Heat Island Effect

Written by: Sara Dennis

A thermometer showing +40 Celsius / +100 Fahrenheit.

In the summertime it’s easy to notice how hot some surfaces, like sidewalks and roads, can get in the sun versus other surfaces, like grass. If you haven’t, and are in a warm sunny area, you can stand (barefoot) with one foot on a sidewalk and one in grass and feel the temperature difference – the sidewalk or road will be much hotter to the touch than the grass. This same phenomenon happens on a much larger scale in our built environments, making urbanized areas much hotter than outlying areas surrounding them. This is commonly referred to as (urban) heat island effect, with the hotter/hottest areas being the heat islands.   

How are heat islands created?

Heat islands are often created in urbanized environments, due to the concentration of certain structures, such as roads and sidewalks (especially with black surfaces and roofs) that absorb the sun’s heat and then release it later, causing higher nighttime temperatures. Many urban areas also have the disadvantage of reduced air flow through the area due to tall buildings and narrow streets, in addition to vehicle exhaust, both of which contribute to the heat island effect.

Natural landscapes and parks with trees and vegetation, as well as water bodies, have the opposite effect, and tend to cool the surrounding air. Trees and vegetation release water from their leaves (transpiration), which then evaporates into the air (evapotranspiration). As the water evaporates, going from liquid to a gas, the surrounding air is cooled, much like how the process of sweating cools us off. Trees also help cool the air by providing shade, and bodies of water tend to cool the air through surface water evaporation.

What impacts does the heat island effect have?

According to the EPA, “a review of research studies and data found that in the United States, the heat island effect results in daytime temperatures in urban areas about 1–7°F higher than temperatures in outlying areas and nighttime temperatures about 2–5°F higher”. The chart below illustrates the differences between surface and air temperatures on various types of land.

US EPA

 

These heat islands in urban areas, make all symptoms of heat waves (or just regular hot weather) worse. In addition to general discomfort, it increases the risk of heat-related illnesses, such as heat stroke or heat exhaustion. These impacts are usually more severe in more disadvantaged communities as well, which generally have less street trees providing shade and less air conditioning. When people do have access to air conditioning, the demand rises with the increased air temperatures as well, creating an increased demand for electricity, putting further stress on the power grid.

In Conclusion

Because of the ways our cities are built, they are susceptible to heat island effect, and often have hotter temperatures during the day and night. These hotter temperatures have real impact day-to-day impacts for people living in those areas. Keep a look out for our future articles that will describe ways to mitigate urban heat island effect and show examples of success stories.