Measuring the impact of trees on localized air temperature in the city

Trees in urban areas are important for their ecological functions as well as for their aesthetic characteristics. They provide shade, which is welcome in hot weather and lowers pedestrian heat stress. They may also lower air temperature: the magnitude of the cooling has been variously claimed to be as much as 6^oC, while other studies report a negligible effect. This disparity may be due to different circumstances, but may also be an artifact of insufficient sheltering of the sensors from radiant fluxes. A systematic field study was performed in hot, dry conditions to assess the effect of trees, comparing air temperature in 10 pairs of adjacent exposed sites and sites shaded by adult trees of 8 different species. Measurements at each site made at 10-second intervals included air DBT by four different methods, humidity, wind speed and direction, net and global solar radiation, and gray bulb temperature, as well as the sky view factor and IR images. Temperature variations due to the properties of the sensor-shield combinations – over 1°C – were similar in magnitude to the cooling effect of the trees. A linear mixed effect model developed to assess the effect of the environmental conditions and tree type on air temperature yielded a correlation of R²=0.54 and RMSE=0.67^oC, with wind speed having the greatest impact. Evapotranspiration from the trees had a minimal effect on humidity below the canopy, probably because the local species are adapted to water scarcity. The contribution of trees to human thermal comfort was substantial, with differences of up to 10 degrees in the PET index between shaded and adjacent non-shaded areas, but these were due primarily to shade. The study confirms the beneficial effect of trees on thermal comfort but reiterates that the focus in many studies on temperature reduction by evapotranspiration may be misplaced.