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

1. Introduction

As urban areas expand, a combination of urban geometry, dense construction materials, limited vegetation, anthropogenic heat emissions, atmospheric pollution, and modified surface and air dynamics patterns contribute to elevated temperatures relative to rural counterparts. This phenomenon, known as the Urban Heat Island (UHI) effect [1,2,3,4,5], has significant implications for thermal comfort, exacerbating discomfort and health risks, and increasing energy demands, especially during extreme heat events [6,7,8].

Thermal comfort is defined as “the condition of mind that expresses satisfaction with the thermal environment” [9] and it represents a subjective evaluation based on multiple environmental and personal factors. In outdoor spaces, thermal comfort is influenced by a wide spectrum of parameters, including air temperature, mean radiant temperature (MRT), wind speed, humidity, clothing insulation, and metabolic rate [10,11,12,13,14]. However, relying solely on quantitative measures is insufficient to comprehensively capture outdoor thermal comfort. While microclimatic variables profoundly influence thermal sensation, they fail to fully account for the divergence between objective measurements and subjective comfort perceptions. Increasingly, psychological adaptation—including factors such as perceived naturalness, individual expectations, prior experiences, duration of exposure, perceived control, and environmental stimulation—plays a critical role in shaping thermal comfort [15,16]. This complex balance between environmental factors and human comfort thresholds is essential in the design of urban spaces that prioritize public health and well-being. Understanding these dynamics is vital for formulating urban design strategies that mitigate heat stress and enhance the livability of outdoor spaces.

Urban morphology encompasses the spatial arrangement, height, and density of built structures and critically shapes local microclimatic conditions. Dense, compact urban configurations restrict airflow and hinder surface heat dissipation, thereby amplifying the UHI effect and diminishing the effectiveness of natural cooling processes such as wind-driven ventilation and convective heat loss. A prevalent feature in these compact environments is the urban canyon, defined by narrow streets bordered by tall building structures (Figure 1) [17,18,19,20]. The geometry of urban canyons is typically characterized by the height-to-width ratio (H/W), also known as the aspect ratio, (AR), which defines the relationship between the vertical height of surrounding buildings and the horizontal width of the street [18,20,21,22]. A higher aspect ratio signifies a deeper canyon with reduced sky exposure, limiting solar radiation penetration, and modifying wind dynamics. Additionally, geometric parameters such as street orientation and the sky view factor (SVF)—which measures the fraction of visible sky within the canyon—are critical for evaluating the potential for radiative cooling and solar heat gain. These factors play a decisive role in shaping local microclimatic conditions and directly influence thermal comfort in urban environments [19,20,22,23,24,25,26].

Urban greening, such as the strategic incorporation of trees, green roofs, and vertical gardens, offers a natural solution to mitigate the thermal challenges posed by urban canyon geometry [27,28,29,30]. Vegetation plays a pivotal role in moderating urban microclimates through shading, evapotranspiration, and influencing local wind dynamics. In deep urban canyons, where solar access is restricted, greenery improves thermal comfort by lowering air temperature and providing localized cooling through shade and enhanced ventilation. In canyons with higher SVF, vegetation helps mitigate heat accumulation by intercepting solar radiation and reducing the radiant heat load on surfaces. Thus, urban greening complements urban geometry by balancing microclimatic conditions and improving thermal comfort in densely built environments. The effectiveness of urban greening, however, depends on several factors such as tree species, canopy density, planting configuration, and the surrounding built environment [14,18,27,28,32,33].

The interaction between vegetation and urban canyon characteristics—such as AR, SVF, and ground surface properties—strongly influences thermal comfort and cooling potential. For example, in narrow canyons with low SVF, shading effects are amplified, whereas tree height and density can significantly modify airflow and thermal conditions at the pedestrian level. Given these complexities, a detailed understanding of how vegetation integrates with specific urban canyon geometries is essential for optimizing greening strategies. Recognizing this intricate relationship between urban geometry and green infrastructure (GI) is crucial for developing sustainable urban planning approaches. By carefully designing green elements to align with urban canyon characteristics, planners can improve thermal comfort, mitigate the urban heat island effect, and create more resilient, climate-adaptive urban spaces [33,34,35,36,37,38,39].

This study conducts an in-depth bibliographic review to achieve two primary objectives: (a) quantify the influence of urban canyon geometry and vegetation on thermal comfort and microclimate regulation in densely built environments, and (b) provide a framework for optimizing urban design and greening strategies through the integration of urban geometry and vegetation to enhance thermal conditions. To achieve its objectives, the review focuses on three critical dimensions. Firstly, it attempts to quantify the impact of urban canyon geometry on microclimate and outdoor thermal comfort. This involves analyzing the influence of critical geometric parameters—including AR (or H/W), street orientation, and SVF—on the local microclimate, solar radiation exposure, and thermal comfort at the pedestrian level. Secondly, it strives to evaluate and quantify the role of vegetation in the regulation of microclimate and thermal comfort. In this respect, the review examines the effectiveness of various urban greening strategies, such as street trees, green roofs (GRs), and vertical gardens, in enhancing thermal comfort and mitigating the UHI effect. Key vegetation attributes leaf area index (LAI), leaf area density (LAD), and planting configuration are analyzed to quantify their cooling contributions through shading, evapotranspiration, and wind modulation. Thirdly, the review investigates synergistic interactions between urban geometry and vegetation, exploring the interplay between urban canyon geometries and vegetation in optimizing thermal comfort. The review aims to establish an optimization framework for identifying the most effective urban design and greening strategies across different canyon typologies, balancing the need for high-density development with environmental sustainability and improved human comfort in outdoor spaces.

The key innovative aspects of this study can be summarized as follows. At first, the review provides a comprehensive synthesis of urban geometry and vegetation interactions. Geometric factors (such as AR, street orientation, and SVF) and vegetation characteristics (e.g., LAI, canopy density, and planting configurations) jointly offer a holistic view of how urban form and greening strategies interact to regulate microclimatic conditions. In addition, the review provides a quantification of combined effects on microclimate regulation and thermal comfort, with the quantification of how geometric parameters and vegetation jointly impact thermal comfort and cooling potential being a significant contribution. Finally, the review offers a framework for optimizing urban design and greening strategies, presenting an evidence-based framework for optimizing the integration of urban canyon geometries and vegetation to enhance thermal comfort.

Peer-reviewed articles and case studies were selected from reputable sources for this review based on a number of criteria. To cover urban canyon geometries, studies were selected for their detailed examination of how key geometric parameters—such as AR, street orientation, and SVF—influence microclimate regulation and thermal comfort in urban canyons. For the integration of urban geometry and vegetation, studies exploring the synergistic interactions between urban form and greening strategies were emphasized, particularly those offering frameworks for optimizing microclimate through integrated design approaches. Finally, to capture a broad range of climate and regional diversity, studies from various climatic regions (especially hot, dry, wet, and temperate environments) were included to assess the interplay between urban form and vegetation across different urban contexts.

2. 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, pollutants dispersion, and thermal comfort conditions in the urban environment [6,10,40]. 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 [18,19,41]. 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 [17]. 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 [24,42]. 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 [18,44,45,46,47,48,49].

$$Q^{*}+Q_F=Q_H+Q_E+{∆Q}_S+{∆Q}_A$$

(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, ${∆Q}_S$ is the heat stored in the urban structure, ${∆Q}_A$ 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 ${∆Q}_S$ and any possible horizontal advection ${∆Q}_A$. The advection term is often considered insignificant for systems within a homogeneous environment [46,50].

2.1. 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 [10]. The canyon width (W) is the horizontal distance between the facades of the buildings on either side of the street canyon [18], which includes the width of the street defining the canyon. These canyon characteristics are shown in Figure 3.

is the street orientation [54].

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) [10,18,19,20]. 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 [44,55,56].

Based on AR (or equivalently H/W) values, a canyon can be characterized as shallow, uniform or medium, and deep [6,57,58,59,60]. 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 [57,58,59,60]: 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 [59,61]. 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 [18,61,62,63]. 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 [19,46,64,65]. 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 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.

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. Optimal 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.

2.2. Orientation

Among the geometric parameters, the orientation of urban canyons—referring to the direction that streets and building facades face—has a significant impact on solar exposure, which in turn influences wind patterns and outdoor thermal comfort [96,97,98,99]. The orientation dictates the intensity and distribution of solar radiation that urban surfaces receive, thereby influencing temperature fluctuations, airflow patterns, shading, and the overall thermal environment within the canyon. This, in turn, affects the thermal balance and microclimatic conditions inside the urban canyon. All in all, the orientation of buildings and streets, plays a crucial role in shaping the microclimate of urban areas.

A thorough understanding of orientation effects is crucial for the design of urban spaces that optimize thermal comfort, especially in the context of global climate change and accelerating urbanization [6,76,100,101]. The importance of street orientation in evaluating outdoor thermal comfort is influenced by two primary factors. East-West (E-W) oriented streets are exposed to extended periods of sunlight, especially in the summer, resulting in elevated temperatures and urban heat island effect enhancement. Additionally, North-South (N-S) oriented streets align with dominant wind directions, enhancing airflow and aiding in the dissipation of heat accumulated throughout the day [88].

Streets with varying orientations experience distinct shading and solar exposure patterns throughout the day and across different seasons [94,102,103]. In the northern hemisphere, the sun’s trajectory results in N-S oriented streets being shaded during the morning and afternoon in the summer, while E-W oriented streets remain fully exposed to sunlight. At solar noon, N-S streets receive full sun exposure, whereas only a small portion of E-W streets is shaded. Streets with intermediate orientations, such as Northwest-Southeast (NW-SE) and Northeast-Southwest (NE-SW), consistently have shaded areas throughout the day. Generally, N-S and intermediate orientation streets benefit from increased shading during the summer months and reduced shading during the winter, in contrast to E-W streets, which experience minimal shading throughout the year [94].

Table 2 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.

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 optimal 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 optimal for taller buildings (above 10 stories) with wider streets (20 meters). Finally, the E-W axis requires robust shading solutions like trees or arcades to maintain comfort in public spaces.

2.3. Sky View Factor

The SVF, is a dimensionless parameter that quantifies the fraction of the visible sky hemisphere that can be seen from a specific point in an urban environment, particularly within an urban canyon. Mathematically, it is expressed as the ratio of the visible sky area to the total hemispherical area when viewed from the ground level and its values range from 0 to 1. An SVF value of 1 indicates an entirely open space with no obstructions while an SVF value of 0 indicates a fully obstructed view, where the sky is completely blocked by surrounding structures or vegetation [10,18,109,110].

SVF is a critical variable in urban climatology as well as in energy applications in the urban environment. SVF affects the amount of solar radiation reaching the ground and the ability of the surface to emit longwave to the sky, thus influencing energy balance, air flow patterns, the UHI effect and urban microclimate [19,23,52]. Moreover, SVF plays a crucial role in influencing human thermal comfort by modulating the balance of longwave and shortwave radiation within urban environments [52,111].

SVF presents significant impacts on the energy balance and thermal regulation, daylight availability, airflow patterns, and outdoor thermal comfort of the urban environment.

Regarding the energy balance and thermal regulation, SVF critically determines the thermal dynamics within urban canyons by controlling solar radiation reaching the ground and the efficiency of radiative cooling. In urban canyons with low SVF (characterized by narrow streets and dense building configurations), reduced daytime solar exposure is counteracted by limited longwave radiation escape at night, resulting in heat retention and elevated nocturnal temperatures, which amplify the UHI effect. Conversely, high SVF areas, with greater sky exposure, enable efficient nighttime heat loss, supporting cooling processes and mitigating UHI impacts [10,18,19,24,45].

Regarding daylight availability, SVF is a critical determinant of natural light penetration within urban canyons. Low SVF values (typically associated with high-rise buildings and narrow streets) obstructs sunlight, significantly reducing daylight availability. This limitation increases dependence on artificial lighting during daytime hours, driving up energy consumption. Furthermore, insufficient natural light adversely impacts circadian rhythms, psychological well-being, and the overall quality of life for urban residents. Optimizing SVF in urban planning is thus essential for enhancing daylight access, reducing energy demand, and improving urban livability [52,112].

Regarding air flow patterns, SVF significantly influences wind flow dynamics within urban canyons, where it interacts with the built environment to modify local wind speeds and patterns. In urban canyons characterized by a low SVF (indicative of narrow street canyons flanked by tall buildings), wind flow is often impeded due to the physical obstruction posed by these structures [18,19,113].

Finally, outdoor thermal comfort in urban environments is strongly influenced by the SVF, as it governs both the radiant temperature and mean air temperature within urban canyons. Areas with lower SVF typically experience elevated temperatures, particularly during the night, due to the reduced capacity for radiative cooling of surfaces. This thermal retention leads to a warmer microclimate, making SVF a critical parameter in the design of thermally comfortable urban spaces [18,19,20].

SVF is evaluated using two principal methodologies extensively detailed in the literature [48,52,114,115]: digital 3D modeling and fisheye lens imaging. Digital 3D modeling, which employs three-dimensional representations of the urban form to quantify structural parameters such as building geometry and street configurations, provided high accuracy for static urban features. However, this method fails to account for dynamic elements like vegetation, limiting its applicability for comprehensive analyses. Fisheye lens imaging captures hemispherical photographs encompassing both the built environment and surrounding vegetation. This approach is particularly advantageous for seasonal SVF analyses, as it accounts for temporal variations in vegetation cover and environmental conditions. By integrating these dynamic factors, fisheye lens imaging delivers a more holistic and temporally sensitive perspective, making it indispensable for studies where the interaction between vegetation and urban morphology significantly impacts microclimatic conditions.

Table 3 synthesizes the critical findings from case studies investigating the role of SVF in shaping outdoor thermal comfort and urban microclimates.

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. Optimizing 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.

3. 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 of heat accumulation in densely built environments [27,28,29,30,32]

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 [130,135,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155].

3.1. 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 [136,137] 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) [34,138]. 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 [34,139].

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

(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 [138].

3.2. Synergistic Effects of Urban Canyon Geometry and Green Infrastructure for Optimizing 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 [31,32,100,135,157].

Table 4 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 optimizing the microclimate and enhancing thermal comfort.

Based on the main findings of the case studies shown in Table 4, a number of synergistic effects are identified. On how AR can act synergistically with vegetation, 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.

3.3. 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 is one such 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 [31,159].

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 [31,160,161,162,163].

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 [164,165,166].

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 [31,167].

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 [31,166,168].

4. Conclusions

This review study has presented an evidence-based framework for optimizing urban design and greening strategies to enhance thermal comfort in urban canyons. By integrating urban geometry and vegetation, the proposed framework offers a scientifically grounded approach to mitigate heat stress, improve pedestrian comfort, and reduce energy demands.

Regarding the role of urban geometry, higher aspect ratios (H/W over 2), optimal street orientations (such as N-S), and adjusted SVF values are essential for balancing shading, ventilation, and microclimatic regulation. These parameters provide a foundation for geometry-aware urban design. There are synergies, such as the interplay between urban geometry and vegetation which amplifies cooling efficiency, with targeted strategies being tailored to specific street orientations, aspect ratios, and SVF conditions. The integration of higher ARs with moderate tree density balances shading and ventilation, while dense vegetation in narrow canyons maximizes cooling. Tailored solutions, such as dense-canopied trees for E-W streets and low-density foliage for deep canyons, exemplify the importance of context-specific design for thermal optimization.

The framework presented in this study bridges urban planning and environmental sustainability, offering actionable insights for creating resilient, thermally optimized urban spaces in diverse contexts. At the same time, it highlights the need for interdisciplinary approaches to integrate design, climate responsiveness, and ecological functionality in future urban developments.