What Is The Urban Heat Island Effect?

An urban heat island, or UHI, is a metropolitan area that’s a lot warmer than the rural areas surrounding it. Heat is created by energy from all the people, cars, buses, and trains in big cities like New York, Paris, and London. Urban heat islands are created in areas like these: places that have lots of activity and lots of people.

What Creates The Urban Heat Island Effect?

There are many reasons for UHIs. When houses, shops, and industrial buildings are constructed close together, it can create a UHI. Building materials are usually very good at insulating, or holding in heat. This insulation makes the areas around buildings warmer.

“Waste heat” also contributes to a UHI. People and their tools, such as cars and factories, are always burning off energy, whether they’re jogging, driving, or just living their day-to-day lives. The energy people burn off usually escapes in the form of heat. And if there are a lot of people in one area, that’s a lot of heat.

Reducing The Urban Heat Island Effect

Green roofs are recognised to have a positive effect on reducing the UHIE (Urban Heat Island Effect).

A green roof, or rooftop garden, is a vegetative layer grown on a rooftop. Green roofs provide shade and remove heat from the air through evapotranspiration, reducing temperatures of the roof surface and the surrounding air. On hot summer days, the surface temperature of a green roof can be cooler than the air temperature, whereas the surface of a conventional rooftop can be up to 90°F (50°C) warmer.¹

Green roofs can be installed on a wide range of buildings, from industrial facilities to private residences. They can be as simple as a 2-inch covering of hardy groundcover or as complex as a fully accessible park complete with trees. Green roofs are becoming popular in the United States, with roughly 8.5 million square feet installed or in progress as of June 2018.²

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Green roofs can absorb both dust and pollutant gases and also offer water attenuation qualities (up to 50% at 100mm depth). They also cool buildings rather than having the hard surfaces radiate heat out at night. There is also evidence of localised cooling, stormwater reduction, less flooding and more insects and wildlife in urban centres.

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