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The urban heat island effect is a phenomenon created by city infrastructure like buildings, roads and rooftops, which absorb heat from the sun and re-emit this heat back into the air. This can cause higher air temperatures in urban areas. While some 27 million Canadians are affected by the urban heat island effect, many Canadian cities are creating green spaces with plants and trees and passing new urban design policies to reduce the impact of the urban heat island effect.
What Is the Urban Heat Island Effect?
The urban heat island is an effect that occurs in cities where tightly packed buildings and paved surfaces like roads trap and re-emit the sun’s heat back into the air, which can cause higher temperatures in the city compared to the surrounding rural area. Cities also produce additional heat through car emissions and the energy used to run office buildings and homes, which can also increase the area's temperature. The heat concentrated in and around cities creates an “island” of high temperatures.
The urban heat island effect is most obvious in the summer when a sunny day can cause paved surfaces like sidewalks and roads to be 27°C to 50°C hotter than the surrounding air. The effect continues into the night when the heat captured by concrete or asphalt surfaces and buildings continues to warm the air well after sunset. A city can be as much as 12°C warmer at night than the surrounding countryside.
Causes
The leading cause of urban heat islands is the albedo effect. Albedo is a measure of the ability of a surface to reflect heat from the sun back into the atmosphere. Light-coloured surfaces like snow, ice and leaves have high albedo and can reflect more of the sun’s rays back into the atmosphere than dark-coloured surfaces like concrete and asphalt, which have low albedo and absorb more rays from the sun. These dark-coloured surfaces that absorb more radiant energy will warm up faster than lighter-coloured objects.
Weather, geography and the layout of a city can impact the severity of the urban heat island. Natural landscape features like trees, plants and bodies of water can provide shade and moisture throughout the day, helping to mitigate high temperatures. A layout of buildings that allows natural wind to flow through the city can also bring a cooling effect. On the other hand, human activities can increase the amount of heat a city creates through the use of cars, air-conditioning units and furnaces, which all increase the surrounding temperature. Calm and clear weather can make heat islands more intense by increasing the amount of heat that reaches the city through solar radiation and decreasing the amount of heat that wind can carry away. Local geographic features like mountains can block wind from reaching a city or help funnel wind down streets, which can also affect the heat island.
Did you know?
Water has a higher heat capacity and specific heat than air. This means more energy is needed to heat water than air. Water also releases heat very slowly when the surrounding temperature has gone down. As a result, bodies of water can absorb a lot of heat from the air without warming up much, cooling the surrounding atmosphere. This is why coastal areas tend to have both milder summers and winters.
Solutions
A city can use many different mitigation strategies or actions to reduce the risk or intensity of urban heat islands. Most of these strategies revolve around increasing the amount of vegetation in an urban area. Trees, plants, bushes, and even green walls and green roofs reduce the amount of dark-coloured materials that absorb heat by replacing them with lighter-coloured plants. Adding a row of trees to a street can reduce the surrounding air temperature by 1°C, and adding a park to a downtown area instead of a building can reduce the surrounding air temperature by 2°C to 6°C. Increasing the moisture level in the soil can also reduce heat since moist soils can cool the air better than dry ones. Pavement does not retain water well, so planting more trees, using interlocking paving stones that allow water to move through them, and planting gardens to trap rainwater can help prevent flooding and keep the soil wet while decreasing the surrounding temperatures.
Did you know?
Trees can have other impacts on cities besides providing shade and helping to mitigate the urban heat island. Trees can help manage air and water quality in cities by absorbing carbon dioxide and other pollutants. They also increase the biodiversity of cities and act as shelters and sources of food for wildlife. Trees are even known to cause an improvement in people’s mental health.
Another strategy is to design more environmentally sustainable cities (see Sustainability in Canada). This includes using reflective construction materials and insulated windows in buildings, as well as adding shade-producing elements to doorways and windows like shutters or roof overhangs. Using more energy-efficient technology, reducing the number of cars on the road, improving airflow in buildings, and using ceiling or floor fans instead of air conditioners can help limit the amount of energy used by a city, which reduces the heat created by homes, cars and businesses.
The Urban Heat Island Effect in Canada
The urban heat island effect impacts over 27 million Canadians who live in urban areas. With climate change, Canadians are predicted to experience four times as many +30°C days by the 2050s than in the past, increasing the likelihood and intensity of an urban heat island. Several Canadian cities have already implemented policies to respond to the urban heat island effect.
Montreal has introduced a roofing by-law in the Rosemont-La Petite-Patrie neighbourhood requiring new or replaced roofs to be green, white or highly reflective, and new parking lots must include plants, bushes and trees in 15 per cent of the space. The city also proposed a new park to increase green spaces that could become the largest municipal park in Canada.
A common strategy in Canadian cities is to increase the amount of tree cover and green spaces. Vancouver is planting trees to increase tree cover, improving rainwater infrastructure to help with extreme rainfall, and bringing back natural above-ground creeks in the city (see Rainfall Extremes). Toronto has planted trees to increase canopy cover and provide shade, both to address public health and in response to the urban heat island effect. Finally, Calgary’s Climate Strategy is trying to increase the average tree cover in the city, while also building green roofs, improving public transportation, and creating a bicycle-friendly city to reduce car emissions.
