Lingshan Li: ‘Understanding how urban design affects thermal comfort can help cities make better decisions as extreme heat becomes more frequent'
Urban parks are often seen as natural refuges from summer heat, but new Concordia research shows that, depending on the time of day, the way trees are arranged within parks can influence whether those spaces cool people down or trap heat.
In a paper published in Urban Forestry and Urban Greening, researchers studied air temperature and humidity in 13 parks across Montreal to understand how different tree planting patterns affect daytime and nighttime cooling.
“We often assume that more trees always mean cooler conditions,” says lead author Lingshan Li, a PhD candidate in the Department of Geography, Planning and Environment. “But our findings show that the configuration of those trees, and the presence of open space, matters just as much.”
Trees cool by day, but not always by night
Using on-site sensors, the team measured temperature and humidity in three types of park environments: open grass areas, dispersed tree clusters and large, dense clusters of trees.
They found that large, aggregated tree clusters provided the strongest cooling during the day, thanks to shade and evapotranspiration, the combined processes which move water from the Earth's surface into the atmosphere. However, those same dense clusters slowed heat release after sunset, keeping temperatures higher at night.
In contrast, open grass areas were often hotter during sunny daytime conditions — sometimes even warmer than nearby paved surfaces. However, these areas cooled down more quickly once the sun set, helping parks release stored heat overnight.
The researchers also calculated wet-bulb temperature, a measure that combines heat and humidity and better reflects human thermal comfort, and found that humidity moderated many of the temperature differences between park areas.
Designing parks for round-the-clock comfort
Li says the results highlight the need for a more nuanced approach to urban greening. While planting trees remains essential for reducing daytime heat exposure, leaving some open areas in parks can help cities avoid trapping heat overnight, particularly during heat waves.
“A park that meets visitors’ needs at different times of day should be structurally diverse,” Li explains. “A balance of mature trees and open grassland can help maintain comfortable conditions both during the day and at night.”
The study also suggests that urban planners and landscape architects should consider trade-offs between tree density and nighttime cooling when designing or retrofitting parks.
Implications for Montreal and beyond
Montreal’s warm summers and its humid, continental climate makes these findings especially relevant locally. But the researchers say similar patterns may apply in other cities with comparable climates.
Beyond immediate design considerations, the work could also improve urban climate models by incorporating differences between dispersed and aggregated tree clusters and by accounting for open grass areas — factors that are often simplified or overlooked.
Li says she hopes future studies will examine how tree species, canopy structure, irrigation and weather conditions such as wind and solar radiation further influence how parks regulate temperature.
“Urban green spaces are essential for climate resilience,” she says. “Understanding how their design affects thermal comfort can help cities make better decisions as extreme heat becomes more frequent.”
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