Urban heat island - what is it, how does it create overheating and how can greenery help?

August 18, 2023

Parks and greenery in built up areas can reduce overheating by 1oC. And people’s actual reception of comfort from parks and greenery equates to nearly 2oC cooling. That is the finding from a recent review paper in Building and Environment scientific journal.[1]

This is important because the number of “heat stress” days is projected to rise in forthcoming years due to climate change and the so called Urban Heat Island (UHI) effect. The review cited an increase from 7-15 heat stress days a year to 45 heat stress days by 2050. This was based on data about Holland, which has a similar climate to UK, and the country about which most of the review was concerned.

The review also noted that the UHI effect can increase urban outside temperatures by 3-10oC. Quite possibly the last thing anyone needs during a heatwave. Certainly not vulnerable people. Death rates increased by 24% during heatwaves and body temperatures were measured to be dangerously high. If there is no chance of cooling then people with high body temperatures will also suffer. If that isn’t enough reason to act, then the paper also notes that work productivity declines during heatwaves.

An interesting conclusion from the review was that the temperature increase due to UHI effect did not depend on the size of the city. They found that the effect could be found in cities, towns and even large villages. The effect is caused by:

  • buildings blocking wind patterns
  • reduced evaporation and transpiration (normally provided by vegetation)
  • buildings radiating heat
  • heat generated by human activities

They found that parks and street trees do in fact counter the UHI, but green roofs and living walls did not. Open water in urban areas was not a clear cut cooler. Once the water warms up, its cooling capacity decreases. A further finding is that, in general, people really appreciate an environment full of greenery, which will enhance their wellbeing.

From a Baker Homes point of view, increased greenery can also provide more habitat areas for biodiversity. In addition, park land is generally more permeable than built up areas and so will help reduce the likelihood of surface water flooding. And one other final benefit, vegetation can clean up our air. This is especially useful in cities, like London, where air quality is drastically below World Health Organisation standards.

Baker Homes provide a range of work on overheating:

  • detailed overheating risk assessment on thousands of homes 
  • we’ve also carried out detailed survey of the extent of overheating in the UK.  The final report is available here.

We have written extensively on the issue and the built environment in research for the Greater London Authority and others. Publications and guidance are here.

  • Your social housing in a changing climate – a detailed assessment of actual works to reduce the effects of climate change in 200 apartments, including overheating
  • The Business Case: Incorporating adaptation measures in retrofits – guidance on the finances of adapting to climate change
  • Measures to incorporate when planning a retrofit – specific measures to include that adapt the home to climate change

These publications can be found downloaded for free from our resources page.

Contact us if you would like any more information.

 

[1] Editorial, Overview of challenges and achievements in the climate adaptation of cities and in the Climate Proof Cities program, Building and Environment 83 (2015) 1-10, https://dx.doi.org/10.1016/j.buildenv.2014.09.006

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