Urban heat islands making our cities hotter

Heat islands could pose a true threat in forming heatwaves within urban areas

Mohsen Hassan is a Mechanical Engineer Consultant at Khatib & Alam.
Mohsen Hassan is a Mechanical Engineer Consultant at Khatib & Alam.

Urban heat islands (UHI) are best described as the temperature gradient difference between developed and undeveloped areas.

This temperature discrepancy induces an upsurge in the air and surface temperatures. The elevation has a deep impact on the physical and chemical properties of the atmosphere.

Islands are mostly common in highly-populated and densely-constructed urban communities. In addition to altering the environment, heat islands are considered to be a main factor in increasing the air cooling costs, air pollution levels, energy demand and heat affiliated illnesses.

The current climate change model predicts an increase in the temperature portfolio throughout the coming years. Although the temperature disparity is minor, heat islands could pose a true threat in forming heatwaves within urban areas. An attempt to diminish these islands would contribute in minimising the adverse outcome on microclimates, human, wildlife and aquatic habitats.

Urbanisation has produced a mass fragmentation of vegetated areas in cities. Compared to urban communities, rural areas are less affected by the heat islands. During daytime, natural surfaces composed of vegetation and soil absorb a large portion of the incident sunlight. This absorption causes the release of water in moisture-trapping soils, a process called evapotranspiration, in which evaporated air cools the surrounding atmosphere. The other fraction of the incident radiation is reflected in the form of heat.

In cities, most exteriors are man-made surfaces formed from non-reflective water-resistant material. Large areas covered with cement have a low SRI (solar reflectance index): they absorb the solar rays. Moreover, non-porous surfaces such as asphalt, tar and concrete possess high thermal conductivity properties. These surfaces block the absorption of rainwater and form hefty heat conductors.

Waste heat from buildings, sidewalks, parking lots, and roads present another factor that contributes to the heat island effect. Energy dissipation from cars, people and concrete buildings constitute a great portion of the thermal inclination.

Modern construction design has focused on creating highly insulated building models. The high insulation prohibited the transfer of heat from the surrounding to the buildings. This method has created a warmer envelope around the buildings. Air quality is also affected. Pollutants from industry and vehicles are trapped in the warm air. These pollutants are trapped within the urban landscape and are not able to scatter. The aquatic environment is also suffering.

Species that have been habituated to a cooler environment would not survive in the newly warm water.

During summer time the heat islands intensify, increasing the need for air conditioning. This in turn increases the need for energy production and electricity. Heat islands could also amend noise propagation. Due to higher temperature profiles, noise pollution increases intensely. Global earth warming is also a main topic that is directly affected by this phenomenon.

Architects and designers are responsible for the mitigation of heat islands. Through sustainable designs and modern urban planning, they are able to reduce the impact of the heat islands and keep cities cooler. Moreover, vegetated areas within the urban communities help in intercepting the incident radiations and assist in atmospheric cooling. Decision-makers must adhere to green and sustainable standards by choosing material with a high SRI. Environmental policies and programmes must stress on reducing the impact of these heat islands in urban communities.

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