People living in urban areas can expect night time temperatures to be on average 7°C above that of the surrounding countryside. This alarming figure comes from the Managing Climate Risks to Well-Being and the Economy report published by the Adaptation Sub-Committee of the Committee on Climate Change (CCC) on 8 July 2014. The temperature difference is down to a phenomenon known as the urban heat island effect. It is a differential that is set to increase under the impact of climate change.
An urban heat island is a manmade area that is significantly warmer than the surrounding countryside, particularly at night. The Met Office says the term stemmed from the analogy of a warm urban island of heat in a cool sea of the surrounding natural environment. Urban heat islands exist because the surfaces in towns and cities, which are generally made of materials such as Tarmac, stone and concrete, absorb and store heat more readily that vegetation.
In addition to solar gains, the heat generated by traffic, air conditioning systems and other energy uses also raise temperatures. The London Climate Change Adaptation Strategy report, published in 2011, estimates that this ‘anthropogenic’ contribution to the urban heat island effect in London can have “a significant local impact in high-density areas, raising summer air temperatures by a further 2ºC”. And if the use of air conditioning were to become more widespread the anthropogenic contribution would increase still further. How can cities and buildings adapt to mitigate the effects of the urban heat island? Building and street orientation can influence surface temperatures and wind movement. North facing facades stay much cooler than those facing other orientations; it can make sense to orientate a building north/south to make shading easier and minimise the surface temperature of the façade.
Wind speeds too depend on the orientation of the street relative to the wind direction; speeds are greatest when streets are parallel to the wind direction and are minimised when the street layout is perpendicular to the wind. In BRE information Paper 19/10: Designing Urban Streets to Minimise Heat Island Effects, James Smith says city streets can be designed to take advantage of the convective cooling effect of the wind by spacing buildings a distance apart that is “1.5 to 3.3 times the average building height”.
Urban heat island temperatures can vary across a city depending on the type of land cover: urban parks, for example, are cooler than adjacent areas covered by buildings. Data generated using the STAR Tools, a tool developed by the University of Manchester and The Mersey Forest and published in the Guardian newspaper, shows that an increase in green spaces within UK cities could help to combat future increases in temperature. Green spaces include trees, parks, gardens, green roofs and waterways. The article cites the example of Greater Manchester, where it says “a 10% increase in green space could keep temperatures at or below present levels into the 2050s, despite a changing climate”.
This article, including its image, has been adapted from an article published by Building4Change.