What is an urban heat island?
An urban heat island is an urban area or metropolitan area that is significantly warmer than its surrounding rural area due to high concentration of high rise concrete buildings, metal roads, sparse
vegetation cover and less exposure of soil.
These factors cause urban regions to become warmer than their rural surroundings, forming an “island” of
higher temperatures.
“Urban heat islands” occur when cities replace natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat.
Urban heat islands are one of the easiest ways to see how human impact can change our planet.
How urban heat islands are formed and what are the causes?
The sun’s heat and light reach the urban area and the rural area in the same way. The difference in temperature between urban and less-developed rural areas has to do with how well the surfaces in each environment absorb and hold heat.
If we travel to a rural area, we will probably find that most of the region is covered with plants. Plants take up water from the ground through their roots. Then, they store the water in their stems and leaves. The water eventually travels to small holes on the underside of leaves. There, the liquid water turns into water vapor and is released into the air. This process is called transpiration. It acts as nature’s air conditioner.
When we visit a big city, we won’t see many plants. Instead, we will see sidewalks, streets, parking lots and tall buildings. These structures are usually made up of materials such as cement, asphalt, brick, glass, steel and dark roofs.
First of all, materials such as asphalt, steel, and brick are often very dark colors—like black, brown and grey. A dark object absorbs all wavelengths of light energy and converts them into heat, so the object gets warm.
In contrast, a white object reflects all wavelengths of light. The light is not converted into heat and the temperature of the white object does not increase noticeably. Thus, dark objects—such as building materials—absorb heat from the sun.
Urban building materials are another reason that urban areas trap heat. Many modern building materials are impervious surfaces. This means that water can’t flow through surfaces like a brick or a patch of cement like it would through a plant. Without a cycle of flowing and evaporating water, these surfaces have nothing to cool them down.
Impact of urban heat island effect
Urban heat island effect increases energy costs (e.g., for air conditioning), air pollution levels, and heat-related illness and mortality.
Heatwaves affect human and animal health, leading to exhaustion, dehydration and increased mortality rate.
It increases the colonization of species that like warm temperatures, such as lizards, geckos and insects such as ants.
What are the ways to reduce the impact of urban heat island?
Increase shade around your home – Planting trees and other vegetation, provides shade and cooling effect through evapotranspiration and it lowers the surface and air temperature.
Install green and cool roofs – Green roofs are an ideal heat island reduction strategy, providing both direct and ambient cooling effects. In addition, green roofs improve air quality by reducing the heat island effect and absorbing pollutants.
‘Lightening’ streets – This is done by covering black asphalt streets, parking lots, and dark roofs with a more reflective gray coating.
Permeable materials – To help cool the heat island, builders can use materials that will allow water to flow through. These building materials—called permeable materials—promote the capture and flow of water, which cools urban regions.
Use energy-efficient appliances and equipments.
Shift all industries away from the urban area.
Reduce emission from automobiles.
PRACTICE QUESTIONS
QUES . Bring out the causes for the formation of heat islands in the urban habitat of the world . UPSC Mains 2013
