The urban heat island (UHI) effect arises due to absorption of incident radiation from the sun by built surfaces of tall buildings, roof, concrete structures and asphalt roads and then releasing it in the form of heat. The term “urban heat island” describes the built-up areas that are significantly hotter than the surrounding open, natural or rural areas. It occurs on the surface and in the atmosphere. The built surfaces are made of high-percentage of non-reflective and water-resistant construction materials. These materials act as heat sinks that absorb the radiated heat and store it for long time.
Lack of sufficient wind, change in thermal properties of the surface materials and lack of evapotranspiration rate in urban areas cause the urban heat island effect. On the other hand, green, wooded and open spaces composed of vegetation and moisture trapping soil use large proportion of absorbed radiation and release them through evapotranspiration process. As evaporation causes cooling effect, the released water vapour contributes to cool the air in the vicinity. On a hot summer day, the urban surfaces are exposed to high temperature of 50–90°F (27–50°C) hotter than the air, where as the temperature of the shades or green open areas surrounding the urban surfaces remain close to air temperature. These changes in temperature between two areas create an “island” of higher temperature in the urban landscape. Normally the temperature difference of higher than 10 degrees forms heat islands.
The increase in temperature in urban areas due to UHI effect can have negative impacts on three pillars of sustainability, i.e. environment, people and economy. Some of the negative impacts include:
- Increase in energy consumption – Increase in temperature leads to increase in demand for cooling, which subsequently puts pressure on electricity supply during the peak periods of demand.
- Increase in emission of air pollutants and GHGs – As more electricity is needed to cool the surfaces, demand on energy supply leads to emissions of air pollutants and greenhouse gases from the power plants. Even use of ozone depleting refrigerants such as CFCs in the air-conditioning system cause depletion in stratospheric ozone layer. Elevated temperature also promotes the formation of ground-level ozone.
- Demand on water – As the surface and air get hotter, people consume more water for both indoor and outdoor usage and it puts pressure on water supply.
- Ecosystem – Hot surfaces transfer the absorbed heat to water features such as rivers, streams, ponds, lake etc. increase the surface water temperature and alerting the aquatic ecosystem structure and functions
- Quality of life – Elevated day and night temperatures along with higher air pollution can cause respiratory diseases, discomfort, heat stress and decrease productivity and increase heat related mortalities.
Ways to Reduce Heat Island Effect
The heat island effect can be reduced by using following strategies.
- Build small – Minimise building footprint and maximise open space
- Minimize hardscape – Design driveways, roads, parking space and hardscape areas smartly by using permeable materials or surfaces such as vegetated roofs, porous pavement and grid pavers. Use open grid pavement system, which is at least 50% pervious and locating the parking space under the building will help reducing the urban heat island effect.
- Use of reflective materials – Use high reflective materials with high solar reflective index (SRI) values for roofs and non-roof exterior surfaces. The SRI value is the combined value of reflectivity and emmitance.
- Shading – Provide shading with existing tree canopy or new trees or with other structures. The surfaces can also be coved by solar panels that produce renewable energy. Shading with some architectural features of SRI of at least 29 will also help to reduce the heat island effect.
- High albedo cool roof and green roofs: Combination of high albedo cool roofs (roofs with controlled SRI) and vegetated roof surface can reduce heat island effect significantly.
Studies have found that the mean daily temperature increase is consistent with increase in urban development. The composition of land cover features can significantly influence the magnitude of land surface temperature. Hence, increase in percent of vegetation is the most essential driver of reducing the land surface temperature and hence the UHI effect. Therefore, proper management of green space is needed to mitigate the UHI effect in the urban cities of arid and semi arid countries.
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