An Urban Heat Island

What is an Urban Heat Island?

An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The temperature difference is usually larger at night than during the day, and is most obvious when winds are weak. This means that less energy is used up evaporating water, that less of the Sun’s energy is reflected and that more heat is stored by buildings and the ground in urban than in rural areas. The heat generated by heating, cooling, transport and other energy uses also contributes, particularly in winter, as does the complex three dimensional structure of the urban landscape.

What are the effects of Urban Heat Island?

Towns and cities are generally a few degrees warmer than rural areas, as a result of the urban heat island (UHI) effect. This urban heat has negative impacts on health, interacts with air pollution, and puts extra pressure on energy systems to deliver cooling. The UHI phenomenon is well known, but the exact characteristics are particular to each city or urbanised area, and vary by geographical location. Globally, urban populations are rapidly growing, and temperatures are rising because of climate change. It is therefore important to fully understand the UHI if we are to be able to manage potential harmful impacts.

How it happens?

When we build and expand cities, we tend to erect buildings with dark surfaces and lay down asphalt pavement. The buildings and the pavement absorb a significant amount of light and radiation and emit it as heat, warming the city. Because more than half of the surfaces in cities are man-made, cities heat up more than rural areas, where structures are less concentrated. This heat absorption is why the temperature difference between cities and rural areas is highest a few hours after sunset. Cities hold on to more heat for a longer period of time than rural areas do.

But that’s not the only thing that causes the urban heat island effect. Scientists believe that vegetation plays a large part in keeping an area cool through a process called evaporative coolingEvaporation is when liquid turns into gas. Plants take in water through their roots and depend on it to live. But after the plant is done with it, dry air absorbs that water by turning it into gaseous water vapour. The air provides the heat that drives this process, so during the process, the air loses heat and becomes cooler. We experience the same type of thing when we sweat — when air hits your sweaty skin, it absorbs the moisture and cools the air around you. Because building a city means replacing vegetation with structures, the city loses the evaporative cooling advantages of vegetation.

Other factors also contribute to the effect. For instance, cars and air conditioners, which are ubiquitous in urban areas, convert energy to heat and release this heat into the air.

Techniques to reduce Urban Heat Island Effect

Following are the techniques to reduce Urban Heat Island (UHI) effect ;

1. By making the roof Green and Cool-

“Green” roofing and “cool” roofing are two ways that densely populated cities are combatting increased temperatures in an effort to mitigate the UHI effect. Green roofs absorb stormwater, provide space to grow food and habitat for pollinators, and reduce ambient heat through evapotranspiration (the process through which plants release water into the surrounding atmosphere). When developing green roofs in urban environments, special attention should be given to choosing vegetation that is native to the local environment and tolerant to local conditions.

A “green roof” is simply a roof that includes plants and vegetation. Green roofs harness the same evaporative cooling effect that cities lose when they hack away vegetation. So a green roof not only prevents the building’s roof from absorbing heat, but cools the air around it, offsetting the urban heat island effect to an extent. Many sustainable buildings use green roofs to reduce their reliance on energy consumption.

 Roof sprinkling is a evaporative cooling solution. Sprinklers on the roof wet the surface so that the air around it cools through evaporation. Urban planners also set up traditional parking lots along lots where trees and vegetation grow. Tall trees not only contribute to evaporative cooling but also provide much-needed shade.

2. Cool Pavements Produce Cold, Hard Benefits

By definition, a “cool” pavement stays cooler than standard pavements because reflectivity is enhanced by using surface coatings and reflective material aggregates. Rethinking which materials are used in critical infrastructure is crucial to mitigating the Urban Heat Island effect. Much like standard roofs, dark pavements and asphalts absorb up to 95% of the sunlight that they are exposed to and radiate that heat out into the immediate surroundings. Hot pavements can aggravate the UHI effect because they make up roughly one third of urban surfaces and also increase the temperature of stormwater runoff. 

3. The Urban Tree Canopy

Urban tree canopy (UTC) is the layer of leaves, branches, and stems of trees that cover the ground when viewed from above. In urban areas, the UTC provides an important stormwater management function by intercepting rainfall that would otherwise run off of paved surfaces and be transported into local waters though the storm drainage system, picking up various pollutants along the way. UTC also reduces the urban heat island effect, reduces heating/cooling costs, lowers air temperatures, reduces air pollution, increases property values, provides wildlife habitat, and provides aesthetic and community benefits such as improved quality of life.

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