Urban Heat Islands

June 18, 2024  |  Kate Rayner

Image by Yinan Chen from Pixabay

Image by Yinan Chen from Pixabay

What are urban heat islands?

Growing up in Toronto in the 2010s, I remember the feeling of excitement when my parents took me downtown on hot summer days. I would get dizzy looking up at the skyscrapers and my ears would ring with the noise of car horns echoing off the concrete walls. I remember the sweltering heat trapped between these walls and asking my parents for cooling breaks in an air-conditioned building.

I also remember my home in a neighbourhood away from downtown being cooler; there are trees lining the streets of two-story houses with green lawns out front. This phenomenon, where heavily built-up areas in cities can be up to several degrees warmer than outlying areas with less built structures, is known as the urban heat island effect.

What causes heat islands?

Heat islands are caused by the make-up of cities, with built roads, commercial and residential buildings and towers, displacing natural surfaces such as trees, vegetation, ponds and soil that help to moderate air temperature.

Trees and vegetation offer a cooling effect through shading and evaporative cooling, whereby heat is absorbed as water evaporates from plants and leaves, much like how our sweat keeps us cool. As cities get built up, trees and vegetation are lost through construction of asphalt pavements and buildings. Tall towers also trap heat by reducing wind flow, and air pollutants can accumulate as well, leading to poor air quality. Pavements are impermeable dark surfaces, which absorb sunlight and heat the surrounding air. Dark roofs similarly cause heating. Such heating results in heat islands.

How do heat islands impact people?

As increasing population leads to denser and larger cities, the heat island effect will increase. The citizens most impacted are those who live in neighbourhoods that are unable to adapt – usually older, multi-residential buildings on streets with little vegetation and nearby parks. These citizens, especially the elderly, are more at risk of heat-related illnesses or even death. These impacts are made worse by climate change, which results in increasing number of hot days every year.

What can cities do?

Cities can implement strategies that involve changes to roads, buildings and other infrastructure that reduce the absorption of sunlight and heat trapping by built surfaces and structures. This includes increases in tree canopies, vegetation cover and parkland, and more reflective pavements and roofs.

City governments can implement such strategies through formal policies and municipal programs. The present trend of increasing number of people moving to cities is global and is likely to continue for years and decades. Cities can develop and implement smart growth strategies where densification to provide housing and infrastructure for increasing population, is balanced by sufficient green space for the health and well-being of citizens.

What can citizens do?

Citizens can get informed on the heat island effect, and implement solutions such as planting trees and vegetation on private lots, and getting involved with the city governments to promote smart growth strategies. We provide below some resources for citizens, including Torontonians, to get started.



MIT Climate Portal – This MIT portal holds explainers on different topics relating to climate, as well as the ‘Today I Learned: Climate Podcast’ which features scientists and experts.

Future Model Toronto – This tool by Stephen Velasco visualizes how the city of Toronto will look once proposed buildings have been built. Focusing on different regions in the Greater Toronto Area (GTA), the site pairs 3D visualizations with short explanations of the city’s plans for development in the area.

Tree Equity Score – This free application by American Forests grades cities and towns in America, and Toronto, based on the region’s tree cover and health and economic status of citizens. This is a useful tool for identifying priority areas in the city for action.

Sweltering Cities – This CBC News article explains the heat island effect, and integrates graphics and modelling results into the descriptive text to give readers a good understanding of the cause of heat islands and their impacts on citizens.

EPA Heat Island Effect – This site by the US Environmental Protection Agency gives a comprehensive overview of heat islands. It includes the science and impacts of heat islands, cooling strategies, and community and citizen actions.

Kate Rayner

Kate Rayner has a BA from the University of Toronto where she specialized in environmental geography with a minor in French. She brings a passion for sustainable resource management and community activism. During her spare time, she’s either watching soccer or refereeing the game with her local club.

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