The impact of spatial pattern and local urban space vegetation on the thermal comfort in cold climate

Because of the expansion of cities, it has become highly challenging to modify  urban structures to address the drawbacks of the planning and design of open spaces, control microclimates, and improve thermal comfort conditions. Due to their effect on the quality of air in cities, urban microclimates are highly significant, and since urban spaces play an important role in creation of urban microclimates, urban designers and planners have the capacity and means to reduce the negative effects of climate on citizens’ health through implementation of proper designs.

Urban spaces affect their users’ thermal comfort via their design elements. Various parameters can induce micro-climatic changes, such as the geometric patterns, vegetation, natural elements, and construction materials of the open spaces. These elements also play important roles in balancing urban thermal comfort during winter. While the effects of vegetation and geometric patterns on thermal comfort in public open spaces during winter have been studied independently, the cumulative impact of vegetation and geometric parameters during winter in cold and dry climates has not been investigated sufficiently. This article sought to address this shortcoming.

The ENVI-met software was used for simulation and comparison of the thermal comfort conditions of the open spaces selected for this study. First, the design parameters of the selected sites were determined via field measurement, and were entered into the software for simulation of the thermal environments. The four climatic parameters of thermal comfort were measured during winter using ENVI-met . Then, a receptor was placed at the center of each selected site for investigation of thermal comfort at the pedestrian scale. Using these receptors, the parameters of space design and their impacts on thermal comfort were studied during the selected season.

The changes recorded for wind speed were larger than those in MRT, air temperature, and RH; hence, the differences in thermal comfort at the central points. According to the results, the Kolapa and Kolanj neighborhoods exhibited more favorable thermal conditions due to their higher PET values. Both Kalpa and Kolanj neighborhoods had north-south orientation. This type of orientation provides top-down access to the sunlight during winter. The H/W ratios at the central receptors of the Kolanj and Kalpa neighborhoods were 0.6 and 0.3, respectively. These H/W ratios provided greater enclosure than those of the other neighborhoods. While a H/W ratio less than one can be said to be suitable for cold climates during winter, it can cause thermal discomfort against wind speed if it lies below a certain threshold. This condition was observed in the Kababiyan neighborhood, where the H/W ratio at the central point was 0.13. As a result, the residents have to take protective measures against the winter wind and storms. In addition, dense vegetation caused a decrease in the SVF of the neighborhoods. The neighborhood centers with vegetation exhibited higher MRT values as well. The trees also raised the RH values of the neighborhoods. Because of the dry climate of Hamadan, Iran, vegetation can certainly improve thermal comfort in this city.

Recent climate studies have explored thermal comfort as an important quality of urban spaces. While thermal comfort can be achieved through a decrease in temperature in hot climates (as emphasized by most studies), it requires an increase temperature to provide thermal comfort in cold and dry climates. The presence of Hamadan’s citizens in the urban open spaces of the city always decreases in the second half of the year because of the cold mountainous climate. Limitation in or even lack of thermal comfort is one of the most important reasons for this change. With their high standards of sociability, the centers of the historical neighborhoods of Hamadan have always served as places for social interaction between the residents of the city. Therefore, this study attempted to assess the thermal characteristics of nine neighborhood centers in Hamadan using the notion of thermal comfort and the relevant variables.