1 Introduction

The advancement of human civilization and economic endeavors is fostered by urbanization [1]. Nonetheless, it gives rise to a detrimental effect known as the urban heat island (UHI) phenomenon [2]. Heat stress risks are rising, and urban heat islands (UHI) are expanding due to climate change in cities [3]. Climate change and population growth expose more than 270 million people who live in cities in the Middle East and North Africa (MENA) to extreme heatwaves [4, 5]. This exposure will increase heat-related morbidities such as headache, drowsiness, and fatigue. Heat stress increases anxiety, aggression, depression, and violent behavior, also escalating the rate of interpersonal violence [6].

Studies in the MENA region show that it is not only the hot temperature that increases mortality risks, but both cold and hot extremes positively affect the mortality rate [7, 8]. Such vulnerability substantiates the need to reduce thermal discomfort in unconditioned spaces. Considering pedestrians' thermal exposure, planning for small-scale urban spaces, such as transitional spaces, is crucial. Transitional spaces account for 10–40% of the total area in various buildings and campuses [9, 10], buffer the pressure of extreme weather conditions, and meet people’s physical activities and social requirements in flexible circumstances.

In moderate climates, people spend considerable time in unconditional transitional spaces, while in climates with hot summers or cold winters, people might use such spaces when they have to. It is indispensable to provide comfortable thermal conditions today and also unpredictable future conditions when the risk of experiencing heat stress is the highest [11, 12].

Transitional spaces are categorized by architectural characteristics [13]. Some are contained within buildings, such as the main atriums. Others are compound to the buildings via covered structures or attached connections, such as corridors and arcades. The other category of transitional spaces essentially embodies the outdoor climate as, courtyards, the frontage of metro station entrances, or trails between terminals. Transitional spaces between terminals and other buildings at the airports and adjacent vacant surrounding lands are considered UHI [14].

Urban renewal projects such as the redevelopment of mixed-use areas, rehabilitation of urban fabrics, regulation of transit-oriented projects, and redesigning transitional spaces provide unique opportunities to mitigate heat islands and generate climatic outcomes that vary from city to city. In this process, different strategies can be helpful including using plants or water bodies.

In 2018, Golasi et al. [15] introduced the Global Outdoor Comfort Index (GOCI), a novel model building upon existing frameworks. GOCI incorporates variations across different regions, factoring in latitude, annual average temperature, and maximum temperature as primary variables. This comprehensive model, designed for the coldest and hottest months, significantly contributes to the advancement of outdoor thermal comfort models by addressing regional distinctions.

In a study conducted by Davtalab et al. in 2020 [16], it was demonstrated that the structure of green spaces significantly influences the Physiologically Equivalent Temperature (PET), with measurement time displaying a noticeable trend. Among various spatial variables, the type of flooring emerged as the most crucial factor impacting PET in the investigated region. As anticipated, microclimatic factors such as air temperature and mean radiant temperature exhibited the strongest correlations with PET. These findings suggest that, particularly in arid regions like Sistan (Iran) facing prolonged droughts, flooring type can serve as a pivotal tool for regulating thermal comfort conditions where green spaces are limited.

Studies have proved the effectiveness of shading created by plants in providing a more comfortable microclimate in summer. For instance, a 4 ℃ difference between uncovered urban areas and well-planted areas was reported by Wong and Yu [17]. Tan et al. [18] showed that shaded surfaces could reduce the glob temperature by 5 to 7 ℃. However, the impact of greenery in open transitional spaces has not received enough attention.

Transitional spaces connect indoors to outdoors. These spaces’ main purpose is transient and, occasionally, a short time rest. Designing such vibrant, dynamic spaces is typically controlled by several priorities including space limit, other functionalities such as fire evacuation, aesthetics, etc. Airport terminals typically consist of a diversity of spaces including open and closed transitional spaces. Indoor transitional spaces at airports have been studied recently [19, 20].

Every day, year along, many passengers walk into or leave airport terminals worldwide. Airport terminals as transient spaces, often not fully indoor, might not meet the travelers’ comfort demands. As passengers are departing or arriving their clothing and even metabolic rates are different [21] from occupants who were studied using other open urban areas. An open transitional area at Mehr-Abad Airport located southwest of Tehran, Iran, was selected as a case study for field measurement and quantitative investigation. In this study, greenery scenarios have been evaluated as effective strategies that affect thermal stresses in open transitional spaces between buildings in hot and cold seasons.

The functional limitations of transitional spaces, in addition to the flight regulations, were reflected in the proposed scenarios. For instance, conventional strategies for increasing vegetation, specifically dense planting in outdoor transitional spaces and the overall landscape of airports have functional technical restrictions because of airport regulations. [14]. Therefore, in this study, grass coverage is more scrutinized than in the previous studies. The influence of grass coverage on thermal comfort in summer and winter time was projected using an empirical approach to simulate and validate the model.

Binarti et al. [22] provided in-depth assessments of tools for modeling outdoor human comfort and thermal stress, the underlying physical equations, and their suitability in different climates, building on the insights gained from prior research. This study benefited from ENVI-met software to simulate air temperature, mean radiant temperature (MRT), air temperature (TA), and sky view factor (SVF), and validated the results using on-site hourly meteorological data. Using Biomet software, we developed three policy scenarios to analyze existing and proposed conditions, relying on physiological equivalent temperature (PET). Studies regarding outdoor thermal perception commonly employ indices such as PET, UTCI, SET, and PMV [23].

The paper is structured as follows: Sect. 2 describes the research methodology in detail. Section 3 is dedicated to the results, while Sect. 4 discusses these findings. Section 5 is the paper’s conclusion and limitations.

In this study, our research objectives are distinctly outlined with a dual focus: firstly, to explore the effects of varied greenery scenarios on outdoor thermal comfort within airport transitional spaces, and secondly, to provide actionable insights for landscape architects and urban planners operating in such environments. Through meticulous examination, our objectives address the nuanced interplay between green elements and thermal dynamics, particularly in open transitional areas. The comprehensive analysis of both hot summers and cold winters in our research provides a holistic understanding of seasonal variations, enriching the existing literature and offering valuable considerations for landscape architects and urban planners in designing transitional spaces.

Our study diverges from existing research in the field by specifically concentrating on the often-overlooked context of open transitional spaces within airport terminals. Unlike many studies that primarily address outdoor thermal comfort in urban areas, our focus on airport environments offers a unique perspective. Moreover, we explore the nuanced impacts of distinct greenery scenarios, especially the role of grass cover, in both cold and hot seasons, an aspect that has received limited attention in the literature.

While previous studies often emphasize the positive effects of greenery on thermal comfort, our research brings forth findings that challenge some claimed notions. For instance, the removal of grass cover is shown to positively affect the Physiologically Equivalent Temperature (PET) index. Additionally, we identify key factors such as Mean Radiant Temperature (MRT) and Wind Speed (WS) as having the highest association with PET, providing a more detailed understanding of the thermal comfort dynamics in transitional spaces.

2 Method

Outdoor thermal comfort is receiving more attention since global warming has become more tangible. More severe weather conditions periods negatively affect humans’ outdoor presence. Staying inside longer does not just increase energy consumption but also deteriorates human physical and mental conditions. As discussed in the `previous section, transitional spaces are vital in urban design, and quality comfortable space encourages visitors to spend more time outside and enjoy their visit better. This research simulated a selected transitional space at Mehr-Abad airport in Tehran to evaluate the impact of vegetation covers on pedestrians’ heat/cold stress.

2.1 Research site introduction

The study site is an international airport serving Tehran (35° 6892′ North, 51° 3890′ East, 1800 m asl), Iran’s capital city, with a population of nearly 9 million. Köppen climate classification categorizes Tehran in the cold semi-arid climate category [24]. The weather is moderate in spring and fall, while hot and dry in summers and cold winters [25]. Tehran has continental climate characteristics and a Mediterranean climate rainfall type. The Mehr-Abad International Airport’s meteorological station reported 17.5 as the mean annual air temperature. Figure 1, shows the site's location is in the west of Tehran inside the Mehr-Abad airport. The total area of the Mehr-Abad airport is almost 1234 Hectares. This study simulates the transitional space between the parking lots and the first and second terminals. Air temperature (Ta) and relative humidity (RH) were measured at 10 points at the site of Mehr-Abad International Airport. As depicted in Figs. (2, 3), the measurement equipment was positioned at 1.5 m AGL (Height Above Ground Level). The points for field measurement were determined based on the most traffic the space of pause, or temporary stop, and distance from the vegetation. The location of each point is depicted in Fig. 2. S Point was placed on the roof of terminal No.1 as a fixed point in validating the measured data. Except for the S point among the nine points, points A, B, C, and D are near the vegetation, and points E, F, G, H, and I, are far from the planted area. Since simulating the whole year is impossible due to the calculation load, one day in summer and one day in winter were selected for simulation. The measured data were collected from 16:20 to 04:20 on January 31st and from 13:20 to 01:20 local time on July 3rd.

Fig. 1
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Mehr-Abad Airport Location

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Fig. 2
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Measurement points

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Fig. 3
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Tree used in the simulation

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2.2 ENVI-met simulation and result validation

Nowadays, numerical simulation leads the research methodologies in the urban green and blue infrastructure (GBI) field. Numerical simulations in this field are either Energy Balance Model (EBM) based or built on Computational Fluid Dynamic (CFD) principles [20]. CFD-based simulations are superior to EBM-based models because they can couple the velocity and temperature fields with humidity and pollution. CFD models also can go beyond scales and couple microscale, mesoscale, and building energy simulations. CFD models have a higher resolution representation of urban geometry than EBM-based models [26]. ENVI-met is a three-dimensional microclimate CFD-based platform that researchers have widely accepted to the extent that more than half the studies that simulated the thermal effect of the vegetation used the software [27]. In this research, ENVI-met (V4.4.2) was implemented as the simulation software to quantify the impact of greenery on the pedestrian's thermal comfort in transitional space at Mehr-Abad airport Fig. 4. ENVI-met simulated the air temperature, MRT, and wind direction. As shown in Fig. 5. The results were validated using the on-site collected data. A total surface of 198,450 square meters was simulated for this study. The area is modeled using 3*3*1 cubic meters cells; therefore, a three-dimensional grid of 150*147*26 cells was formed at the end. The greenery covers around 20% of the total simulated area and is mainly around the pedestrian pathways. As can be seen in Fig. 4, the trees on the site are mainly pine-cypress approximately 12 m high and 4 m crown diameter. Since the airport works 7–24, the simulation was done 24 h to cover daytime and nighttime comfort. Tehran has cold winters and hot summers; therefore, thermal comfort should be considered in both summer and winter. The simulation duration in our study was carefully chosen to capture critical periods at Mehr-Abad airport, aligning with our research objectives. We have extended the simulation timeframe, taking into consideration industry-standard durations observed in studies like Karimi et al. [28], Teshnehdel et al. [29], and Ghaffarianhoseini et al. [30]. The Rayman model was used to calculate the physiologically equivalent temperature (PET). [31]. The Rayman model inputs are reported in Table 1. In this study, the input parameters for the simulation, including Dry Bulb Temperature (℃), Relative Humidity (%), Wind Speed 10 m AGL (m/s), and Wind Direction (degree), were sourced from the Mehr-Abad Ecological Station in Tehran. These meteorological data were collected during the specific period relevant to our study. It is essential to note that the values used in the simulation were directly obtained from the station without any further manipulation or adjustment. Here, the heat transfer resistance of clothing was considered to be 0.5 Clo (properly dressing for people in summer), 0.9 Clo (properly dressing for people in winter), and the activity to be 80W (walking in an outdoor environment) [32, 33, 34]. In our study, we employed the Biomet tool, allowing for the consideration of both male and female respondents. To maintain consistency and draw parallels with certain studies, we opted to use a male participant for simulation, a practice observed in studies like Karimi et al. [28] and Toy and Yilmaz [35] where a male individual was specifically chosen for similar purposes. It is important to note that our methodology supports the inclusion of either gender in the simulation, and our decision to focus on a male participant was a deliberate choice for the sake of consistency and comparison with existing literature. Since the main objective of this research is limited to the transitional area’s pedestrians, validation data is collected from that area, as shown in Fig. 3. To validate the simulation, air temperature (Ta), mean radiant temperature (MRT), wind speed (WS) as well as relative humidity (RH) were measured at ten different locations inside the transitional space at Mehr-Abad airport, as shown in Fig. 3. Mean radiant temperature (MRT) is a parameter that measures the heat load on a person’s body and is the biggest driver of thermal comfort, especially in an arid and semi-arid environment [36]. The physiologically equivalent temperature (PET) unit is ℃; which makes it easier to use and understand than other similar indicators [37].

Fig. 4
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Visualization of the three different simulation scenarios on ENVI-met

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Table 1 ENVI-met parameters
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Architects and energy engineers can calculate and communicate the results for the projects more conveniently. PET is based on the Munich Energy-balance Model for Individuals (MEMI), which describes the thermo-physiological heat balance of humans [38]. If Ta, WS, RH, and MRT are known, the thermal comfort index can be computed by the Rayman model. These factors are acquired from the ENVI-met output data.

2.3 Simulation scenarios

Three scenarios were simulated in this study to quantify the impact of greenery on transitional spaces’ thermal comfort. There are summer and winter simulations for each scenario.

  1. 1

    Base scenario: the existing landscape, which includes pavement, 287 trees, 1087 square meters of grass cover, and buildings, is collected through field measurement

  2. 2

    Tree-only scenario: it is assumed that trees have small footprints

  3. 3

    Grass-only scenario: in this case, all trees were eliminated. The site has the same area of grass-covered lawn as the base scenario

By comparing the results, we will be able to evaluate the impact of greenery on PET and outdoor thermal comfort in summer and winter.

3 Results

3.1 Measured data and data validation

The simulation was performed on January 1st, for winter, and on July 31st, for summer. Air temperature and relative humidity were recorded 24 h at ten points on both dates to validate the results. Figure 5 compares the average recorded data in the summer and winter with the simulation results, which highlights a great agreement between measured and estimated data. The model efficiency was estimated using all points measured and simulated data to calculate the correlation coefficients and the root means square error (R2). Acquired results, depicted in Figs. 6 and 7 show the acceptable efficiency of the model, with R2 over 0.82.

Fig. 5
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Left side: comparing the measured data during 24 h and the simulated data by ENVI-met. Right side: correlation of the data measured and simulated in winter

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Fig. 6
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Average wind speed, mean radiant temperature, air temperature, PET values in the current situation, and the two scenarios during the two seasons

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Fig. 7
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MRT, Ta, and Ts maps at 18:00 p.m in summer

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3.2 Greenery impact on microclimate

To estimate the human-biometeorological reference height, The simulation results regarding Ta and MRT are described for a height of 1.5 m AGL [39] and illustrated in two ways: (i) 2-D maps obtained from the Leonardo in ENVI-met, depicted in Fig. 7, (ii) box plots of the data simulated in all points, depicted in Figs. 6. The Ta range is 28.40–47.94 °C in summer and 2.89–10.59 °C in winter. The impact of greenery on microclimate is shown in the box plots (Fig. 6) of Ta and MRT at the pedestrian level (1.5 m AGL) during the simulation (24 h) at all points. The average Ta and MRT during the CS scenario day were 35.86 and 50.70 °C. Regarding the impact of the different greening scenarios on summer heat mitigation, results display an increase in the WT (where tree cover is removed from the base case) scenario compared to the base case, with an average Ta increase of 2.14 °C and an average MRT increase of 9.03 °C, shown in Fig. 6. The current situation (CS) model and WG demonstrate a striking difference from the WT. this is because trees in the CS (and WG) are distributed unevenly, therefore, MRT varies across the site. Regarding these results, scenario WG (where the grass cover is removed from the current situation) reduces Ta by 0.06 °C and increases MRT by 0.15 °C. As anticipated, adding the tree number in the transitional space also slightly increased the relative humidity, even though it could be assumed insignificant. Based on Fig. 6, the results demonstrate that increasing trees reduces the wind speed in comparison to the current situation in two seasons.

3.3 Effect of greenery on thermal comfort conditions

The thermal conditions can be deemed comfortable when the PET is between 18 and 23 ℃ [ 37]. Simulation results demonstrate that the average PET in the current situation is 39.81 ℃ in summer and 5 ℃ in winter. According to the PET index, the area experiences hot stress in summer and cold stress in winter. Figure 6 illustrates the box plots of computed PET during the simulation time (13:00–13:00) for three scenarios. Based on the simulation, the best scenario for summer thermal comfort is WG with 35.80 ℃ (slightly warm sensation). At the same time, WG provides only a slightly better thermal sensation in winter (WG: 6.68 ℃). For better comprehension, Table 2 summarizes the average differences of Ta, MRT, and PET compared to the current situation (CS) in two seasons.

Table 2 Average changes of Ta, MRT, and PET in comparison to the current situation in two seasons
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The present research has divided the vegetation cover into two categories, grass (herbaceous) and tree vegetation. Comparing the simulation results enables us to evaluate each greenery type's impact. In the case of the removal of grass cover (WG), we can usually see an increase in the average of total points by 0.04 Celsius, and the maximum temperature increase in the (C) station is equal to 1.24 Celsius. Regarding the grass cover, due to the shiny surface of the leaves and the angle of radiation, the thermal comfort index was reduced at some points of the day during summer. For instance: at 6 A.M. at (C) station, due to grass cover removal, the PET index dropped by approximately 1.90 degrees Celsius.

According to Fig. 8, among the nine points in summer, at the points of E, F, G, H, and I, (far from the planted area) we can observe the slightest change in the data of (WG) and (WT) conditions as compared to the existing condition. Removing the tree cover and increasing SVF after the diffusion of daily latent heat on the surfaces during the night can improve thermal comfort up to 0.4 Celsius.

Fig. 8
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Comparing the thermal comfort index in different scenarios during summer and winter

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Also, since most of the airport’s tree cover is Tehran pine and is evergreen, their removal will cause a reduction in shading and an improvement in average thermal comfort. As shown in Fig. 9, The acquired data in the summer demonstrates that the thermal comfort index (PET) has the highest correlation with MRT during the daytime (the R2 coefficient reaches up to 0.72 at 3 P.M). In contrast, during the night, the correlation between the MRT index and thermal comfort (PET) significantly decreased (the R2 coefficient dropped to 0.028 at midnight). the SVF parameter has the highest correlation with PET during the night, while the wind speed (WS) is the second (The R2 coefficient for SVF and WS at noon is equal to 0.61 and 0.47, respectively). This is because of the determining role of SVF in releasing the latent heat of the surface. However, the impact of SVF on PET in the daytime is quite limited. As depicted in Fig. 9, among the parameters, WS(m/s) had the highest correlation with PET in both day and night periods. (Correlation coefficient at 09:30, 12:30, 15:30, and 24:00 are 0.97, 0.96, 0.97, and 0.95, respectively). In both seasons, during the night, SVF has a correlation of almost 0.62 with the PET index, which shows the importance of this parameter in thermal comfort during the night. Air temperature (TA) correlates lowest with thermal comfort in cold and hot seasons during the day and night. Based on the calculated PET index, the minimum temperature recorded in the summer was 23.2 Celsius at 2 A.M in station B. The maximum temperature where is surrounded by façade glass and Granit rock materials pavement in the station was 53 Celsius at 2 P.M. In winter, the minimum temperature at point C and the maximum temperature of PET at point H were, respectively recorded as 0.24 and 11.9 Celsius. The two stations I and H at the entrance and exit of terminal No.2 are the hottest points since these points are surrounded by glass surfaces and transparent Granit rock pavement and are not shadowed middays.

Fig. 9
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Correlation of the PET with MRT, WS, and SVF, in summer and winter

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4 Discussion

Due to the escalation of climate change and spreading heat islands in cities, outdoor thermal comfort has become one of the most critical issues in landscape architecture. Numerous studies have been conducted to improve outdoor comfort [40, 41] as it looks like it is deteriorating faster than expected [42]. Besides, due to swift urbanization and global warming, Urban Heat Islands (UHI) intensity and frequency have increased. Many cities are experiencing tense UHI [43] that reduces the quality of urban resident’s life. [18, 44]. It is worth mentioning that the Mehr-Abad airport area is recognized as one of the biggest thermal islands in the Tehran metropolis. Among the studies conducted on this subject, vegetation cover is recorded as one of the most effective solutions to improve thermal comfort. It has many benefits, such as controlling disturbing sounds [45], improving air quality [46, 47], reducing stress [48], increasing social interactions in urban environments [49], and more importantly mitigating the impact of UHI [34, 50]. Urban greenery can also affect human thermal comfort during cold seasons, especially in climates with a significant temperature difference between summer and winter. However, Literature does not adequately address the issue of thermal comfort during the winter months [51,52,53]. Also, studies on the beneficial effects of greenery in semi-arid climates are limited. Transitional spaces are locations where the physical environment bridges the interior and exterior environment. Transitional space will help to save energy if it can be developed according to their climatic needs. People have to use these spaces not necessarily by choice. On the other hand, the space could be limited and spatial conflicts can occur. Therefore, the choices for fighting thermal discomfort are limited for architects. Current comfort standards do not address transitional spaces. These spaces should be considered independently, and not use the same standard for spaces inside or outside of buildings. By examining the microclimates and outdoor comfort in a transitional area in Tehran, Iran, which has a cold semi-arid climate, this study seeks to fill these gaps. Airports consist of different spaces including transient spaces, that are working round the clock. Daily, pedestrians, including passengers, crew members, staff, and taxi drivers, use this transitional open space at the Mehr-Abad airport. In this study, a transitional space located at Mehr-Abad airport was selected to evaluate the outdoor thermal comfort for pedestrians under different circumstances. Our study holds practical significance for landscape architects and urban planners by contributing valuable insights into the enhancement of thermal comfort in open transitional spaces, particularly those found in airport terminals with extreme climatic conditions. The specific case study at Mehr-Abad airport provides applicable knowledge for airport terminal design, aiming to create more comfortable spaces for passengers throughout the year. The identified mitigation strategies, including the impact of different greenery scenarios such as removing grass cover and adding deciduous trees, offer tangible guidelines for architects to reduce heat stress in transitional spaces. The findings are beyond theoretical exploration, providing actionable insights that can be applied in real-world scenarios, aligning with the practical needs of professionals in the field. Urban design should be based on systematic and accurate local microclimate and pedestrian comfort calculations. This can be achieved using numerical simulation tools like ENVI-met, validated using collected data [54]. In this study, ENVI-met was used to simulate different greenery scenarios for the studied area. To calculate the physiologically equivalent temperature accurately, in addition to environmental factors, the role of human factors should be considered. The Metabolic rate and clothing coverage are two important human factors in evaluating thermal comfort, in both seasons. The clothing insulation value was considered according to the clothing of employees and vehicle drivers in summer and winter. Also, the activity rate, according to Biomet software standards, Basal rate, and Metabolic work rate were considered based on a person walking at the age of 35 at a speed of 1.2 m/s. This paper explores the effect of grass and tree cover on thermal comfort. Thermal comfort was assessed in two seasons, winter and summer, to evaluate the role of the different vegetation cover in both cold and hot seasons, while most similar studies only examined the hot season. In the present article, a model of the transitional space was simulated using ENVI-met v4 and validated against data of Ta and RH measured in summer and winter (24 h in summer and 24 h in winter). The base scenario (CS) discussed the simulation of the current condition. Unlike other studies, where the size and rate of the vegetation cover can be increased, for example, in urban parks and plazas, in the airport areas, as mentioned, there are restrictions regarding planting more trees. In other words, although shading and aesthetics are important, they are not the primary considerations in this case. Therefore, this research, after simulating the vegetation cover of the current situation (CS), investigated two other scenarios, including removing different greenery elements (tree, grass) to evaluate the role of greenery elements in summer and winter in thermal comfort. In other words, the effects of the area’s microclimate on the physiologically equivalent temperature were studied. According to the simulation results, the role of a limited green scheme for outdoor comfort in summer and winter could be discussed. Our study introduces several novel elements to the existing body of knowledge in outdoor thermal comfort research. Firstly, by concentrating on open transitional spaces within airport terminals, we address a specific and often overlooked context, providing valuable insights into a setting where passengers frequently experience varying climatic conditions. Secondly, the exploration of different greenery scenarios, particularly the emphasis on grass cover removal and the incorporation of deciduous trees, contributes a unique perspective to the literature. This approach sheds light on the nuanced effects of specific vegetation elements on thermal comfort, offering practical considerations for landscape architects and urban planners. Additionally, our study stands out by encompassing a comprehensive analysis of thermal comfort conditions in both hot summers and cold winters, providing a holistic understanding of the seasonal variations that transitional spaces may experience. Together, these distinct features make our research novel and valuable in advancing the understanding of outdoor thermal dynamics in transitional spaces.

  • The current situation

Simulating the current situation (CS) does not confirm any significant association between PET and Ta. Karimi et al. [28] also claimed that it is evident from the results that the reduction of UHI does not contribute significantly to improving thermal comfort. Further, because of the effect of radiations on both Ts and PET, the reduction of Surface Urban Heat Island (SUHI) in sunny hours will enhance the thermal comfort condition. Compared to the CS scenario, the results indicate that each vegetation element plays different roles during cold and warm periods of the year. This fact is worth investigating since the literature has overlooked it, especially for winter. As seen in Fig. 3, among the nine measured points, points A, B, C, and D are located near the vegetation, while points E, F, G, H, and I are far from the vegetation. For this reason, we analyzed these points in two separate categories.

  • The current situation without trees (WT scenario)

In summer, in the WT scenario, at the points A, B, C, and D located near the vegetation, the PET rate has increased (except D point, because it is in the shadow) by 3.59,1.79,0.15 Celsius respectively, at 6:00 AM, shown in Fig. 10. It caused weakening thermal comfort according to the direct sunlight and larger unshaded areas. After dark, contrary to the day results, we can see a decrease of 0.6 Celsius in PET (which means an improvement in thermal comfort) according to the increase of SVF and release of long-wave radiation mainly at the points close to the vegetated area. Point D experiences the maximum decrease during the night, and at point B, the maximum increase in PET happens during the day mainly because point D is shaded by the adjunct building. By removing the tree cover in winter, we see an increase in the PET rate and an improvement in the thermal comfort conditions because of direct sunlight and larger unshaded areas. It is because most of the tree cover is the evergreen Tehran pine, and their removal will cause a reduction in shading and an improvement in thermal comfort at the average points.

  • The current situation without grass (WG scenario)

Fig. 10
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PET values the busiest hours of the airport for three scenarios

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In summer, by removing the grass cover, the rate of PET increases slightly during the day, which can be negligible and remains almost constant during the night and does not change compared to the existing scenario. At point C, we see the most changes in the thermal comfort index because this point only has grass cover. Also, by eliminating grass cover in winter, the thermal comfort index rate has increased at all points, and the thermal comfort conditions have improved. However, the increase rate of PET is lower compared to the WT scenario due to the shading of the tree cover compared to the grass cover. The critical point here is that contrary to expectations, having grass does not affect thermal comfort positively. Based on these findings, the PET index can be slightly improved by removing the shrub cover (grass) not only during the winter but also during the summer. That means solely based on outdoor thermal comfort, grass cover is not recommended. Looking at the thermal comfort conditions, it is inferred that increasing the coverage of trees generally improves the thermal comfort zones and levels in summer. But in winter, Despite the undeniable benefits of trees, there could be negative impacts in semi-cold climates for different reasons: trees can reduce wind speed [55], and their canopies can block the net outgoing long-wave radiations [38]. Looking at the thermal comfort conditions, it is inferred that increasing the coverage of trees generally improves thermal comfort. However, the coverage of the open transitional area with grass does not significantly impact thermal comfort.

5 Conclusion

Outdoor thermal comfort research studies in urban areas describe how trees play vital roles; including leveling sunlight, creating microclimates, and reducing the impact of heat islands. There has been little attention given to the different roles played by vegetation elements, especially grass cover, in cold and hot periods of the year in the literature, which is why it merits further investigation.

Our study introduces novel insights by focusing on open transitional spaces in airport terminals, addressing a context often overlooked in thermal comfort research, and providing a unique perspective compared to urban outdoor thermal comfort studies. The exploration of distinct greenery scenarios and a comprehensive analysis of both hot summers and cold winters contribute unique perspectives, advancing our understanding of the thermal dynamics in these crucial spaces.

This study has analyzed the factors related to thermal comfort in the open transitional area at an airport. Several simulations were done for different greenery scenarios at Mehr-Abad by using the ENVI-met software, validated against measured Ta and RH data in summer and winter. Three distinct greenery scenarios were evaluated based on the pedestrians’s comfort using the transitional space. Contrary to some other studies, the results of this study support the findings by Karimi et al. [28] on the insignificant correlation between SVF and PET. According to the results, MRT and WS had the highest association with PET as the indicator for thermal comfort.

According to these simulated results, we can claim that shrub cover (grass) removal positively affects the PET index. Substituting the grass cover with material with lower albedo such as Pea gravel can improve thermal comfort mainly in winter but also in summer. Adding more deciduous trees to the transient space to prevent solar shading in winter also is another suggestion for designers. However, vegetation alone can’t solve discomfort issues in cold and hot hours. Extending the conditioned spaces or confining the transient spaces might be solutions that architects might consider in hot summer/ cold winter climates.

This study created a better understanding of the impact of greenery on the thermal comfort of open limited spaces and demonstrated the need for a more detailed examination of the characteristics of vegetation during the design process. It also shows the importance of considering radiation-related indicators and thermal comfort conditions to have more serviceable open spaces in urban areas.

Our study provides practical insights for landscape architects and urban planners in enhancing thermal comfort in transitional spaces, particularly in challenging environments like airport terminals. The identified strategies, including varying greenery scenarios, offer tangible guidelines for mitigating heat stress and improving the design of these spaces in real-world applications.

While our study sheds light on the positive impacts of integrating greenery into transitional spaces, it is crucial to recognize inherent limitations. Spatial constraints may restrict the optimal placement of green elements, and maintenance challenges can affect their sustainability. Seasonal variations in plant responses and diverse user preferences pose additional considerations. Acknowledging these limitations provides a nuanced understanding of the complexities involved in enhancing transitional spaces with green features. Future research should delve into these aspects for a more comprehensive exploration of the role of greenery in such environments.

Furthermore, projects can discuss further design features by referring to the characteristics of deciduous and evergreen trees and accurate inspection of the physical characteristics of vegetation. Also combining retractable canopies with greenery scenarios in transitional spaces needs to be investigated. Finally, although research studies have been done on the effect of typology and greenery on outdoor thermal comfort, this area still needs more attention.