The London Urban Heat Island – Upwind Vegetation Effects on Local Temperatures

Vegetation has the potential to cool the air in contact with it through evaporation of surface and transpirated water. This effect may have significant benefits in reducing the need for mechanical cooling of a building or increase comfort in outdoor spaces. The actual depression in air temperature is variable and uncertain in its degree and reach. Data from a large measurement exercise in London, UK, allowed a detailed analysis of the effect of greenness on the nearby temperatures. 24 measurement sites in urban areas were used to gather hourly air temperature data for two years. Aerial photography was used to analyse the percentage greenness in 16 sectors surrounding each measurement station. These results were used as the basis for calculating an “upwind greenness” value as the wind direction changed. This allowed a dynamic index to be used for calculating the effect of wind direction on temperature in the presence of evaporative cooling and demonstrate the link more effectively. Inner and outer greenness were analysed separately to assess the extent of the cooling effect. This has application to urban planning when deciding on more sustainable development incorporating deliberate greening of an area.