The term ‘urban heat island’ refers to the localized increase in temperature associated with an urban area. The UHI is an example of unintentional climate modification when urbanization changes the characteristics of the earth’s surface and atmosphere. It was observed that the UHI effect might result in minimum urban temperatures being 5-6° greater than the surrounding countryside. In the case of London, mean annual temperature was 11°C, while the surrounding countryside was 9.6°C and the suburbs was 10.3°C in the period between 1932—1960. In Kew, London, it has an average of some 72 frost-free days than rural Wisley The reason the city is warmer than the country comes down to a difference between the energy gains and losses of each region. The obvious reason is that the urban structure has changed the atmospheric composition and altered the UH budget.
Firstly, during the day in rural areas, the solar energy absorbed near the ground evaporates water from the vegetation and soil. Thus, while there is a net solar energy gain, this is compensated to some degree by evaporative cooling. In cities with less vegetation, urban structures like buildings and sidewalks absorb most of the solar energy input. As cities have drainage systems like gutters, less surface water remain on the impervious pavements resulting in less evaporation cooling, contributing to higher air temperatures. The lack of expenditure of latent heat (heat used for evaporation) means that by day, 70-80% of net radiation may be transferred to the atmosphere as sensible heat (heat which can be felt and increase air temperatures)
Secondly, the intensity of the UHI is enhanced by the emission of heat trapping pollutants into the urban atmosphere along with the heat lost from homes, factories and vehicles. As urban areas are often inhabited by large numbers of people, heat generation by human activity also contributes to the UHI. Such activities include the operation of automobiles, air conditioning...
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