On hot summer afternoons, the centres of large cities can be several degrees warmer than the surrounding countryside. This "urban heat island" effect occurs because dark surfaces such as asphalt and roofing absorb sunlight and release it slowly as heat, while buildings trap warm air and reduce the cooling effect of wind. A lack of vegetation makes the problem worse: trees and plants cool the air by providing shade and releasing water vapour.
In a recent study, researchers measured the average afternoon surface temperature in July across four neighbourhoods of one city, alongside the percentage of each area covered by trees. Their results are summarised below.
Neighbourhood | Tree cover (%) | Average July afternoon surface temperature (°C) Northgate | 8 | 41 Riverside | 15 | 38 Elm Park | 27 | 34 Hillcrest | 35 | 32
The researchers caution that tree cover is not the only factor: Riverside, for instance, is next to a large river, which may have cooled it independently. Nevertheless, they conclude that expanding urban tree cover is a promising strategy for reducing extreme heat, particularly in neighbourhoods where many residents lack air conditioning.
