Urban Canyon Geometry and Green Infrastructure: A Review of Strategies for Enhancing Thermal Comfort and Microclimate

Abstract

Urban canyons, integral components of the built environment, significantly influence microclimatic conditions and thermal comfort. This review investigates their combined effects with green infrastructure on thermal comfort, offering a comprehensive framework for supporting urban design and greening strategies. The review is based on a structured literature analysis of peer-reviewed studies retrieved from major scientific databases (Scopus and Web of Science), following defined selection and screening criteria. Urban canyon orientation determines solar exposure and its interaction with prevailing wind patterns, affecting ventilation and heat dissipation. The urban canyon aspect ratio influences shading and airflow regulation, while their sky view factor moderates radiative cooling and daylight availability. Urban greening—encompassing street trees, green roofs, and vertical green walls—complements urban geometry by reducing air temperatures, enhancing evapotranspiration, and modifying local wind dynamics. Tree shading can reduce the physiological equivalent temperature in urban canyons, mitigating extreme heat stress. Key vegetative parameters, such as leaf area index and canopy density, are critical for quantifying cooling contributions. Key findings underscore the role of higher aspect ratios in enhancing shading and ventilation while they emphasize the critical influence of street orientation and sky view factor on microclimatic regulation. Vegetation emerges as a vital component, with tree shading contributing substantially to cooling effects and reducing physiological equivalent temperature. The beneficial synergistic interaction between urban geometry and vegetation optimizes thermal comfort. Tailored strategies based on urban canyon typologies balance urban development with environmental sustainability. The proposed framework provides actionable strategies for designing resilient and thermally optimized urban spaces, promoting climate-adaptive urban planning by addressing the dual challenges of the urban heat island and thermal discomfort in cities.

1. Introduction

Urbanization has intensified the modification of local climatic conditions, leading to elevated temperatures in cities compared to their rural surroundings. This phenomenon, widely known as the Urban Heat Island (UHI) effect [1,2,3,4,5,6], is driven by dense construction materials, limited vegetation, anthropogenic heat emissions, atmospheric pollution, and altered surface–atmosphere interactions. These conditions significantly affect energy consumption, public health, and outdoor thermal comfort, particularly during extreme heat events [7,8,9].

Outdoor thermal comfort represents a complex interaction between environmental, physiological, and psychological factors. It depends on key microclimatic variables—including air temperature, mean radiant temperature (MRT), wind speed, and humidity—as well as personal parameters such as clothing insulation and metabolic rate [10,11,12,13,14,15]. However, thermal perception cannot be fully explained by physical variables alone. Psychological adaptation, including perceived naturalness, expectations, prior experience, and environmental control, also plays a critical role in shaping human thermal response [16,17]. This complexity highlights the need for integrated urban design approaches that address both physical and perceptual dimensions of thermal comfort.

Urban morphology strongly influences local microclimate and thermal conditions. In dense urban environments, compact configurations restrict airflow and reduce heat dissipation, intensifying thermal stress and amplifying the UHI effect. A fundamental morphological unit in this context is the urban canyon, defined by streets flanked by buildings (Figure 1) [18,19,20,21]. Its geometry—typically described by the aspect ratio (H/W), street orientation, and sky view factor (SVF)—controls solar radiation exposure, wind patterns, and radiative exchange processes, thereby shaping microclimatic conditions and thermal comfort at the pedestrian level [19,20,21,22,23,24,25,26,27].

Urban greening has emerged as a key strategy for mitigating adverse thermal conditions in urban canyons. Vegetation—through shading, evapotranspiration, and airflow modification—reduces surface and air temperatures and improves outdoor comfort conditions [28,29,30,31,32]. In deep canyons, greenery enhances cooling by providing shade and influencing ventilation, while in high-SVF environments, it reduces radiant heat loads by intercepting solar radiation [15,19,28,29,33,34]. The effectiveness of green infrastructure (GI), however, depends on multiple factors, including tree species, canopy density, planting configuration, and the surrounding built environment.

The interaction between vegetation and urban canyon characteristics—such as AR, SVF, and surface properties—plays a critical role in determining thermal comfort outcomes. For example, in narrow canyons with low SVF, shading effects are amplified, whereas vegetation structure can significantly modify airflow and thermal conditions at the pedestrian level. These interactions highlight the need for integrated analysis of urban geometry and vegetation in order to improve thermal performance and support climate-responsive urban design [34,35,36,37,38,39,40,41,42].

Despite extensive research on individual parameters, existing studies often examine urban geometry and vegetation separately, limiting the understanding of their combined effects. A systematic synthesis that links geometric characteristics and vegetation parameters to thermal comfort outcomes across different climatic contexts remains needed.

This study addresses this gap by conducting a comprehensive literature review with two primary objectives. First, it systematically analyzes and quantifies the influence of urban canyon geometry and vegetation on outdoor thermal comfort and microclimate regulation in densely built environments. This includes examining key geometric parameters—such as aspect ratio (AR), street orientation, and sky view factor (SVF)—as well as vegetation characteristics, including leaf area index (LAI), leaf area density (LAD), and planting configuration. Second, the study develops an evidence-based and structured framework that integrates urban geometry and vegetation into a conceptual decision-support scheme, highlighting their synergistic interactions and supporting climate-responsive and context-sensitive urban design. A detailed description of the literature selection methodology, including the databases consulted, search strategy, time frame, and inclusion and exclusion criteria, is provided in Section 2.

The novelty of this work lies in: (i) the integrated assessment of urban geometry and green infrastructure, (ii) the synthesis of quantitative evidence on their combined effects on thermal comfort and microclimate, and (iii) the formulation of a structured framework that supports the design of thermally improved urban environments across diverse climatic conditions.

2. Methodology and Literature Selection Strategy

This study adopts a structured narrative review approach, enhanced by systematic elements to ensure transparency, reproducibility, and methodological rigor in the literature selection process. Such an approach allows for a comprehensive synthesis of existing knowledge while maintaining flexibility in integrating diverse types of evidence, including experimental studies, numerical simulations, and field investigations.

The bibliographic search was conducted using major scientific databases, including Scopus and Web of Science, selected for their extensive coverage of peer-reviewed literature in urban climatology, building physics, and environmental sciences. The search strategy was developed to capture the multidimensional nature of the topic, combining keywords related to urban geometry, thermal comfort, and green infrastructure. Specifically, combinations of terms such as “urban canyon” and “thermal comfort,” “aspect ratio,” or “sky view factor” and “microclimate,” “urban greening” or “green infrastructure” and “thermal comfort,” as well as “street orientation” and “urban climate” were employed to ensure broad yet targeted coverage of relevant studies.

The review primarily focuses on literature published between 2000 and 2024, reflecting the rapid development of research in urban microclimate and outdoor thermal comfort over the past two decades. Earlier seminal studies were also included where necessary to establish theoretical and methodological foundations.

To ensure the relevance and scientific quality of the selected studies, explicit inclusion and exclusion criteria were applied. Eligible studies were required to address outdoor urban environments, explicitly consider urban canyon geometry (including parameters such as aspect ratio, sky view factor, and street orientation), and assess thermal comfort using established indices such as the Physiological Equivalent Temperature (PET), Universal Thermal Climate Index (UTCI), or Mean Radiant Temperature (MRT). Furthermore, studies investigating the role of vegetation or green infrastructure in modifying urban microclimate were prioritized. Only peer-reviewed journal articles and high-quality conference proceedings were considered. Studies focusing exclusively on indoor environments, lacking quantitative or qualitative assessment of thermal comfort, presenting purely theoretical formulations without validation, or published in languages other than English were excluded.

The initial search yielded a broad pool of publications, which were subsequently screened through a multi-stage process involving title and abstract evaluation followed by full-text assessment. Greater emphasis was placed on studies providing quantitative results, validated numerical modeling, or field measurements. Particular attention was given to studies examining the interaction between urban geometry and vegetation, as well as those covering a wide range of climatic conditions to ensure the generalizability of the findings.

The final dataset comprises approximately 60 studies, which were systematically analyzed and categorized according to key parameters, including aspect ratio, street orientation, sky view factor, and vegetation characteristics. This structured methodology ensures that the review captures a representative and comprehensive body of knowledge while maintaining clarity, consistency, and scientific robustness in the selection process.

3. Role of Urban Canyon Geometry

Urban canyons are fundamental geometric units representing two-thirds of urban areas and they significantly affect microclimate, energy balance, air quality, pollutant dispersion, and thermal comfort conditions in the urban environment [7,11,40]. The energy balance within urban canyons is primarily governed by the absorption, reflection, and re-emission of solar radiation by the surfaces of buildings and streets. Due to the vertical orientation of surfaces in urban canyons, solar radiation is often trapped, leading to increased heat retention and contributing to the UHI effect [19,20,43,44]. The geometry of urban canyons, referring to characteristics such as AR or orientation, plays a crucial role in determining the distribution of shortwave and longwave radiation and, subsequently, the thermal behavior of the canyon [18]. Additionally, anthropogenic heat emissions from vehicles, air conditioning units, and industrial activities further enhance the heat fluxes within urban canyons, affecting both microclimate and air quality [25,45]. Understanding these heat fluxes is essential for the design of sustainable urban environments, where mitigation strategies can be implemented to reduce heat stress and energy consumption. Figure 2 provides a graphical summary of the primary energy fluxes within the built environment. In addition to illustrating these flows, Figure 2 highlights the key impacts of human activities, with distinct areas categorized based on different land use types.

Understanding the urban energy balance is crucial for comprehending the mechanisms that drive the development of urban climate and microclimate and of the urban heat island effect as well. The following two main equations are used in the scientific literature to express the main heat flux sources [19,47,48,49,50].

$$Q^{*}+Q_F=Q_H+Q_E+\Delta Q_S+\Delta Q_A$$

(1)

$$Q^{*}=Q_{SW}^{\downarrow }+Q_{SW}^{\uparrow }+Q_{LW}^{\downarrow }+Q_{LW}^{\uparrow }$$

(2)

where $Q^{*}$ is the net radiation (all waves), $Q_F$ is the anthropogenic heat, $Q_H$ is the turbulent sensible heat flux, $Q_E$ is the turbulent latent heat flux, $\Delta Q_S$ is the heat stored in the urban structure, $\Delta Q_A$ represents possible horizontal advection fluxes, $Q_{SW}^{\downarrow }$ is the incoming shortwave radiation, $Q_{SW}^{\uparrow }$ is the outgoing shortwave radiation, $Q_{LW}^{\downarrow }$ is the incoming longwave radiation, and $Q_{LW}^{\uparrow }$ is the outgoing longwave radiation. These equations describe how the balance between the net radiation $Q^{*}$ and anthropogenic heat $Q_F$ is maintained by the turbulent sensible $Q_H$ and latent heat $Q_E$ fluxes, along with the heat stored in the urban structure $\Delta Q_S$ and any possible horizontal advection $\Delta Q_A$. The advection term is often considered insignificant for systems within a homogeneous environment [46,50].

3.1. Aspect Ratio

The physical dimensions of the canyon are usually described by its length, its height, and its width. The canyon length (L) is defined as the distance measured along the canyon between two major street intersections or the distance between two cross streets that enclose the canyon [5]. The canyon heigh (H) is defined as the average height of the buildings that line both sides of the street canyon. When there are buildings of different height, H is typically taken as the average height of the two opposing building facades [11]. The canyon width (W) is the horizontal distance between the facades of the buildings on either side of the street canyon [19], which includes the width of the street defining the canyon. These canyon characteristics are shown in Figure 3.

The AR of an urban canyon is a key concept in urban climatology that describes the geometric relationship between the height of the buildings and the width of the street within an urban canyon. Referring to Figure 3, the AR in the context of urban canyon equals the ratio of the canyon height (H) to the canyon width (W) [11,19,20,21]. A high AR indicates narrow streets with tall buildings, while a low AR indicates wide streets with short buildings. AR is a critical dimensionless quantity, typically expressed as H/W, that describes the geometry of the urban canyon and influences various environmental and energy factors such as air flow inside the canyon, solar radiation, sunlight penetration, air temperature inside the canyon, air quality and thermal comfort, and pollutant dispersion [47,52,53,54,55,56,57,58].

Based on AR (or equivalently H/W) values, a canyon can be characterized as shallow, uniform or medium, and deep [7,59,60,61,62]. Shallow urban canyons have H/W values below 0.5, uniform or medium urban canyons have H/W values close to unity without any significant openings in their walls, while deep urban canyons have H/W values equal to 2 or higher. An urban canyon’s length (L) and high (H) can provide a further sub-classification [59,60,61,62]: short urban canyon have length-to-height ratios (L/H) equal to about 3, medium urban canyons have L/H values equal to about 5, and long urban canyons have L/H values equal to about 7.

AR serves as a fundamental parameter in urban planning and sustainable urban design, as it governs microclimatic dynamics and environmental conditions within street canyons. Its influence extends to air temperature regulation, solar radiation distribution, airflow characteristics, and pollutant dispersion processes. Urban geometry, typified by AR, modulates wind patterns and pollutant behavior, with significant implications for air quality and thermal comfort [61,63]. High AR values in deep urban canyons, characterized by tall buildings and narrow widths, create sheltered microenvironments where reduced wind speeds hinder ventilation, fostering the accumulation of heat and pollutants near ground level. Such configurations frequently induce robust recirculation zones or vortices that further entrap pollutants. In contrast, shallow canyons, with lower AR values, facilitate enhanced wind penetration, improving ventilation efficiency and mitigating pollutant stagnation through weaker or absent recirculation zones [19,63,64,65]. In addition, AR values profoundly affect solar radiation penetration. Deep canyons restrict direct sunlight due to prolonged shadowing from tall structures, resulting in cooler but dimly lit environments that necessitate increased reliance on artificial lighting. Conversely, shallow canyons enable greater solar radiation access, elevating daytime surface temperatures and enhancing natural illumination, thereby reducing the energy demand for artificial lighting [20,49,66,67]. The intricate balance between solar gain, shading, and thermal comfort conditions emphasizes the crucial role that ARs play in urban design, highlighting their importance in creating sustainable and livable urban environments.

Table 1. Impact of AR on outdoor thermal comfort and urban microclimate.

Reference	Case Study Location	AR	Methodology	Affected Parameters (Objectives)	Key Findings
[68]	Goteborg, Sweden	H/W equaled 1.4, SVF equaled 0.5.	Experimental investigation	Air temperature	No great temperature differences were found between the canyon and the open urban area at least at the center of the city
[21]	Ghardaia, Algeria	Different canyons with H/W equal to 0.5, 1, 2, and 4 for a north–south and an east–west orientation. Northeast-southwest and northwest-southeast orientations were considered for H/W equal to 2.	Simulation with 3-dimensional numerical ENVI-met models	Thermal comfort	A comparative analysis of all scenarios indicated that the timing and duration of extreme heat stress events were heavily influenced by AR and street orientation. Both geometric urban factors could effectively mitigate extreme heat stress when combined appropriately.
[48]	São Paulo, Brazil	Sensitivity analyses were performed for two cases: (a) H/W equal to 0.5, 1, 2, 3, 4, 5, 7 and 10, and (b) Nine different canyon orientations with an aspect ratio equal to 1	Simulation where an urban canopy layer model was coupled with a one-dimensional second-order turbulence closure model	Energy fluxes and urban air temperatures	The AR of the urban canyons greatly influenced the energy fluxes and temperatures in the urban areas
[69]	Seven idealized street canyons for simulation	ARs values from 0.5 to 8, two different air velocities 2.5 and 6.5 m/s, and various diurnal heating scenarios	Simulation with a computational fluid dynamics (CFD) numerical model coupled with photochemistry calculating transport equations for NO, NO2 and O3	NO, NO2, and O3 pollutant concentrations	Different diurnal heating scenarios significantly influenced the exchange of reactive gases between street canyons and the air above. Additionally, higher building ARs and stronger ambient wind speeds generally lead to increased entrainment of O3 concentrations into street canyons along windward walls, regardless of diurnal heating conditions.
[56]	Theoretical case studies	Three different urban canyon ARs were considered for the case studies: 0.5, 1, and 43. Canyon orientations of 0, 45, and 90 degrees were used for each AR.	Simulation with a 3D CFD numerical model. Model results were validated through wind tunnel experiments.	Vertical velocity profiles	Air velocity values remained unaffected by the AR of the canyon. Furthermore, a shift in wind direction led to an increase in air velocity values, ranging from approximately zero to 2.5 m/s, despite an incoming air velocity of 2 m/s.
[70]	Central area of Thessaloniki, Greece	ARs fluctuated between 0.6 and 3.3 and were divided into four groups: very wide (0.6 to 0.7), medium wide (1 to 1.1), medium deep (1.7) and very deep (2.8 to 3.3). Different orientations were considered.	Experimental investigation and simulations with ENVI-met	Outdoor thermal comfort conditions (PET)	During summer midday, variations in canyon orientation could result in comfort index differences of up to 22 °C, while differences in aspect ratio could cause variations of up to 4 °C. During the winter, these respective differences were equal to 4 and 7 °C.
[71]	Guangzhou, China	Scaled models of urban canyons with H/W of 1, 2 and 3 (height equal to 1.2 m and width varying from 0.3 to 1.2 m), and different thermal storage capacity (hollow concrete buildings and buildings filled with sand for higher capacity)	Experimental measurements	Outdoor air temperature profiles	During the day, street canyons with a lower H/W (equal to 1) tended to be warmer than those with higher ratios (H/W between 2 and 3) because a greater proportion of solar radiation is absorbed by the exposed wall and ground surfaces. At night, wider street canyons often cool down more rapidly due to enhanced longwave radiation emission from the larger surface area and improved night-time ventilation, which facilitates heat loss
[72]	Campinas, Sao Paulo, Brazil	36 scenarios were simulated for winter and summer conditions for avenue canyons (H/W smaller than 0.5), regular canyons (H/W equal to 1), and deep canyons (H/W over 2). Short canyons (L/H less than 3), medium canyons (L/H equal to 5), and long canyons (L/H over 7) were also considered.	Simulation with ENVI-met	Air temperature, wind speed, and pedestrian thermal comfort	Canyons with a higher H/W enhanced wind speed and provided more shading from buildings, leading to improved thermal comfort for pedestrians, particularly during the summer. Furthermore, increasing the L/H did not significantly impact thermal comfort at the pedestrian level.
[73]	A typical urban area with residential buildings	Two scenarios with four configurations were simulated based on shallow (H/W less than 0.5) and deep (H/W over 3) street canyons	Simulation with CFD calculations in ANSYS-CFX 18	Wind velocity, air temperature, urban heat island intensity	The configurations with H/W equal to 1 and L/W equal to 2 were the most effective for reducing air temperature and controlling UHI effects
[74]	Guangzhou, China	Five different H/W ratios were considered: 0.5, 1, 2, 3, and 6. Each ratio included six street canyons, except for the case where H/W was 6, which included four.	Experimental investigation.	Air temperature, west and east wall temperatures within 2D urban canyons	As H/W increased, no significant changes were observed in the canyon air temperature. Additionally, the temperatures of the east and west walls for H/W equal to 2, 3, and 6 were lower (ranging from 26.1 to 26.9 °C) and had a smaller diurnal temperature range (between 11.7 and 18.4 °C). H/W values of 0.5 and 1 had temperatures of 26.7 to 28.7 °C and diurnal temperature range (DTR) of 16.0 to 26.1 °C.
[75]	Guangzhou, China	Street canyons with different aspect ratios (H/W equal to 1, 2, 3, and 6)	Experimental investigation	Surface temperature values, wind velocities, short and long wave radiation fluxes.	Compared to H/W equal to 1, H/W values of 2, 3, and 6 resulted in an increase in daily total wave radiation by 11.41%, 14.41%, and 19.40%, respectively. Furthermore, during the daytime, surface temperature showed a negative correlation with the AR, whereas at night, the correlation was positive.
[76]	Toronto, Chicago, New York City, and Detroit (North America)	Three scenarios were considered for each case study, with H/W equal to 0.5, 2, and 8	Simulation with numerical model coupled with energy balance	Air temperature and urban heat fluxes	During the daytime, a significant reduction in heat flux and air temperature occurred only in very large Urban Canyons Aspect Ratios (UCAR). In addition, an increase in the value of UCAR intensified UHI. However, no clear correlation was found between UCAR and the temperature of individual grid cells.
[77]	Colombo, Sri Lanka	One rural and five urban canyons were selected for the experiment with H/W varying between 0.1 and 1.2	Experimental investigation	Air and surface temperature, wind speed, and humidity	Maximum temperatures showed a tendency to decrease with higher H/W and closer proximity to the sea. Additionally, a nocturnal UHI effect was observed at all urban sites. Finally, the temperature differences between sunlit and shaded surfaces in urban areas reached up to 20 °C, underscoring the importance of shade in urban canyons.
[22]	Fez, Morocco	Measurements were conducted in a deep canyon (H/W equal to 9.7) and a shallow canyon (H.W equal to 0.6)	Experimental investigation	Climatic parameters and thermal comfort	Daytime T differs between deep and shallow canyons; deep canyons cooler by day but warmer at night (heat retention). In hot–dry climates, deep canyons are beneficial, while in colder climates wider streets are preferable for solar access.
[78]	Athens, Greece	Air temperature measurements were conducted within three deep urban canyons (H/W equal to 3, 2.1, and 1.7), during the nighttime in the summer and autumn.	Experimental investigation	Air temperature, nocturnal urban heat island intensity	Reducing the AR from 3 to 1.7 led to higher median cooling rates (1.1 and 1.85 °C), maximum cooling rates (0.57 and 0.86 °C), and minimum cooling rates (1.5 and 2.8 °C).
[79]	Constantine City, Algeria	Street canyons in a very dense urban structure area with H/W between 1 and 6.7.	Experimental investigation	Air temperature, ground surface temperature, and UHI intensity	A significant air temperature difference between the urban canyons and the neighboring rural areas was found of approximately 3 to 6 °C
[80]	North Africa	H/W varied from 0.5 to 4 with several selected orientations	Experimental investigation; measurements of temperature and shading simulations	Solar shading, air temperature in the canyon	Correlations were proposed between urban canyon geometry and microclimate, which can be beneficial in creating urban design guidelines that dictate street dimensions and orientations for urban planners
[81]	Athens, Greece	10 urban canyons in the city center with H/W fluctuating between 1 and 2.5	Experimental investigation; measurements of air temperature, wind speed and direction.	Night ventilation energy performance of a standard room in an urban setting, considering both air-conditioned and naturally ventilated conditions, with single-sided and cross-ventilation during the night	The effectiveness of the studied techniques was considerably diminished due to the rise in air temperature and the drop in wind speed within the canyons
[82]	Athens, Greece	An urban canyon at the center of Athens with H/W equal to 3.3 (7 m in width, 40 m in length, and 23 m in height). Its orientation was 327.8 degrees from the north.	Experimental investigation, with measurements of air temperature, surface temperature, wind speed and direction. The air flow rate was measured in a ventilated building inside the canyon.	Natural ventilation in the urban canyon	Natural ventilation significantly reduced; airflow decreased by 82% (single-sided) and 68% (cross-ventilation).
[83]	Osaka, Japan	Two actual urban canyons were examined with H/W varying between 1 and 1.5	Simulation with a simple urban canyon model that was used assuming uniform heigh for buildings	Net solar radiation gains in the canyon (roofs, walls, street)	AR was one of the most important parameters affecting solar radiation gains inside the canyon
[84]	Campinas, Brazil	The model canyon had a length of 500 m, with widths of 9, 21, and 44 m. Its height increased incrementally by 2.5 m, ranging from 5 to 40 m. Several orientations were considered.	A 3D street canyon model was created using RayMan Pro software [84] to simulate the impact of urban design on the thermal comfort conditions	Thermal comfort in urban canyons	Urban design factors like width, height, and orientation significantly impacted the thermal conditions within street canyons. Additionally, compared to other configurations, a NE–SW orientation was particularly effective in reducing PET during the day.
[85]	Tinos island, Greece	Two canyons: traditional (H/W ≈ 2–4) and modern (H/W ≈ 0.7–0.9); simulations covered H/W = 0.6–3 with multiple orientations.	Experimental investigation and simulation for analyzing shading.	Shading conditions and solar access	Across all H/W ratios, summer solar access ranked as follows: (E) < (W/NW) < (SE/NE) < (SW/N) < (S). Thus, E–W axis streets were most effective in reducing solar gains during summer. However, this advantage decreased significantly with higher H/W, as taller buildings and narrower streets limited its effectiveness.
[86]	Nanjing city, China	64 urban canyon scenarios were simulated with different aspect ratios and orientations	Simulations with ENVI-met [87] and Rayman Pro [84]	Outdoor thermal comfort conditions	There was an inverse relationship between the street aspect ratio and the air temperature (Ta). This occurred because an increased aspect ratio can lead to reduced solar access and improved shading within the canyon, consequently lowering Ta.
[87]	Riyadh, Saudi Arabia	Two urban canyons were studies: a traditional deep canyon (H/W equal to 2.2) and a modern shallow canyon (H/W equal to 0.42) in a hot and arid climate. Both canyons run roughly in a northeast-southwest direction and were flanked by residential buildings.	Experimental investigation with measurements carried during the summer. Ambient air temperatures were measured within the canyon, and at roof level. The surface temperature of walls, roofs, and streets was also measured.	UHI and thermal comfort conditions	The UHI effect intensified as the H/W ratio decreases. Additionally, the air temperatures in deep and shallow canyons exceeded those in rural areas by 5% and 15%, respectively. Finally, the significant temperature rise in shallow canyons was due to extensive surface exposure to intense solar radiation.
[88]	Tunis, Tunisia	Different H/W were studied, from 4 to 0.25	Simulation with ENVI-met.	Outdoor thermal comfort in a Mediterranean subtropical climate (hot, dry summer and cool, rainy winter)	A high H/W could provide favorable thermal comfort conditions during the summer. As the H/W increased, comfort levels improved, for example, comparing two scenarios with H/W values equal to 4 and 0.25 revealed a 8.48 °C difference in the Universal Thermal Climate Index (UTCI).
[89]	Harbin, China	Shallow (H/W less than 1), medium (H/W between 1 and 1.5), and deep canyons (H/W over 1.5) were studied	Measurements and simulations	Outdoor thermal comfort conditions (PET) in a severe cold climate of northeast China	In severe cold climates, shallow to medium canyons with H/W between 0.5 and 1.5 are more advantageous. Furthermore, deeper canyons with H/W greater than 1.5 were not recommended due to their insufficient solar absorption.
[90]	Tehran. Iran	27 canyons were simulated with H/W from 0.6 to 2.5 to determine the suitable H/W for minimizing the effect of UHI on the microclimate.	Numerical simulations using ANSYS and thermal satellite images	Air temperature and UHI effect	ARs between 1 and 1.5 resulted in the lowest and most consistent temperatures under varying physical conditions
[91]	Santiago, Chile	The basic model consisted of an urban canyon with an H/W of 1 and a SVF of 0.3. Additional simulations were conducted for various urban canyon configurations, including H/W ratios of 0.5, 1.5. and 2.5, as well as several orientations.	Simulations with Rayman Pro	Thermal comfort conditions (PET)	The pattern observed indicated that, in the winter, heat stress increased as AR decreased, while in the summer, performance was stabilized with H/W values greater than 1.5.
[92]	Nagpur, India	Four urban canyons with a wide spectrum of aspect ratios reaching up to 1.6 and with different orientations and SVF	Measurements and simulations with Rayman Pro (climatic parameters)	Thermal comfort conditions (PMV, UTCI, and PET), and impact on microclimate	mPET strongly linked to solar-driven microclimate; AR effects significant for N–S canyons, negligible for E–W.
[93]	Ahvaz, Iran	Six urban canyons were examined with H/W varying from 0.2 to 0.6.	Simulations with RayMan (PET), site micrometeorological measurements, and questionnaire survey	Outdoor thermal comfort in a hot climate	Decreasing H/W resulted in higher PET values
[94]	Western Sydney, Australia	Four representative case studies featured narrow and semi-wide streets (H/W from 0.5 to 2)	Simulations with ENVI-met	Outdoor thermal comfort (PET) in a humid subtropical climate	Varying the AR from 0.5 to 1 increased comfort hours by 30.59%, ranking second in its impact on thermal comfort after orientation.
[95]	Damascus, Syria	Three urban areas with H/W rations equal to 0.31 (lowest), 0.83 (moderate), and 2.95 (highest) were studied	Simulations with ENVI-met	Outdoor thermal comfort (PET) in a hot dry climate	Deep canyons: AR–orientation–vegetation strongly affect T and comfort; detached layouts: AR/orientation minor, vegetation dominant.

provides a comprehensive summary of case studies examining the influence of urban canyon ARs on climatic conditions and thermal comfort. Each case study outlines the canyon’s geometry, applied methodology (experimental or theoretical), affected parameters, and key (quantitative) findings. The Table is organized thematically, focusing on studies that examine the impact of AR on microclimate and thermal comfort, allowing comparative evaluation across different climatic contexts, methodologies, and performance indicators.

Table 1 highlights critical insights into the interplay between urban geometry and thermal behavior. On the interaction between solar radiation and heat storage, low AR canyons (H/W equal to 1) experience elevated daytime temperatures compared to canyons with higher AR (H/W ranging from 2 to 3) due to greater solar radiation penetration. This intensified absorption and storage of heat by building facades and ground surfaces amplify ambient temperatures during the day. Regarding nocturnal cooling efficiency, lower AR canyons cool more effectively at night, as their larger SVF enhances longwave radiation emission and radiative cooling, complemented by improved ventilation. On the interaction of wind flow and shading, higher AR canyons improve pedestrian thermal comfort, particularly in the summer, by facilitating airflow and providing substantial shading. However, variations in the L/H ratio have limited influence on pedestrian-level thermal conditions. Regarding UHI dynamics, the UHI effect is exacerbated as the H/W ratio decreases. Suitable configurations (with H/W equal to 1 and L/W equal to 2) strike a balance between shading, ventilation, and heat retention, moderating urban air temperatures effectively. Regarding urban-rural temperature differentials, air temperatures in urban canyons exceed rural surroundings by approximately 5% in deep canyons and 15% in shallow canyons. Finally, on the inverse AR-temperature relationship, high ARs diminish solar radiation penetration and increase shading, resulting in reduced air temperatures within the canyon.

3.2. Orientation

Urban canyon orientation—defined by the alignment of streets and building façades—constitutes a key geometric parameter governing solar exposure, airflow dynamics, and outdoor thermal comfort [96,97,98,99]. By controlling the intensity and spatial distribution of incoming solar radiation, orientation directly influences surface heating, shading patterns, wind behavior, and the overall thermal balance within the canyon. Consequently, it plays a fundamental role in shaping urban microclimatic conditions.

The importance of orientation becomes particularly critical in the context of climate change and increasing urban density, where effective passive design strategies are required to mitigate heat stress [7,78,100,101]. Two dominant mechanisms explain its impact. First, East–West (E–W)-oriented streets are subjected to prolonged solar exposure—especially during summer—resulting in increased surface and air temperatures and intensified urban heat island (UHI) effects. Second, North–South (N–S) oriented streets tend to align more favorably with prevailing wind directions, enhancing ventilation and facilitating heat dissipation [88].

Orientation also governs diurnal and seasonal shading dynamics [94,102,103]. In the northern hemisphere, N–S streets are partially shaded during morning and afternoon hours in summer, while E–W streets remain largely exposed to direct solar radiation. At solar noon, N–S canyons receive peak solar input, whereas E–W canyons exhibit limited shading. Intermediate orientations, such as NW–SE and NE–SW, provide more balanced and continuous shading throughout the day. Overall, N–S and oblique orientations generally enhance summer shading while maintaining adequate winter solar access, whereas E–W orientations tend to experience persistently higher solar loads and reduced shading potential [94].

Table 2. Impact of orientation on outdoor thermal comfort and urban microclimate.

Reference	Case Study Location	Orientation and Other Canyon Characteristics	Methodology	Affected Parameters (Objectives)	Key Findings
[81]	North Africa	Several canyon geometries were applied, with street orientations varying in steps of 15° from north to east and H/W varying from 0.5 to 4	Experimental investigation with measurements of temperature and shading simulations	Solar shading, air temperature in the canyon	A N–S street orientation with an H/W of 1.5 or greater could achieve street shading between 40% and 80% of the total area. Additionally, diagonal orientations (NW–SE and NE–SW) only provided 30% to 50% shading year-round.
[21]	Ghardaia, Algeria	Two primary solar orientations (N–S and E–W) were selected and analyzed across different H/W (0.5, 1, 2, 4). Intermediate orientations (NE–SW and NW–SE) were evaluated for an H/W of 2.	Simulation with a 3D numerical ENVI-met model	Outdoor thermal comfort (PET) in a hot and dry climate	Wide canyons (H/W ≈ 0.5) show high stress, worst in E–W; N–S with H/W > 2 improves conditions (lower PET, shorter stress).
[104]	Sede-Boqer, Negev desert, Israel.	Different urban canyon geometries were tested varying street axis orientations aligned and perpendicular to the prevailing wind	Simulations for calculating the cooling load of a building	Energy consumption for cooling in a dry climate	High AR N–S streets could enable buildings to shade each other’s façades and windows, leading to a reduction in cooling demands. On the other hand, wide streets oriented along an E–W axis, with buildings facing north and south, maintained relatively low cooling loads even in the absence of mutual shading.
[105]	Tinos Island, Greece	Two urban canyons in a traditional and a modern urban area. N–S, E–W, NA–SW, and NW–SE orientations were selected for the simulations while the H/W ratio varied between 0.6 and 3.	Simulations with Rayman Pro (PET)	Outdoor thermal comfort and microclimate	Best comfort in covered streets, then north-facing E–W sidewalks; N–S favorable for H/W = 0.8–1.3, comparable to diagonal at higher AR (2–3).
[85]	Tinos island, Greece	Two urban canyons were investigated with the same street axes orientation. Parametric simulations were conducted for various street orientations (N–S, E–W, NE–SW, and NW–SE. Several ARs were tested.	Experimental investigation and simulation for analyzing shading	Shading conditions and solar access	Summer solar gain: E < W/NW < SE/NE < SW/N < S; E–W streets minimize gains, but effectiveness decreases with higher H/W.
[88]	Tunis Tunisia	N–S, W–E, NE–SW and NW–SE orientations with H/W ratio values varying from 4 to 0.25	Simulation with ENVI-met	Outdoor thermal comfort Mediterranean subtropical climate (hot, dry summer, and cool, rainy winter).	For all H/W configurations, streets oriented in the N–S direction tended to provide the highest degree of comfort, whereas those oriented in the W–E direction offered the least favorable conditions.
[70]	Central area of Thessaloniki, Greece.	Experimental investigation was conducted in 18 street canyons for the winter and the summer periods. Different orientations were considered (NWSE, NESW, EW, and NS). ARs fluctuated between 0.6 and 3.3.	Experimental investigation and simulations with ENVI-met	Thermal comfort conditions of pedestrian (PET)	NW–SE orientations most comfortable overall; seasonal effects vary by canyon depth and façade exposure (SW/NW sides dominant).
[94]	Ahvaz, Iran	Six urban canyons with predominant street orientations NNE–SSW and WNW–ESE. Among the studied sites, three were oriented NNE-SSW or near NS, while the other three were aligned WNW–ESE or near EW. H/W varied from 0.2 to 0.6.	Simulations with RayMan-(PET) as well as site micrometeorological measurements and questionnaire survey	Outdoor thermal comfort in a hot climate	Canyons with orientations closer to NS exhibited lower air temperature and MRT. In the NS-oriented canyons, the west-facing sidewalks showed reduced MRT and PET values in the morning, while the east-facing sidewalks experienced lower values in the afternoon.
[95]	Western Sydney, Australia	Four representative case studies featuring narrow and semi-wide streets with two main orientations: N–S and E–W, both with a 9-degree deviation. H/W varied from 0.5 to 2.	Simulations with ENVI-met	Outdoor thermal comfort in a humid subtropical climate	The orientation of street canyons was the most significant factor, accounting for 46.42% of the influence on PET.
[90]	Harbin, China	Basic orientations studied were N–S, W–E (for branches) and NW–SE for shallow, medium, and deep canyons	Measurements and simulations	Outdoor thermal comfort conditions (PET) in a severe cold climate of northeast China	N–S: shallow–medium AR best; E–W: favorable at H/W ≈ 1–1.5; NW–SE poor in winter; NE–SW needs summer shading but benefits winter warming.
[106]	Isfahan, Iran	The existing orientation was 76 degrees (E–NE). For simulations, orientation was altered to (N–S) and (E–W) (0 and 90 degrees). Several scenarios were applied: 60 (NE–SW), 150 (NW–SE), 120 and 60 degrees.	Simulations with ENVI-met and RayMan (PET). Validation with field measurements and questionnaires.	Outdoor thermal comfort (PET) in a hot and dry climate	An orientation of 150 degrees was suitable for thermal comfort, as it provided effective shading and significantly lowered MRT.
[92]	Santiago, Chile	Base case: H/W = 1, SVF = 0.3; simulations covered H/W = 0.5–2.5 with E–W, N–S, NE–SW, and NW–SE orientations.	Simulations with Rayman Pro	Thermal comfort conditions (PET)	Preferred orientations: N–S (best for >6 stories), NW–SE (second-best), NE–SW (favorable for >10 stories, wide streets); E–W requires shading.
[93]	Nagpur, India	Four urban canyons with N–S and E–W orientations and a wide spectrum of ARs	Measurements and simulations with Rayman Pro (climatic parameters)	Thermal comfort conditions (PMV, UTCI, and PET), and impact on microclimate	Outdoor thermal comfort conditions were strongly influences by the orientation of the street canyon, being particularly notable for the N–S orientation but negligible for the E–W orientation. Furthermore, a N–S oriented street with a high H/W resulted in the least physiological stress for the longest part of the day.
[107]	Tel-Aviv, Israel	Two canyon type courtyards were selected with H/W ratios equal to 0.6 and 0.48 and orientations close to N–S	Simulations and parametric analysis, and comparison with measured values	Microclimatic parameters and cooling effect of green areas.	The impact of orientation on air cooling in N–S oriented wooded clusters was found to be only marginally more effective than in E–W-oriented clusters. In a cluster with a H/W ratio of 1 and high wall albedo, the N–S orientation was approximately 0.64 °C cooler than the E–W orientation.
[108]	Bangkok, Thailand	One street canyon with a H/W ratio of 1.1 and N–S, NW–SE and NE–SW orientations	Simulations with ENVI-met BioMET and measurements for validation	Outdoor thermal comfort (PET)	N–S canyons offered the most comfort hours (31 to 46%), followed by NW–SE (23 to 46%) and NE–SW (23–38%) orientations. Additionally, E–W canyons provided the least favorable conditions and require further study.

provides an overview of the most representative case studies demonstrating the impact of urban canyon orientation on outdoor thermal comfort conditions for different climatic conditions. The table is organized thematically, focusing on studies that examine the influence of street orientation on outdoor thermal comfort, enabling comparative analysis across different climatic conditions, urban configurations, and methodological approaches.

Several noteworthy insights may be drawn from the studies tabulated in Table 2. Street orientation emerges as the most influential factor, contributing approximately 46.42% to the overall impact on thermal comfort. This dominant role of orientation underscores its centrality in urban planning. The efficiency of shading is shown by the fact that N–S-oriented canyons with an AR of 1.5 or greater provide 40 to 80% shading, outperforming diagonal orientations (NW–SE and NE–SW), which offer only 30 to 50% shading annually. Regardless of orientation, wide streets (with an H/W of 0.5) exhibit heightened thermal stress, with E–W streets being particularly problematic due to limited wall shading, even at high AR values (e.g., H/W equal to 4). On the cooling advantage of N–S orientation, high AR N–S streets (with a H/W over 2) deliver improved thermal comfort by moderating PET peaks, shortening stress periods, and enabling mutual shading, thereby reducing cooling demands. During the summer, orientations experience increasing solar gain in the following order: East (E), West/Northwest (W/NW), Southeast/Northeast (SE/NE), Southwest/North (SW/N), and South (S). E–W streets exhibit reduced solar gains, but this advantage diminishes in narrow streets with tall buildings. On the relationship of comfort dynamics to orientation and depth, in medium-wide canyons, NW–SE orientations yield the most comfortable year-round conditions. For deep canyons, the south side of E–W canyons is preferable in the summer, while the southwest side of NW–SE canyons excels in the winter. A careful examination of comfort hours showed that N–S canyons offer the highest proportion of comfort hours (31 to 46%), followed by NW–SE (23 to 46%) and NE–SW (23 to 38%). E–W canyons are the least favorable, necessitating additional shading measures. Finally, there is a notable correlation between the mean physiological equivalent temperature (mPET) and microclimatic factors like solar radiation. This correlation is particularly strong for N–S oriented canyons, emphasizing the need to account for orientation in thermal comfort assessments.

Summarizing the favorable conditions for different orientations, the N–S axis is preferred for buildings taller than six stories, as it provides the best thermal comfort and minimizes physiological stress. The NW–SE axis is suitable for buildings above six stories and neighborhood canyons with buildings up to six stories. The NE–SW axis is suitable for taller buildings (above 10 stories) with wider streets (20 m). Finally, the E–W axis requires robust shading solutions like trees or arcades to maintain comfort in public spaces.

3.3. Sky View Factor

The sky view factor (SVF) is a dimensionless parameter that quantifies the fraction of the visible sky hemisphere from a given point within an urban environment, particularly in street canyons. It is defined as the ratio of visible sky area to the total hemispherical area, with values ranging from 0 (fully obstructed) to 1 (completely open) [11,19,109,110].

SVF is a key descriptor in urban climatology and building energy studies, as it regulates both shortwave solar radiation input and longwave radiative exchange with the sky. Through these mechanisms, it directly influences the urban energy balance, airflow patterns, the urban heat island (UHI) effect, and local microclimatic conditions [20,24,55]. In addition, SVF strongly affects outdoor thermal comfort by modulating the radiative environment, particularly mean radiant temperature (MRT) [55,111].

From an energy balance perspective, SVF governs the trade-off between solar gains and radiative cooling. Low SVF conditions—typically associated with narrow, dense urban canyons—limit daytime solar access but also restrict nocturnal longwave radiation loss, resulting in heat storage and elevated nighttime temperatures that intensify UHI effects. In contrast, high SVF environments facilitate efficient radiative cooling during nighttime, contributing to lower air temperatures and improved thermal regulation [11,19,20,25,48].

SVF also plays a critical role in daylight availability. Low SVF values significantly reduce solar penetration, increasing reliance on artificial lighting and potentially affecting visual comfort and well-being. Conversely, higher SVF enhances daylight access but may increase solar heat gains, highlighting the need for balanced design strategies [55,112].

In terms of airflow, SVF interacts with urban morphology to influence wind dynamics. Low SVF configurations, typically characterized by tall buildings and narrow streets, tend to suppress ventilation due to physical obstruction, whereas higher SVF conditions generally support improved airflow and dispersion processes [19,20,113].

The combined radiative and aerodynamic effects of SVF have direct implications for outdoor thermal comfort. Low SVF environments often exhibit increased nocturnal heat retention and elevated MRT, while higher SVF promotes cooling but may increase daytime heat exposure. Therefore, intermediate SVF conditions are often associated with more balanced thermal environments [19,20,21].

SVF can be quantified using two primary methodologies [52,55,114,115]. Digital 3D modeling provides accurate geometric representation of urban form and is well suited for analyzing static configurations. However, it typically neglects dynamic elements such as vegetation. In contrast, fisheye lens imaging captures hemispherical photographs that include both built and natural elements, enabling more comprehensive and seasonally sensitive assessments. This method is particularly valuable in studies where vegetation–geometry interactions significantly influence microclimatic behavior.

While increased shading improves outdoor thermal comfort, it may reduce daylight availability in adjacent buildings, including biologically effective (melanopic) light. This highlights the need for integrated design strategies that balance outdoor cooling benefits with indoor daylight requirements.

Table 3. Impact of SVF on outdoor thermal comfort and urban microclimate.

Reference	Case Study Location	Climatic and Urban Environment Characteristics/SVF Methodology	Main Objectives	Key Findings
[111]	Goteborg, Sweden	A total of 17 stations were examined, including 16 in an urban area and one in an open area. The fisheye photographic method was used to derive SVFs at different heights above ground level.	The impact of SVF on the urban air temperature was examined and analyzed with regression analysis.	There was a relatively strong correlation between SVF and air temperature during clear, calm nights. This relationship was evident not only in specific case studies but also on an annual average basis.
[79]	Constantine City (Algeria)	Street canyons were considered, in a very dense urban structure area with different geometric configurations. Fisheye photographs were taken at each station at a height of approximately 1.5 m above ground.	Examine air temperature within the canyons	A notable air temperature difference was observed of about 3 to 6 °C between the urban canyons and the surrounding rural areas. Furthermore, it was noted that higher SVF values generally corresponded to higher recorded air temperatures with few exceptions.
[89]	Tunis Tunisia	Three factors were considered for the urban canyons in a Mediterranean subtropical climate: AR, SVF, and street orientations. Fisheye photographs were taken.	Outdoor thermal comfort conditions were examined	Of the morphological indicators examined, H/W and SVF stood out as having a significant influence on external thermal comfort
[109]	Bari, South Italy	The study location was in a mediterranean climate with hot dry summers and mild winters. Geometric methods and SVF maps were generated using a 3D database within a geographical information system (GIS).	The relation between SVF and land surface temperature (LST) was investigated	A positive correlation between LST and SVF was established, with the trends being nearly identical for images taken by the same sensor but differing for those at varying resolutions. The differences were caused by micro-scale factors, such as the thermal properties of building materials, anthropogenic heat, humidity, pollutants, etc.
[24]	Isfahan, Iran	The climate was arid with hot and dry summers and cold winters. The fisheye photographic method was used. SVF and thermal comfort (PET) were computed by entering these fisheye photos and additional meteorological data into RayMan.	Field measurements were used to examine the relationship between SVF and various micrometeorological variables	Regression analysis revealed that SVF had the least impact on air temperature, while it significantly influenced mean radiant temperature and surface temperature. Additionally, a positive and significant correlation was found between SVF and PET.
[116]	Barcelona (Spain), Berlin (Germany), London (UK), New York (US), Nanjing (China), and Paris (France).	Six street models were studied, representing the core urban morphology (characteristic street patterns) of the six cities. SVF was computed by using fisheye lens photography of the urban street canyons as well as other relevant meteorological data.	The study used simplified street models to represent the typical urban morphology of the six major cities. Each model included a central main street with intersecting secondary streets.	SVF negatively correlated with BD, closing, and symmetry ratios; positively with opening ratio.
[117]	Harbin, China	The study area was in the Dwa climatic zone (Köppen climate classification), with dry summers and cold winters (some of the lowest temperatures in the region.). The SVF was determined using fisheye lens photography of urban street canyons, along with measurements of meteorological parameters.	SVF was used as an index to adjust and analyze the landscape morphology. The study focused on understanding the relationship between SVF and the thermal environment of a typical street canyon in Harbin.	The following were found for typical street canyons with an AR of 0.5: (a) Both temperature and mean radiant temperature initially decreased and then rose as the SVF decreases; (b) RH showed an initial increase followed by a decline; and (c) There was a statistically significant quadratic relationship between SVF and the temperature, RH, and MRT within the street canyon.
[94]	Ahvaz, Iran	Six urban canyons were examined in a hot and arid climate in Koppen Geiger classification (designated as BWh). Measurements, and simulations with ENVI-met and Rayman.	The impact of SVF on outdoor thermal comfort conditions was investigated	There was a strong correlation among SVF and both PET and MRT across various locations, with Pearson correlation coefficients at noon ranging between 0.75 and 0.93. Furthermore, no significant correlation was found between SVF and RH at any of these sites. Finally, shading had no significant effect on air temperature (Ta), leading to a lack of correlation between Ta and SVF.
[52]	Austin, Texas, USA	The case study area exhibited significant variation in building heights, with open and green spaces scattered throughout. Fisheye lens photography was used.	The influence of urban geometry and of the most important meteorological parameters on the urban thermal environment and microclimate was investigated in this case study	The investigated urban geometry factors, such as SVF, floor area ratio (FAR), and Building Coverage Ratio (BCR), affected the microscale thermal environment in urban street canyons. Building on these results, a model was developed to estimate the microscale UHIs, which influenced the microscale thermal environment.
[93]	Nagpur, India	The study focused on tropical wet and dry climate according to the Köppen climate classification. Four urban canyons with orientations N–S and E–W and a wide spectrum of Ars were studied with fisheye lens photography used for the SVF.	Impact of thermal comfort conditions (PMV, UTCI, and PET), on microclimate	The minimum SVF played a significant role in both studied street orientations by blocking solar radiation. Additionally, as a combination of AR, trees, and other built structures, SVF can be adjusted without changing the AR.
[118]	Beijing, China	The study considered an urban area in the humid continental monsoon climate. Fisheye lens photography was used for SVF. Climatic parameters (air temperature) were measured for day and night, summer and winter.	Multiple regression was used to analyze the impact of landscape design parameters on urban air temperature variability	It was found that (a) Greater building area correlated with higher air temperatures; (b) Increased vegetation cover correlated with lower air temperatures; (c) Site geometry significantly influenced temperature regulation; (d) Daytime air temperature rose with higher SVF; and (e) Nighttime air temperature decreased with higher SVF.
[119]	Hong Kong	The study was carried out in a humid-subtropical climate. 3D GIS technology was used for SVF.	The study addressed the impact of SVF on UHI. A parametric study established a connection between SVF and two key planning parameters: site coverage ratio and building height.	A 10% increase in the average SVF could lead to a decrease in air temperature by approximately 0.48 °C. Urban planners can strategically manage site coverage ratio and building height to enhance the SVF, thereby reducing UHI effects in densely populated urban areas.
[120]	Mendoza, Argentina	The study climate was semi-arid, characterized by hot summers, mild winters, and low annual rainfall. The nighttime UHI effect reached up to 10 °C during all seasons. Fisheye lens photography was used for SVF.	Thermal comfort and energy balance were calculated	The absorbed solar radiation and the re-emitted radiation were the key components influencing the energy budget in thermal comfort calculations. These variables were affected by (a) the SVF, which is shaped by the arrangement of urban elements, buildings, and green spaces, and (b) the thermophysical properties of materials, which dictated surface temperatures.
[121]	Athens, Greece.	The mediterranean climate of the study area is characterized by hot, dry summers and mild, wet winters. Selected urban areas for the analysis featured various configurations of trees and buildings.	The study addressed the impact of SVF, environmental layout, and vegetation coverage on outdoor thermal comfort conditions (PET).	Sites with lower SVF values and dense vegetation provided better human biometeorological conditions. Additionally, higher SVF values and nearby buildings were associated with less favorable conditions. Finally, significant correlations were observed between SVF values and various biometeorological indices.
[122]	Huwei, central Taiwan	Subtropical climate; SVF measured at six sites (0.04–0.81) using fisheye images.	The study addressed the impact of shading on the long-term thermal environment by assessing the comfort levels of local residents using 10 years of meteorological data.	Locations with minimal shading (high SVF) were uncomfortable in the summer, while highly shaded locations (low SVF) were uncomfortable in the winter. Locations with moderate shading provided the longest periods of thermal comfort throughout the year.
[123]	Shanghai, China	The study area had a humid subtropical climate. Extensive field measurements were carried out.	The study considered the microscale impacts of urban form and density (including buildings and greenery) on outdoor ventilation potential by utilizing empirical data gathered from extensive field measurements.	A 10% increase in SVF could lead to a 7 to 8% rise in wind velocity ratio.
[124]	London, UK	Three areas high, medium, and low building densities were selected in central, west, and north London.	The study examined the connections between urban geometry factors and solar availability indicators over various time periods. The seasonal solar performance of urban form façades and ground surfaces was also examined.	Ground SVF and diffuse irradiance depend on spacing, coverage, directionality, and layout; direct irradiance is season-dependent (solar altitude).
[125]	Curitiba, Brazil	The study area climate was temperate oceanic with dry winters. Measurements of climatic parameters along with fisheye images were taken at each monitoring point in order to calculate the SVF.	The study aimed to study the impact of SVF (as an urban geometry indicator) on pedestrian thermal comfort conditions	On hotter days, areas with higher SVF, meaning less sky obstruction, tended to cause greater heat discomfort. However, these same areas could offer comfort on cooler days. Furthermore, no significant correlation between the diurnal urban heat island effect and SVF was observed. Finally, a relatively strong correlation (with a Pearson correlation coefficient of 0.71) was found, indicating that SVF is a significant factor in air temperature variations.
[126]	Beijing, China	The study area had a typical humid continental monsoon climate	The study targeted the impact of SVF on outdoor thermal conditions and PET in Beijing’s central business district	Compared to less shaded areas, highly shaded areas (SVF less than 0.3) experienced fewer instances of hot conditions during the summer while enduring extended periods of cold discomfort in the winter. Moderately shaded areas (with an SVF between 0.3 and 0.5) and slightly shaded areas (SVF over 0.5) on the other hand tended to have a more balanced thermal perception, with less extreme variations between hot and cold conditions.
[127]	Atisaz in Tehran, Iran	Semi-arid climate; ENVI-met analysis with SVF classes: open (0.75–1), semi-open (0.5–0.75), semi-dense (0.25–0.5), dense (<0.25).	The study targeted the impact of SVF on air temperature	The following were found: (a) Before and after warm hours, air temperature increased as SVF decreased (negative correlation); (b) During warm hours, higher SVF values correlated with positively higher air temperatures; (c) Open spaces with high SVF had higher temperatures, while shaded, low-SVF spaces remained cooler; (d) A weak SVF-air temperature correlation was noted at 9 am; and (e) Improving SVF was a key to achieving balanced day and night temperatures.
[128]	Beijing, China	The study area had a humid continental monsoon climate. Seven building indicators were considered, including average building height (BH), average BD, ratio of building surface area to plan area (λB), FAR, SVF, frontal area index (FAI), and building shading (BS).	The study aimed to examine the relationship between urban morphology and urban microclimate across various spatial scales (30 m to 1 km) and temporal scales (diurnal and seasonal).	The influence of morphological indicators on annual wind speed weakened with spatial scale, while their effect on air temperature (Ta) and RH initially decreased and then increased. Additionally, morphological factors most strongly affected meteorological parameters (Ta, wind speed, and RH) at the 30 m scale, with SVF being the dominant factor. Finally, urban microclimate effects varied diurnally as vegetation impacted air temperature more at night, and SVF was especially critical at night in the winter.

synthesizes the critical findings from case studies investigating the role of SVF in shaping outdoor thermal comfort and urban microclimates. The table is organized thematically, focusing on studies that investigate the influence of SVF on thermal and microclimatic conditions, enabling comparative analysis across different climatic contexts, urban morphologies, and methodological approaches.

Insights from Table 3 underscore the interdependencies among SVF, energy balance, and climatic parameters such as air temperature, relative humidity (RH), and wind speed. Regarding outdoor thermal comfort, strong positive correlations exist between SVF and thermal indices like PET and MRT, demonstrating consistent trends across diverse urban locations. Low SVF areas with high shading are more thermally comfortable in the summer but less so in the winter, whereas moderate SVF ensures balanced comfort year-round. Low SVF combined with dense vegetation enhances biometeorological conditions, while high SVF near buildings exacerbates thermal discomfort. Finally, high SVF areas amplify heat stress on hotter days but may offer improved comfort on cooler days, highlighting its dual role in thermal dynamics.

Turning to the issue of impacts on urban microclimate, SVF shows strong correlations with air temperature, particularly during clear, calm nights, and near the annual average. Higher SVF values generally lead to increased daytime air temperatures but lower nighttime temperatures, emphasizing its role in diurnal thermal regulation. In canyons with low aspect ratios (e.g., H/W equal to 0.5), air temperature and MRT initially decline with decreasing SVF but increase beyond a threshold. While RH shows limited correlation with SVF in some studies, others identify a quadratic relationship influenced by specific street canyon geometries. Improving SVF is essential for achieving consistent air temperature moderation throughout the day and night. Finally, factors such as site coverage, outdoor distance, and layout complexity strongly influence the interaction between mean ground SVF and diffuse irradiance.

In addition to geometric parameters, the thermophysical properties of urban materials play a crucial role in modulating the thermal behavior of urban canyons. Surface characteristics such as albedo, thermal conductivity, and heat capacity directly influence solar radiation absorption, heat storage, and longwave radiation exchange. Materials with low albedo and high heat capacity, such as concrete and asphalt, tend to absorb and store substantial heat during the day and release it at night, thereby intensifying nocturnal urban heat island effects. In contrast, high-albedo materials reduce solar heat gains, while materials with lower thermal inertia facilitate faster cooling. Therefore, the combined effect of urban geometry and material properties governs the overall energy balance and thermal regime within urban canyons, highlighting the importance of integrated design strategies that couple morphological characteristics with appropriate material selection.

4. Role of Greenery

Urban greening is increasingly recognized as an effective strategy for mitigating adverse thermal conditions in urban canyons. Incorporating vegetation into urban infrastructure—such as through street trees, GRs, green walls, vertical gardens, and pocket parks—plays a pivotal role in moderating microclimates and enhancing thermal comfort. Vegetative elements act as natural regulators by providing shade and solar control, facilitating evapotranspiration, mitigating air pollution, and influencing local wind patterns, which collectively contribute to the reduction in heat accumulation in densely built environments [28,29,30,31,33]. The effectiveness of green infrastructure is strongly climate-dependent, as vegetation performance varies with local environmental conditions; evapotranspiration-driven cooling is particularly effective in hot–dry climates, while in humid climates its contribution may be reduced, and in colder regions excessive shading may negatively affect winter thermal comfort [30].

Research has shown that strategic urban greening can significantly lower surface temperatures, reduce the UHI effect, improve thermal comfort, and create more habitable outdoor spaces for city dwellers [1,129,130,131,132]. Vegetation serves as a natural moderator of urban microclimates, primarily through mechanisms such as shading, enhancing evapotranspiration, and altering local wind dynamics [133]. By intercepting solar radiation, vegetation reduces the amount of heat absorbed by urban surfaces, while evapotranspiration dissipates heat, thereby cooling the surrounding air [134]. Furthermore, the presence of trees and green surfaces can modify wind flow patterns, promoting ventilation that can further alleviate heat stress in densely built environments [9,120,130,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153].

4.1. Key Structural Parameters of Vegetation for Canopy Characterization

The most significant vegetation canopy morphological parameters used to quantify the impact of vegetation on outdoor thermal comfort conditions include LAI and leaf area density (LAD). LAI is a dimensionless quantity that describes the amount of leaf area per unit ground surface area [135,136] and equals

$$\mathrm{LAI}={\displaystyle \frac{\mathrm{Ground} \mathrm{surface} \mathrm{area}}{\mathrm{Total} \mathrm{leaf} \mathrm{area}}}$$

(3)

where the total leaf area refers to the one-sided or total surface area of all leaves within a given ground area, depending on how it is measured, and the ground surface area is the horizontal area of the ground beneath the vegetation. LAI is a critical parameter in ecological, meteorological, and environmental sciences as it directly influences various processes such as photosynthesis, transpiration, and energy exchange between the land surface and atmosphere.

LAD is defined as the total one-sided leaf area (in m²) within a given volume of vegetation (in m³) [34,137]. It provides a vertical profile of how leaf area is distributed across different layers of the canopy, which influences radiation penetration, energy exchange, and microclimatic conditions within urban environments.

The relationship between LAI and LAD can be expressed by integrating the LAD across the vertical extent (height) of the plant canopy [34,138].

$$\mathrm{LAI}={{\displaystyle \int }}_0^{H}LAD\left(z\right)dz$$

(4)

where H is the total height of the canopy.

Further to LAI and LAD, tree heigh, trunk heigh, and crown diameter are additional parameters that could be considered as regards tree canopies [137].

4.2. Synergistic Effects of Urban Canyon Geometry and Green Infrastructure for Enhancing Thermal Comfort

The interaction between urban canyon geometry and GI presents a highly effective approach for enhancing thermal comfort in dense urban environments. Strategic integration of green elements—such as trees, GRs, and vertical gardens—within specific urban canyon configurations leverages synergistic effects to optimize shading, modulate airflow, and enhance evapotranspiration. These interactions provide critical opportunities for mitigating heat stress and creating more livable and sustainable urban spaces [33,34,100,154,155].

Table 4. Synergistic interactions between urban geometry parameters and vegetation.

Reference	Case Study Location	Urban Geometry and Vegetation	Main Objectives	Key Findings
[156]	Seoul, South Korea	Simulations were based on an AR of 1.5 and a tree spacing of 6 m, resembling real-world conditions. Additionally, the study explored various ARs (H/W equal to 0.5, 1, and 2) and a denser tree spacing of 2 m to assess their impact on thermal comfort and microclimate.	Thermal comfort and microclimatic parameters calculations with ENVI-met (PMV)	The following were found: (a) An urban canyon with an AR of 1.5 and 2 m tree spacing showed the lowest temperature, MRT, and PMV; (b) A 2 m tree spacing at an AR of 2.0 would likely enhance thermal comfort further, as both PMV and MRT improved with this AR; and (c) At an AR of 1.5 and 2 m tree spacing, temperatures were lowest at a value of 35.91 °C (12:00 p.m., 0 degrees wind) and 36.09 °C (90 degrees wind).
[154]	Bilbao (Basque Country, Spain)	Three types of canyons were selected: (a) B/T of 0.8 and H/W of 3.5 (low-rise), (b) B/T of 0.6 and H/W of 1.5 (mid-rise), and (c) B/T of 0.4 and H/W of 1.3 (high-rise), where B/T represents the ratio of building surface to the total area of the canyon. For each type of canyon, seven vegetation scenarios were considered: S0 (current situation), S1 (finishing materials of ground surfaces), S2 (grass), S3 (grass and trees), S4 (GRs), S5 (grass and GRs), and S5 (grass, GRs and trees).	Thermal comfort and microclimatic parameters calculations with ENVI-met (PET)	High ARs increased shadowing but could also limit ventilation, impacting surface temperature, MRT, and the PET index. Additionally, H/W and ground surface materials significantly affected the intensity and duration of discomfort periods (PET over 23 °C). Finally, greening combined with appropriate ARs was essential for mitigating thermal stress in different canyon configurations.
[33]	Port Phillip, Melbourne, Australia	Two street canyons (A & B) were considered: (a) Street A was oriented east–west, wide (30 m), with low-rise buildings (two stories, 6 m), an H/W of 0.2, and scattered small trees unlikely to develop large canopies; (b) Street B was also oriented east–west, narrow (5 m), with low-rise buildings (two stories, 6 m), and an H/W of 1.2. No other urban GI was present.	Developed a five-step framework for prioritizing GI to improve urban microclimate	The following were proposed: (a) For street canyon A, plant wide, dense-canopy trees at higher frequency, particularly on the sun-exposed southern side; (b) For street canyon B, install a GW or narrow hedge on the north-facing wall to improve thermal comfort; (c) Street trees effectively lowered surface temperatures in canyons with H/W less than 0.8, but their cooling effect diminished as H/W exceeded 0.8; (d) In narrow canyons with sufficient light, green walls, facades, and ground-level vegetation should be prioritized over trees due to space limitations; and (e) Higher H/W ratios reduced light availability and increased wind turbulence, challenging plant survival.
[137]	Wuhan, central China	Eight street canyon geometries were considered by varying two orientations (N–S, E–W) and four H/W ratio values (1, 1.5, 2, and 3.0). The morphological parameters of street trees included LAD (0.2 to 3 m2/m3), tree height (8 to 12 m), trunk height (greater than 3 and less than or equal to 3 m), and tree crown diameter (4 to 8 m).	Thermal comfort calculations (PET) with various combinations of urban geometry and tree forms were carried out. The study developed a framework for tree species selection, considering tree characteristics and canyon geometry, e.g., Ars and orientations.	Favorable configurations depend on orientation and AR. For E–W canyons, H/W ≈ 1–1.5 requires dense vegetation on both sides, while higher AR (≥2) reduces vegetation priority, favoring north-side planting. For N–S canyons, dense vegetation is most effective at H/W ≈ 1–1.5 (both sides), while at higher AR (≥2–3) priority decreases, with emphasis on west-side planting.
[105]	Tinos Island, Greece	A traditional and a modern urban canyon in two different urban areas were studied. N–S, E–W, NA–SW, and NW–SE orientations were selected for the simulations, while H/W varied between 0.6 and 3. Street trees were used for the simulations.	Outdoor thermal comfort and microclimate simulations with ENVI-met	Trees had a stronger influence on reducing PET values on E–W streets, particularly on the south-facing side. Additionally, planting large trees (5 m in diameter) closely spaced along E–W streets created a cooling effect similar to that of covered streets. Finally, on N–S streets, trees still improved thermal comfort, but the effect was less dramatic due to thermal conditions being already adequate.
[35]	Hong Kong	The base scenario modeled four street canyons with varying ARs (ARB of 1, 2, 3, and 4), along with trees of three different ARs (ART of 1.7, 3.3 and 5). The modeled trees all had the same LAI of 3 but varied in LAD, distributed across three different tree heights (5, 10, and 15 m). Each tree had a consistent crown diameter of 3 m and a trunk height of 2 m. Sensitivity analysis was carried out for an ARB of 2 and an ART of 3.3, meaning that treed were 10 m tall with a 3 m crown width. LAI was set to 3, but the effect of LAI being equal to 1, 2, 4, 5, and 6 was also investigated.	Outdoor thermal comfort (PET) and microclimate simulations with ENVI-met	It was found that trees, irrespective of their configuration, lowered thermal sensation from “very hot” to “hot” across all street canyon ARs. Additionally, the enhancement in thermal comfort, indicated by PET, diminished as the street canyon aspect ratio increased. Finally, variations in tree aspect ratios influenced the PET reduction, suggesting that the distribution of LAD at varying tree heights played a more significant role in improving thermal comfort than the height of the tree trunk height or LAI.
[36]	Hong Kong	Four different configurations were considered for the simulations: open area without trees, open area with trees, street canyon without trees, and street canyon with trees. Four ARs were evaluated, combining the selected building heights with typical street widths. All streets were modeled with a symmetrical N–S orientation and a length of 60 m.	Outdoor thermal comfort (PET) and microclimate were simulated with ENVI-met	Trees improve comfort in all canyons, but effects depend on geometry: deep canyons favor tall trees with low LAI and sparse canopies, while shallow canyons benefit from denser, medium-height trees with wide crowns; impacts vary with species, time of day, and street layout.
[37]	Seoul, South Korea	Studies street canyons were narrow and wide with an E–W orientation, featuring pedestrians on the northern sidewalks. Vegetation included trees of varying size (small, medium, large, and very large).	Simulations for calculating MRT were carried out. MRT was assessed by examining how tree size and spacing influence solar shading and longwave radiation exchange.	It was found that MRT decreased significantly with narrower tree spacing, especially for smaller trees. Additionally, on the south-facing side of E–W streets, tree size impacted thermal comfort more than street size. Furthermore, MRT reduction was primarily due to trees blocking direct shortwave radiation, lowering reflected and emitted radiation from walls and sidewalks. Finally, favorable tree spacing enhanced pedestrian thermal comfort: smaller trees required closer spacing for shading, while larger trees remained effective with wider spacing.
[108]	Bangkok, Thailand	Street canyons with an H/W of 1.1; N–S, NE–SW, and NW–SE orientations; and a SVF of 0.592 were studied	Outdoor thermal comfort (PET) was simulated with ENVI-met. Measurements were used for validation.	In hot–humid climates, shading is critical: trees reduce PET by up to 8.6 °C (buildings up to 14.2 °C) and increase comfort hours, especially in shallow canyons (H/W ≈ 0.5–0.7); N–S orientation performs best, and tree effects are strongest in avenue canyons.
[37]	Hong Kong	A total of 54 generic tree forms were integrated with 10 distinct urban morphology types characterized by their SVF	Thermal comfort (PET) was simulated with ENVI-met	Tree species regulated temperature differently, with daytime cooling effects of 0.3 to 1 °C and nighttime effects of 0 to 2 °C, depending on the tree form and the SVF. Additionally, the heat reduction potential (HRP) of trees ranged from +5% to –20%, with negative values indicating reduced heat and improved thermal comfort. Finally, in areas with lower SVF, tree HRP declined due to shading competition from nearby buildings, with effectiveness varying by species.
[38]	Taipei City, Taiwan	Street canyon: H/W = 1, with buildings on both sides standing 40 m tall. The sidewalks on both sides of the street are each 2 m wide. Orientations: NS and EW. Vegetation: Different LAI trees were considered	Outdoor thermal comfort (PET) and microclimate. Simulations with ENVI-met	Increasing LAI reduces Tmrt and PET (stronger in E–W than N–S); central planting has minor effect; high LAI improves comfort and can provide insulation in cold, windy conditions.
[39]	Guangzhou, China	Street canyons with H/W values equal to 1, 2, and 3 were studied. The primary vegetation factors considered were tree crown coverage and planting density ($\rho$).	Thermal comfort simulations and measurements (PET) were carried out	High tree density (ρ = 1) with large crowns reduces daytime PET (up to 4 °C); lower density increases PET. Effects vary by crown size and AR; narrow canyons show lower PET due to stronger wall shading.
[40]	Hong Kong	Street canyon orientation included NW–SE and EW. To investigate tree species selection under the 30% GCR condition, nine scenarios were evaluated. Eight of those scenarios focused on a single tree species, chosen from the eight most common species in Hong Kong. The final scenario incorporated a mixture of tree species to examine their collective impact on thermal comfort and energy efficiency. Finally, an SVF-based tree selection scenario used SVF to guide tree species selection for urban planting.	Thermal comfort (PET) and energy efficiency were simulated with ENVI-met	Vegetation provides vertical and horizontal cooling (up to 20 m), with total reductions of 5–11 °C depending on GCR; LAI is the dominant factor, and suitable tree type depends on SVF (sparse in low SVF, dense in high SVF).

presents a selection of the most representative case studies, illustrating the key characteristics and synergistic interactions between urban geometry parameters (AR, orientation, and SVF) and vegetation in improving the microclimate and enhancing thermal comfort. The Table is organized thematically to highlight studies that investigate the combined effects of urban geometry and vegetation, enabling comparative analysis of their synergistic impact on outdoor thermal comfort and microclimatic conditions.

Based on the main findings of the case studies shown in Table 4, a number of synergistic effects are identified, including how AR can act synergistically with vegetation and urban canyons with higher aspect ratios (H/W equal to 2) can achieve a balance between sufficient shading and ventilation through the strategic use of moderately dense tree cover. This configuration minimizes overheating while maintaining adequate air circulation, critical for pedestrian comfort. As for thermal comfort in narrow canyons (H/W less than 1.0), dense vegetation, such as trees with high LAI and wide crowns, provides effective cooling through shading and evapotranspiration. However, in deeper canyons, excessive shading may obstruct airflow, necessitating careful vegetation placement to optimize both cooling and ventilation. As AR increase though, the resultant wind turbulence and reduced light availability create less favorable conditions for vegetation growth, particularly for species requiring higher light exposure. This necessitates selecting resilient plant species capable of thriving in low-light, high-turbulence environments.

Regarding the synergy of orientation and vegetation, streets aligned along the E–W axis, which experience prolonged solar exposure, benefit significantly from dense-canopied trees planted strategically along the southern side. This configuration achieves PET reductions comparable to fully shaded streets, demonstrating the critical role of vegetation in mitigating thermal stress. To enhancing comfort in N–S streets which inherently experience lower thermal stress, planting trees with higher LAI can enhance shading and reduce heat stress. This strategy proves particularly effective in areas with intense solar exposure during midday.

On the synergy of SVF and vegetation, in deep canyons with low sky view factors (SVF), trees with tall trunks and sparse lower foliage effectively balance moderate shading with sufficient ventilation. This approach maintains air circulation while offering relief from direct unlight. To maximize shading in shallow canyons with high SVF, dense tree foliage, which significantly reduces PET and air temperatures, offers benefits. By obstructing direct shortwave radiation and minimizing reflected and emitted heat, vegetation in these settings provides comprehensive thermal comfort.

4.3. An Evidence-Based Framework for Supporting Urban Canyon Design

The synthesis of the reviewed literature enables the formulation of an evidence-based framework for supporting urban canyon design through the combined consideration of geometric parameters and green infrastructure. Unlike isolated analyses of individual variables, this framework integrates AR, street orientation, and SVF with vegetation characteristics to support informed and context-specific decision-making. The framework is structured as a conceptual decision-support scheme that organizes the relationships between urban form, vegetation, and thermal comfort into clearly defined components and interactions.

The proposed framework is organized into three interconnected layers, each representing a key component of the urban thermal system. The first layer concerns urban geometry, where AR, orientation, and SVF define the baseline microclimatic conditions by controlling solar access, radiative exchange, and airflow patterns. The second layer involves vegetation characteristics, including parameters such as LAI, LAD, canopy structure, and spatial configuration, which regulate shading, evapotranspiration, and local wind modification. The third layer corresponds to thermal comfort outcomes, expressed through indices such as PET, UTCI, and MRT.

The interaction between these layers determines the overall thermal performance of the urban canyon. Based on the reviewed evidence, the framework identifies key design pathways. In high AR canyons, moderate vegetation density is required to balance shading and ventilation, whereas in shallow canyons, dense vegetation with high LAI is more effective in reducing thermal stress. Street orientation further modulates these effects, with E–W canyons requiring enhanced shading strategies (e.g., dense tree canopies), while N–S canyons benefit from more flexible vegetation configurations due to inherently improved shading patterns. Similarly, SVF conditions dictate vegetation design, where low SVF environments require careful vegetation placement to avoid airflow obstruction, while high SVF environments benefit from increased canopy density to reduce radiative loads.

The framework can be interpreted as a decision-support tool that links urban morphology with vegetation strategies under different climatic conditions. Rather than prescribing universal solutions, it emphasizes context-dependent design, where the appropriate combination of geometry and greening is selected based on local climatic constraints, urban density, and design priorities.

The application of the framework follows sequential logic. First, the baseline microclimatic conditions are defined by the geometric configuration (AR, orientation, and SVF). Second, vegetation strategies are selected and adapted based on these geometric constraints and local climatic conditions. Third, the resulting thermal performance is evaluated using appropriate thermal comfort indices (e.g., PET, UTCI, MRT). This stepwise structure enables the systematic interpretation of the interactions between design variables and thermal outcomes.

This structured and systematic approach provides a clear bridge between theoretical understanding and practical urban design, enabling planners and engineers to systematically evaluate trade-offs between shading, ventilation, and radiative exchange in order to enhance outdoor thermal comfort.

4.4. Challenges and Critical Aspects of Using Green Infrastructure

GI is increasingly recognized for its potential to mitigate UHI, improve air quality, enhance thermal comfort, reduce urban noise, and bolster urban resilience. However, its integration into urban canyons—a setting defined by restrictive geometries, dense land use, and entrenched infrastructure—presents significant challenges. These environments require innovative, resource-efficient, and context-specific solutions to ensure the feasibility and sustainability of GI. Taking into account the interaction between urban canyon geometry and vegetation, a number of key challenges emerge.

Space limitations and density constraints represent one challenge. Urban canyons often lack sufficient space for traditional green solutions such as parks or expansive GRs, particularly in areas dominated by vertical development. Narrow streets and closely spaced buildings further limit light and airflow, critical for plant growth. While compact solutions like GRs, GWs, and modular planters offer space-efficient alternatives, their localized benefits are often less impactful than larger vegetated areas. Maximizing the ecological and social contributions of GI in such confined spaces requires integrating multifunctional systems, such as coupling vegetation with renewable energy or stormwater management, to align with sustainable urban living principles [32,157].

Water scarcity poses an additional critical challenge to GI, particularly in arid and semi-arid regions where irrigation is essential to sustain vegetation. Conventional water-intensive methods often undermine the ecological benefits of GI by exacerbating resource constraints. Innovative water conservation strategies—such as rainwater harvesting, dew and fog collection, graywater reuse, and condensate recovery from HVAC systems—are increasingly essential. Employing drought-resistant plant species and efficient irrigation systems can further enhance resilience, ensuring GI remains viable in water-limited environments while supporting long-term urban sustainability [32,158,159,160,161].

The high installation and maintenance costs associated with the installation and upkeep of GI often hinder its adoption in urban canyons. Implementing features such as GRs and permeable pavements demands specialized materials, structural adaptations, and skilled labor, resulting in substantial initial investments. Additionally, ongoing maintenance activities—irrigation, pruning, pest control, and system repairs—requires sustained the allocation of resources, raising concerns about long-term feasibility. Strategic planning, cost-effective designs, and durable materials are crucial to making GI financially viable and accessible in dense urban contexts [162,163,164].

Retrofitting GI into established urban infrastructure frameworks presents additional challenges. Urban canyons often lack the structural capacity to support additional loads or water retention systems, and integration may disrupt existing utilities such as drainage, electrical, and communication networks. Effective implementation demands adaptive engineering and flexible design systems, alongside strategic planning that ensures compatibility with existing infrastructure. Collaborative, interdisciplinary approaches are key to overcoming these obstacles, fostering resilient urban greening initiatives [32,165].

GI in urban canyons must achieve a delicate balance between aesthetic enhancement and functional performance. Features such as GRs and GWs can simultaneously improve visual appeal, regulate temperature, purify air, and manage stormwater. However, prioritizing aesthetics can sometimes compromise ecological efficiency or demand higher maintenance, straining sustainability. Designing adaptable systems that meet environmental standards while maintaining visual appeal requires input from landscape architects, engineers, and urban planners to ensure solutions are both practical and harmonious with urban aesthetics [32,164,166].

In light of these challenges, it becomes evident that the effective implementation of green infrastructure in urban canyons requires a context-sensitive and integrative approach, rather than generic solutions. Based on the reviewed literature, practical strategies should prioritize the alignment of vegetation characteristics (e.g., canopy density, leaf area index, and spatial configuration) with canyon geometry and local climatic conditions. In parallel, urban planning interventions—such as building setbacks, variation in building heights, and the introduction of ventilation corridors—can complement greening strategies by improving airflow and reducing heat accumulation. While the present study does not aim to prescribe site-specific design solutions, it provides a structured synthesis of key parameters and interactions that can support informed, climate-responsive decision-making in urban environments.

5. Conclusions

The present study provides a comprehensive synthesis of the combined effects of urban canyon geometry and green infrastructure on outdoor thermal comfort and urban microclimate. By systematically analyzing key geometric parameters—aspect ratio (AR), street orientation, and sky view factor (SVF)—alongside vegetation characteristics, the review advances current understanding from isolated parameter analysis toward an integrated, multi-factor perspective.

A primary contribution of this work lies in the formulation of an evidence-based framework that links urban geometry and vegetation characteristics to thermal comfort outcomes. Unlike previous studies that focus on individual parameters, this framework highlights the interdependent nature of shading, radiative exchange, and airflow processes, providing a structured basis for climate-responsive and context-sensitive urban design strategies.

The findings demonstrate that no single parameter can be considered independently. Instead, thermal performance emerges from the combined interaction of geometry, vegetation, and climatic conditions. For example, higher aspect ratios enhance shading but may limit ventilation, requiring careful integration with vegetation of appropriate density and canopy structure. Similarly, street orientation strongly influences solar exposure, with E–W canyons requiring targeted shading interventions, while N–S configurations generally provide more balanced thermal conditions. The role of SVF is shown to be dual, influencing both daytime heat gain and nocturnal cooling, thereby necessitating balanced design approaches rather than extreme configurations.

From a practical perspective, the review translates these insights into design-oriented considerations. Dense vegetation with high leaf area index is particularly effective in shallow canyons with high solar exposure, whereas in deeper canyons, vegetation strategies must prioritize maintaining airflow while providing localized shading. The selection of vegetation characteristics, canopy structure, and spatial configuration should therefore be directly linked to canyon geometry and climatic context. In addition, the thermophysical properties of urban materials—such as albedo, thermal conductivity, and heat capacity—further influence heat storage and release processes, reinforcing the need for integrated approaches that combine urban form, vegetation, and material selection.

Despite these advances, several research gaps remain. First, there is a need for more standardized methodologies to enable consistent comparison across different climatic conditions and modeling approaches. Second, the integration of dynamic and data-driven methods, including machine learning and real-time monitoring, remains limited and represents a promising direction for improving predictive capabilities. Third, further research is required on the long-term performance and maintenance of green infrastructure under varying environmental and socio-economic conditions. Finally, the interaction between thermal comfort, air quality, and energy performance should be explored within unified and multi-scale frameworks.

Overall, this study highlights the importance of moving beyond fragmented analyses toward integrated, multi-parameter approaches. By bridging urban geometry, vegetation, material properties, and climate within a coherent framework, it provides a robust foundation for enhancing thermal comfort and supporting sustainable urban design in diverse environmental contexts. The proposed framework offers a structured basis for interpreting complex interactions and can support more informed, context-sensitive decision-making in future urban planning and design practices.