of surfaces; reduced evapotranspiration, and the emission of waste energy in the form of heat from industrial plants (Ruth 318). Destruction of trees and vegetation is one of the causes of urban heat effects. Trees and other vegetation are very important in controlling atmospheric temperature because of their contribution to cooling effects of the surrounding air. If these trees are destroyed, the mechanism for the provision of cooling effect will not take place. The removal of soil and replacement of pavement is also another cause of urban heat. Soil do not absorbs and retain much heat as compared to pavements that have high heat absorption and retention capacity. Due to these properties, urban cities with pavements everywhere will exhibit a rise in temperature as compare to the neighboring rural areas (National Research Council U.S 88)

Urban heat effect may also be caused by human beings themselves because in the previous century only two percent of the total population was in the urban regions but currently there are about twenty five percent of the total populations of the whole world who are residing in towns and cities. In the developed countries with vast industrialized capacity, more than fifty percent of the population lives in town. For instance, in United States of America, a study indicated that human effect in small towns had a significant cause to increase in temperatures. The difference in temperature was noted to be not less than 0.1o C the United States, mean temperatures and the global mean temperatures. This shows that human population affect temperature change both in small towns and big cities. (Michael, 2004).

Another study to determine the rate at which land use causes climatic change as opposed to urbanization gave a result that land use will only contribute to a mean daily temperature increase of 0.35 o C per century which is lower as compared to temperature increase due to Urbanization (Kalnay and Cai (2003).There was also a research to determine on urbanization effects on temperature which has taken place since nineteen century. It indicates that changes in the temperature difference were recorded in different periods from the year1900 to the year 2000. For example, global warming was experienced from the year 1900 to 1940; global cooling was also experienced from the year 1940 to 1965 and another period of global warming was again experienced from 1965 to 2000. City with more than a population of one million people also experiences annual mean temperature of 1o C to 3 o C higher than its surrounding whereas evening temperatures can have a difference of 12o C between city temperatures and the rural areas surrounding it. All these change are dependant on urbanization level (Hansen, Ruedy, Imhoff, Lawrence, Easterling, Peterson, and Carl, 2001).

Cities have microclimate which are linked to surface composition and structures which are found within. There is interdependency between the urban and the atmosphere in the City therefore they should be treated together. An example is the microclimate is the shade effect caused by tall buildings. The building styles together with materials being used are destructive to the traditional designs and practices which adapted to the natural climate. It was noted that three dimension building structure systems absorbs much heat radiations from the sun than two dimension horizontal surface under cloudless condition (Wener and Stellar 181). Other causes of urban include buildings and other materials such as concrete and asphalts which are also found in these cities. They are good absorbers of heat radiation and can store large amounts of heat as compared to natural vegetation found in the surrounding rural regions. A lot of heat is also being released from heating sources and air conditioning machines; power generating plants, industrial plants, vehicles and several heating equipments such iron boxes and cookers. These sources releases bedspread of polluting materials that covers the whole city. These materials are released together with small particles, carbon dioxide and carbon monoxide gases and water in the form of vapor among other evaporates. These chemical gases released are the green house gases which are the major cause of global warming. These gases also act as nuclei for condensation process which will further encourage precipitation. It therefore shows that the cities are not only hot but also experiences continuous precipitation (Michael, 2004). Heavy loads of solid particles, gases and other liquid contaminants that are found in the urban atmosphere are estimated to be ten times more in particulates in the city than in the rural areas. These reflect back incoming solar radiations and also reduces the rate of out flow of heat from the earth atmosphere causing temperature build up (Hough 32)

Effects of Urban Heat Island
When there are high temperatures in the cities, it affects the environment in two different ways. First it perpetuates the development of compounds like smog. This occurs when pollutants in the air are subjected to higher temperatures. The smog is destructive to the environment and also is dangerous to human health. There is also high demand for the cooling as well as air conditioning equipment to solve problems of urban heat Island. This will further affect the consumer through high prices of these equipments. With the increase in number of these air conditioners, there will be a lot of energy required to run these equipment which will finally increase the amount of chemical gases being released into the atmosphere. The cost of high energy and equipment is all transferred to the consumers. It is estimated that about fifteen percent of electric energy consumed in Los Angeles are used to solve the consequences of urban heat island (Mathias, Kieran, & Paul, 2006). No major study on the energy balance climatology has been done with an exception of ration of artificial and natural radiation heat in relation to the solar radiation and pollution of air (Wener 31-53).

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