| ABSTRACT | In the face of local as well as regional warming trends, Mediterranean cities that are overheated for much of the year must be continually reimagined to make them more walkable and sustainable. While the “urban heat island” effect has been studied for many years through both modeling and empirical observation, the intensity of the UHI is typically quantified by spatial differences in air temperature, a measure which may not reflect the actual thermal stress experienced by pedestrians. On clear summer days, both biophysical heat stress and the perception of thermal discomfort are dominated by the body's exposure to solar radiation – and therefore one of the most effective ways to combat urban heat stress is the provision of shade, ideally by covering large portions of the pedestrian realm with healthy, broad tree canopies. This is a complex and expensive endeavor, however, and our research is aimed at developing tools with which urban planners can prioritize their interventions in order to plant trees where they can have the maximum benefit. This presentation summarizes two parts of the ongoing work: in the first part, we use an extensive campaign of mobile on-site microclimatic measurements in the city of Tel Aviv to calibrate a model for the mapping of pedestrian thermal stress, and in the second part we address the “walkability” of different urban street segments based on a large-scale observational study of pedestrian behavior and the tendency to choose shaded paths over those with differing extents of solar exposure. In the first part of the study we evaluate the correlation between exposure to solar radiation and overall heat stress on summer days, using a mobile monitoring station with a 6-direction net radiometer setup that allows for the collection of a large urban climatic database without compromising the accuracy of the results. The results indicate that solar exposure is responsible for a high percentage of the variation in pedestrian thermal stress according to three different thermal comfort indices, while air temperature differences in adjacent shaded and unshaded locations are found to be very small. The second part of the presentation builds on these findings by quantifying the relative volume of pedestrian traffic on sidewalks that are in shade, relative to those which are unshaded. Using high-frequency photographic documentation of pedestrian and bicyclist activity at 36 monitoring locations in central Tel Aviv during the summer, we identified a total of over 5,000 individuals – approximately 60% of whom travelled in the shade. At locations where the size and functionality of shaded and unshaded spaces were most closely comparable, this proportion rose to 71% of all pedestrians on foot. The relative tendency for traveling in shade was also found to increase systematically with the level of pedestrians’ solar exposure, as a function of the sun’s angle of incidence and intensity at a given time. These findings, together with the study’s extensive sample size, increase our confidence in suggesting that in hot Mediterranean climates people are more likely to use sidewalks and public open spaces if they are well-shaded. |