Microscale vegetation effects on outdoor thermal comfort in a hot-arid environment

The effect of irrigated vegetation on human thermal stress in a hot-arid region was tested in two semi-enclosed urban spaces with varying combinations of mature trees, shading mesh, lawn and paving. The Index of Thermal Stress was calculated hourly from measured data to evaluate thermal comfort in the spaces, and was expressed on a scale of thermal sensation. While thermal stress in a paved unshaded courtyard was severe during mid-day hours, both grass and shading, either by trees or by mesh, contributed significantly to thermal comfort. A combination of the two strategies resulted in comfortable conditions at all hours – though trees alone provided more efficient cooling in terms of water use, as measured by the rate of evapotranspiration. The main effect of both grass and shade was to reduce radiant loads, while differences in air temperature were small.