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Urban Heat Islands: A Cause of Concern

 

Haphazard development in urban areas has led to undesirable changes in the landscape. The areas which were once open land or covered with vegetation have been converted into concrete forests, asphalt covered roads, sidewalks and other infrastructure. The urbanization leads to the formation of Urban Heat Islands (UHIs) - the phenomenon referring to warmer temperature in the city relative to outlying rural areas. According to the Environment Protection Agency, bigger cities in US have air temperature up to 5ºC more than the nearby natural land cover. UHIs effects are most intense during the clear sky and calm winds as heavy cloud cover prevents solar radiation to reach earth surface, reducing day time warming in cities and strong winds accelerate atmospheric mixing, decreasing urban rural temperature gradient.

How does it form?

There are various factors that contribute to the formation of UHIs but the significant causes of UHIs formation are urban developments by changing in land surface and waste heat generation by energy consumption in day to day lives.

When you travel to rural areas, you will find that most of the regions is occupied by natural cover or vegetation and farmland covered with crops. In rural areas, plants and trees acts as nature’s air conditioner by providing cooling effect of shading and evaporation of water from leaves and soil, also known as evapotranspiration. On the contrary, in urban areas tall buildings, roads, sidewalks and other infrastructure built of asphalt, concrete, bricks, glass and steel, displaces natural vegetation. All these building materials are mostly dark in colour, non-reflective and water resistant in nature which absorb incident solar radiations resulting in surface temperature and overall ambient temperature to increase significantly.



Urban areas are densely populated, as more and more people around the globe moving to big cities in search of work and opportunity. To accommodate the growing population in the city, engineers creates tall buildings or skyscrapers. These buildings are constructed in a manner that there is no room left for air flow. As a result, heat that escapes insulation trapped in and between tall buildings, further aggravating UHI effect.


Moreover, other factors such as “waste heat” emitted by people’s activities also additionally contribute to UHIs effects. The production of waste heat from ACs and refrigerators, as well motor vehicles and factories which are continuously emitting heat energy and just other day to day activities are also recognized as contributing factors to UHIs effects.

Types of UHIs

Generally, earth’s surface and atmospheric air tend to have difference in temperature. That’s why heat islands are of two types: surface heat islands and atmospheric heat islands. They are classified on the basis of ways they are formed, the techniques used to identify and measure them, their impacts and to some extent, the methods available to alleviate them.

 


·         Surface heat islands

Surface heat islands forms when on a hot, sunny summer day urban areas such as roadways and rooftops absorb and emit heat to a larger extent and are hotter up to 27-50ºC relative to air temperature. On the other hand, natural surfaces which are more often found in rural areas remain close to air temperature. Surface heat islands generally occurs day and night but tend to be most intense during day time when the sun is shining.

·         Atmospheric heat islands

These are formed as a result of warmer air in urban areas as compare to rural surroundings in vicinity. Atmospheric heat islands are most intense after sunset due to the slow release of heat from urban structure 

Impacts of UHIs

v  Worsen water quality

v  Higher energy consumption

v  Increased emissions of greenhouse gases and air pollutants

v  Risk of human health and comfort.

Mitigation Strategies

v  To increase surface reflectivity (higher albedo) by covering the construction material surface with light colour or white paint (white topping) in order to reduce incident radiation absorption by built surfaces.

v  To increase vegetation cover mainly in the form of forests and parks.

 

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