A woodland site in Guildford averaged 4.5°C cooler than a built-up comparison site during the daytime across ten extreme-heat events. That is a site-specific result, not a universal promise.
What did the researchers measure?
The University of Surrey’s 24 August research summary describes monitoring between June 2023 and August 2025. Sensors compared woodland, grassland, lakeside, riverside, a pocket park and a green wall with a built-up location.
At peak summer hours, woodland was around 2.5°C cooler; grassland and lakeside differences were about 2.3°C and 2.1°C. The larger 4.5°C woodland contrast refers specifically to the analysed extreme-heat events, not the entire summer record.
Each landscape type had only one monitored example. Local conditions could affect the comparison; it is not a universal league table.
Why do trees cool their surroundings?
The US Environmental Protection Agency explains two complementary mechanisms. Leaves intercept sunlight, reducing the energy reaching shaded surfaces. Vegetation also moves water into the atmosphere through evapotranspiration, which uses heat as water evaporates.
That is why a shaded pavement and an exposed pavement can feel very different, even in the same neighbourhood. Tree shade on buildings can also reduce the need for air conditioning. Cooling is one of several potential benefits, rather than the only reason to preserve urban vegetation.
Are satellite temperatures the same as air temperatures?
No. The study’s published highlights distinguish satellite land-surface temperatures from the in-place monitoring. Surface measurements add spatial context but should not be substituted directly for the air temperature a person experiences.
What counts as a heatwave?
The Met Office definition requires at least three consecutive days meeting a county-specific daily maximum temperature threshold. A single very hot afternoon is not enough. This helps explain why a site’s heatwave record and its average temperature difference answer different questions.
What does this mean for urban planning?
The practical lesson is to measure local performance rather than buy a headline temperature reduction. This study offers long-term evidence from a real town; repeated examples and monitoring of neighbouring streets would make it easier to judge how far cooling extends. Green space is a useful part of adaptation, not a substitute for every other response to extreme heat.
Reporting basis: The university summary, journal highlights, EPA background and Met Office definitions, checked on 31 August 2026.
Illustrative urban greenery; not one of the Guildford monitoring sites. Photo by Hailey Tong on Unsplash