Urban design strategies for summer and winter outdoor thermal comfort in arid regions: The case of historical, contemporary and modern urban areas in Mashhad, Iran

There is an increasing demand for urban planning and design strategies to decrease air temperatures in present and future cities. In this study, the outdoor thermal comfort (OTC) in Mashhad, Iran, is examined during summer and winter days by investigating the impact of urban form on pedestrian thermal comfort in three neighborhoods (historical - HA, contemporary - CA, and modern - MA) and focusing on the effects of such urban factors as height-to-width (H/W) ratio, canyon orientation (N-S, E-W, NNE-SSW, and EES-WWN), trees, and building materials. The analysis is conducted with the ENVI-met and Rayman models, which together provide reasonable estimates of OTC. The results are as follows. (1) OTC is improved in HA and CA by the presence of both trees and mid H/W ratios (H/W=1.5), particularly in E-W- and EES-WWN-oriented canyons. (2) During both summer and winter, the mean radiant temperature (MRT) is the most effective climatic factor. In winter, when MRT is high, the Physiological Equivalent Temperature (PET) is closer to the comfort range. The reverse is valid for the summer. (3) The Sky View Factor (SVF) has more significant effects in the summer than in the winter. (4) Wind velocity is the second most important factor influencing OTC. (5) The latent heat effect is less prominent in the HA and CA, which use indigenous materials (brick), than in the MA, where modern construction materials with lighter colors are more common. In winter, on the other hand, MA has a relatively lower PET in comparison to the other two neighborhoods. Lower H/W ratios, material properties (stone) and dominant wind directions are the main reasons for this poor OTC situation in the MA, with N-S canyons with low H/W ratios experiencing the worst cold stress values (<18°C). When considering both winter and summer, the results show that using indigenous materials and planting a combination of windbreaker and deciduous trees in shallow canyons will help urban planners and designers improve OTC.