Measuring the impact of microclimatic changes of planting vegetation on comfort indicators

Abstract
Objective: With the expansion of urbanization in recent years, the climate of various regions has undergone significant changes and consequences. One effective strategy for mitigating the impacts of microclimatic alterations involves evaluating the influence of vegetation cover on key microclimatic parameters. This study aims to investigate the planting patterns of various tree species along an east-west-oriented corridor between two residential building blocks in the climate of Sabzevar, Iran.
Method: To achieve the research objectives, different tree planting scenarios were modeled using ENVI-met software, and the key microclimatic variables, as well as the thermal comfort index—Physiological Equivalent Temperature (PET)—were analyzed.
Results: The results indicate that a planting configuration consisting of two outer rows of deciduous trees (10 meters in height and 5 meters crown width) and one middle row of evergreen trees (18 meters in height and 11 meters crown width) provides favorable summer conditions. In winter, a combination of one upper row of deciduous trees (11 meters in height and 9 meters in crown width) and one lower row of low leaf-density coniferous evergreens (15 meters in height and 7 meters in crown width) provides optimal thermal comfort in the Sabzevar climate.
Conclusion: The supply of required moisture and greater shading in the first pattern is due to planting one additional row of trees compared to the second pattern, as well as the environmental conditions being closer to the comfort zone during the summer season. On the other hand, in the second option, the use of lawn cover and deciduous trees with low height in the central courtyard has led to an increase in the mean radiant temperature and a rise in the PET index during winter.
 
 
 
English Extended Abstract
Introduction
Although the influence of vegetation on the urban microclimate has been examined extensively in a broad range of studies, it is widely recognized that the assessment of thermal comfort in outdoor urban environments is inherently context-sensitive. The performance and effectiveness of vegetation in moderating thermal conditions can vary significantly depending on a city's unique geographical characteristics, local climate, urban morphology, and cultural patterns of space usage. Therefore, generalized conclusions drawn from studies conducted in one region may not be directly transferable to other urban contexts without accounting for site-specific variables.
In light of this, the present research is specifically designed to evaluate the role of urban vegetation in enhancing thermal comfort within open public spaces in a selected district of Sabzevar, a city located in the semi-arid climatic zone of northeastern Iran. This study adopts a simulation-based methodology, utilizing the ENVI-met microclimate modeling software to explore and compare the microclimatic performance of two distinct vegetation planting scenarios. By analyzing a range of environmental indicators—including air temperature, mean radiant temperature, wind speed, and the Physiological Equivalent Temperature (PET) index—this study seeks to provide empirical evidence on how strategic vegetation placement can mitigate adverse thermal conditions and improve outdoor comfort levels in Sabzevar's urban environment.
 
Method
This study investigates the key factors influencing urban microclimate and explores the role of vegetation cover as a passive design strategy to enhance spatial quality and thermal comfort in outdoor environments. The research employs advanced numerical simulation using ENVI-met, a well-established three-dimensional microclimate modeling tool designed to assess the interactions between urban morphology, vegetation, and atmospheric conditions at the pedestrian level.
The analysis is conducted through the evaluation of two distinct vegetation configurations implemented along an east–west oriented pedestrian corridor located within the courtyard of a residential complex in the cold and semi-arid climatic context of Sabzevar, Iran. This region is characterized by significant diurnal temperature fluctuations and limited annual precipitation, necessitating context-specific design solutions to mitigate thermal stress in open spaces.
In the first design scenario, the vegetation layout comprises two outer rows of deciduous trees, each reaching a height of 10 meters and featuring a crown width of approximately 5 meters, flanking a central row of tall evergreen trees with an average height of 18 meters and a broad crown span of 11 meters. This arrangement is intended to optimize shading and evapotranspiration during the summer months while allowing for some solar penetration during the winter.
The second scenario adopts a vertical stratification strategy, featuring an upper row of deciduous trees with a height of 11 meters and a crown width of 9 meters, combined with a lower row of coniferous evergreen trees characterized by low leaf density. These conifers attain a height of 15 meters and possess a narrower crown width of 7 meters. This composition is aimed at providing a balance between winter solar gain and summer shading, while also minimizing visual obstruction and facilitating airflow within the pedestrian space.
Through comparative simulation and analysis of these two vegetation schemes, the study seeks to quantify their respective impacts on critical microclimatic parameters such as air temperature, mean radiant temperature, wind flow, and thermal comfort indicators, with particular emphasis on the Physiological Equivalent Temperature (PET). The findings are intended to inform climate-responsive landscape design strategies applicable to similar arid and semi-arid urban settings.
 
Results
Due to the use of three rows of trees in the southern part of the blocks (east-west pedestrian axis), which is one row more than the other option, the comfort index number in summer is reported to be about 2 degrees Celsius lower (22.26 degrees Celsius). Also, in this option, soil cover is used in more places, which causes the ground surface temperature to decrease in these areas in the summer.
In the second option, the central courtyard is covered with grass and low-height deciduous trees, which causes the sun's rays to pass through the branches of the trees and increases the average radiant temperature, and higher PET is observed in winter.
 
Conclusion
The incorporation of evergreen trees within the central courtyards of residential blocks has demonstrated a notably positive influence on thermal comfort conditions and corresponding comfort indices during the summer season. These trees contribute to microclimatic improvement through two primary mechanisms: first, by increasing relative humidity via transpiration processes, and second, by providing extensive shading, which significantly reduces surface and air temperatures at the pedestrian level. As a result, environmental parameters are shifted closer to the physiological comfort zone, as reflected in improved values of indices such as the Physiological Equivalent Temperature (PET).
Furthermore, during colder months, the strategic use of high-reflectivity surface materials can enhance outdoor thermal perception. These materials reflect incident solar radiation toward the ground and surrounding vertical surfaces, including the body of individuals standing or walking in the space (approximately 180 cm above ground level). This redirected radiation increases the mean radiant temperature (Tmrt) at human height, thereby improving thermal perception and comfort without the need for active heating systems.
In contrast, the application of materials with lower albedo values on horizontal and vertical surfaces can lead to increased heat absorption during warmer seasons, exacerbating thermal stress in pedestrian zones. Thus, by carefully selecting surface materials based on seasonal performance—favoring high-reflectivity materials in winter and low-albedo materials in summer—urban designers can effectively modulate thermal conditions and enhance outdoor comfort in both hot and cold periods.