What is the Urban Heat Island Effect?

There is substantial opportunity for cool roofs to reduce the urban heat island effects occurring across Canada and within its major cities. Studies have consistently shown that installing cool roofs across urban landscapes can play a significant role in reducing the urban heat island effect. But, what is the urban heat island effect?

Cities increase in temperature not only from climate change, but they also create their own areas where surface and ambient air temperatures are higher than those in surrounding undeveloped or rural land. These areas are called heat islands. Rooftops, roads, parking lots, and other paved surfaces increase all add to this effect .

Cities with as few as 100,000 people can be impacted by the effects of an urban heat island. For larger metropolitan areas, the effects can be severe: Over a recent 3 year period, temperatures in highly developed city cores in the North east of North America like Toronto, New York and Boston, were an average of 7° to 9°C (13° to 16°F) higher than temperatures in nearby rural areas. On a hot, sunny day the surface of a conventional roof can exceed the ambient air temperature by up to 50°C (90°F). Urban heat islands can also elevate night time temperatures significantly, with this effect sometimes exceeding the daytime effect.

Urban heat island effect

The increased temperature in our urbanized areas leads to a number of negative impacts to our environment such as:

Poor air quality. Concentrations of fine particles, inhalable coarse particles, and ozone (or “smog”) can cause a variety of significant health problems. Increased air temperatures resulting from the urban heat island effect only exacerbate these conditions.

Increased energy use and GHG emissions. As urban temperatures increase, we use more electricity to cool buildings than would be necessary without the effects of urban heat islands or climate change. This results in higher levels of greenhouse gas emissions. Since air conditioners and HVAC systems vent heated air into the atmosphere, their added use can further increase outside air temperatures, resulting in an increased need for building cooling, and more GHG emissions.

Increased water consumption and stress on ecosystem health. Many plants and animals are sensitive to the increased temperatures that occur in urban cores—for example, increased temperatures can interfere with photosynthesis—and the warmer temperatures can result in more water being used for irrigation to support stressed vegetation.

Do Cool Roofs Reduce the Urban Heat Island Effect?

Lab research confirms that white roofs reduce global warming, proving to be three times more effective at countering climate change than even green roofs, thanks to all that reflected sunlight. Studies simulating widespread cool roof adoption have shown reductions in urban ambient temperatures on the order of 1° to 2°C on hot summer afternoons. Lowering temperatures citywide results in significant air quality improvements and cooling energy savings. For some scenarios, that is comparable to the effect of replacing all gasoline-powered vehicles with electric models.

Cooling ambient air temperatures both locally on the roof surface and on a citywide basis may produce a secondary cooling benefit: The lowered temperatures can increase the efficiency of roof-mounted central air-conditioning systems whose air intakes are located near the green roof or cool roof.

Finally, cool roofs may help to cool the earth itself by acting as mini-reflectors and reducing the balance of heat in our atmosphere, directly counteracting the effects of greenhouse gases. Like the polar ice caps, higher-albedo roofs reflect energy out of the atmosphere and thereby have the potential to result in a cooler planet. By making use of white or light-coloured aggregate in soil matrices, green roofs may be able to be designed to contribute some of this same benefit.

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