The significance of shade provision in reducing street-level summer heat stress in a hot Mediterranean climate

The rise in air temperature in urban areas has been attracting the attention of urban climate researchers for years now, focusing on the intensity of the so-called Urban Heat Island (UHI) phenomenon. Recently, though, it has been rightly questioned whether the concept of UHI intensity is helpful in assessing mitigation of urban heat stress, especially since the concept does not reflect the spatial and temporal microscale climatic differences between adjacent intra-urban locations and the much lower magnitude of the UHI effect during daytime. Moreover, while UHI research is focused on near-surface air temperature levels, the existing evidence makes it clear that air temperature does not fully describe the intensity of street-level daytime heat stress during the hot season. This study attempted to evaluate the correlation between exposure to shortwave radiation and significant increases in heat stress under summer conditions through extensive monitoring. Using a mobile measurement setup we monitored summer daytime outdoor conditions in 148 unique locations in Tel Aviv-Yafo, a coastal city with a hot-summer Mediterranean climate. Analysis of the resulting dataset showed the decisive effect of solar exposure on significantly increased heat stress according to three common thermal comfort indices. A much weaker correlation was observed between comfort levels and air temperature, relative humidity, and wind speed levels. These findings support the conclusion that under summer daytime conditions in similarly hot climates the provision of shade has a far stronger positive effect on reducing heat stress than, for example, feasible reductions in air temperatures or increases in wind speed.