A study, part funded by VALUE, by the University of Manchester and Red Rose Forest has found that tree shade is one of the best ways to provide effective local cooling in urban heat islands (UHI). UHIs occur because of the increased capacity in cities for trapping and absorbing heat resulting from the high concentration of buildings, roads and pavements. This causes cities to be several degrees warmer than the surrounding rural landscape and is a particular problem on hot summer days. This problem is likely to get worse with climate change.
The study looked at the role that trees and grass can play in reducing temperatures in urban areas over the summers of 2009 and 2010 in Manchester. Two different temperatures were measured on small plots of land composed of concrete and grass in the presence or absence of tree shading; surface temperature and air (globe) temperature (used to measure radiant heat). The same measures were also recorded at midday on larger expanses of asphalt and grass in an urban park.
The results found that both surface type and shade greatly affected surface temperatures. Grass reduced maximum surface temperatures by up to 24C while tree shade reduced them by 19C. In contrast, surface type had little effect upon globe temperatures, whereas shading reduced them by up to 5-7C. These results show that both grass and trees can effectively cool surfaces and can provide cooling, helping to reduce the urban heat island in hot weather. In contrast, surprisingly grass has little effect on local air temperatures, so should have little effect on human comfort, whereas tree shade can provide effective local cooling.
One aspect that wasn’t studied, but is useful to consider, is the three dimensional nature of trees; as well as having a cool canopy, they also shade adjacent trees. Importantly shading affects human comfort, since it will alter our perception of how hot we feel. So the more trees there are in urban areas, not only the cooler it will be but people will feel cooler and therefore be more comfortable. This could have far reaching affects not only for our health and mental wellbeing but to encourage us to live, work and socialise more in our cities especially in hot summer months. A few more reasons to show the importance of urban trees!
To read the full study in the journal of Urban Forestry & Urban Greening visit:
For more information about urban trees or donating to the Gift That Grows tree planting scheme, please contact:
South Yorkshire Forest Partnership,
5th Floor Howden House,
1 Union Street,
Tel: 0114 2571 199
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South Yorkshire Forest Partnership /
Sheffield City Council (UK)
C: South Yorkshire Forest
T: +44 (0)114 257 1199