Urbanization and climate change are among the most defining issues of our time (Dreyfus, 2015). The Urban Heat Island (UHI) effect exacerbates the impact of rising temperatures in cities due to global warming ((Dogan et al., 2019); (Ramamurthy & Bou-Zeid, 2017)), which has led to growing interest in studies of outdoor thermal comfort. Over the past decade, this field has gained significant importance, becoming a key indicator of urban space quality and a crucial factor in designing climate-adapted and healthy urban environments ((Elnabawi & Hamza, 2020); (Lachir, 2022)). Poor outdoor thermal comfort can adversely affect human health and well-being while also reducing social and commercial outdoor activities. High temperatures and humidity can make working or spending time outdoors difficult (Smith et al., 2014). Ensuring appropriate thermal comfort can extend the duration of outdoor activities, enhance urban life and vitality, and decrease energy consumption in built environments (El-Bahrawy, 2023).

The planning and design of urban spaces has been shown to significantly impact outdoor thermal comfort ((Targhi & Dessel, 2015); (Yahia et al., 2018)). Effective strategies, including optimizing urban geometry, incorporating vegetation, using appropriate materials, and considering street orientation, can significantly enhance outdoor thermal comfort in urban areas. ((Bedra et al., 2023); (Hedayatnia et al., 2023); (Korkut & Rachid, 2024)) In particular, urban morphology, including the compactness and layout of buildings, as well as the presence of open spaces, has been shown to cause significant variations in microclimatic conditions, affecting thermal comfort differently across various urban settings. For instance, a study in Isfahan demonstrates that different block designs lead to varying microclimatic and thermal comfort outcomes (Sadeghian et al., 2024). Therefore, urban design must account for local microclimatic conditions to enhance thermal comfort, fostering more sustainable cities and improving urban quality of life.

Assessing thermal comfort in outdoor environments is highly complex due to the interplay of various environmental, personal, psychological, and cultural factors. Key environmental parameters, such as solar radiation, air temperature, wind speed, and relative humidity, significantly influence thermal comfort and are strongly altered by urban microclimate processes ((Shooshtarian & Ridley, 2016); (Zhang et al., 2022)). The heterogeneity of urban geometry and surface characteristics leads to high spatial variability in thermal comfort within urban areas, highlighting the need for micro-scale models focused on the neighborhood scale to accurately depict interactions between buildings and their surroundings. Several studies have developed and used micro-scale models for outdoor thermal comfort, aiming to provide insights for sustainable urban design and decision-making, emphasizing the importance of considering microclimate factors for improving outdoor thermal comfort ((Aghamolaei et al., 2023); (Jänicke et al., 2021); (Lam et al., 2021)). Available simulation tools include ENVI-met (Huttner & Bruse, 2009), RayMan (Matzarakis et al., 2007), and Ladybug (Sadeghipour Roudsari & Pak, 2013). These tools quantify outdoor environmental conditions by estimating thermal comfort indices. The most frequently used indices are the Physiologically Equivalent Temperature (PET) and the Universal Thermal Climate Index (UTCI). Both PET and UTCI were specifically developed for measuring thermal comfort in outdoor environments and have shown reliability and potential as useful tools in urban planning (Staiger et al., 2019).

In the Moroccan context, few studies have addressed outdoor thermal comfort. (Ouali et al., 2020) showed that urban design choices, such as the shape, size, and spacing of buildings, can significantly improve outdoor thermal comfort in moderate climates. A study in Fez, Morocco, found that urban geometry plays a crucial role, with deep street canyons offering better comfort in hot climates but necessitating wider streets or open spaces for solar access during colder seasons (Johansson, 2006). (Jihad & Tahiri, 2016) investigated the influence of the aspect ratio in Agadir, Errachidia, and Fez. Existing literature demonstrates the importance of outdoor thermal comfort in the Moroccan context and highlights the significance of urban design in enhancing this comfort. However, there is still a significant knowledge gap in this field, particularly regarding how different design options influence thermal comfort. Studies in various climatic regions demonstrate that effective urban design must be tailored to local conditions (Ruefenacht et al., 2022). This case study in Marrakech, Morocco, examines key design parameters related to urban forms that impact the microclimate and provides guidelines to assist architects and urban planners during the early design stages of new districts in a city where extreme temperatures are a significant concern.

This paper aims to contribute to existing knowledge by analyzing how various urban form features impact the urban microclimate and thermal comfort. We investigate three urban form features: building height, street aspect ratio, and street orientation, and quantify their effects on the urban heat island and thermal comfort in the climate context of Marrakech, Morocco. These urban form features are identified as key geometrical characteristics influencing outdoor microclimate conditions. Previous research indicates that their interactions with air temperature and thermal comfort can vary, with some outcomes being debated in the literature. For example, compact urban areas may reduce daytime temperatures and improve thermal comfort by providing shading and limiting direct solar exposure, which is beneficial during warmer periods. However, these same features can also retain heat at night, thereby intensifying the UHI effect and reducing nighttime cooling ((Cha & Oh, 2020); (Hedayatnia et al., 2023)). This paper provides valuable insights for urban planners aiming to enhance thermal comfort and mitigate urban heat through effective design strategies. To achieve this goal, we analyze various design scenarios of an urban canyon, focusing on key geometrical characteristics and using validated simulation models to estimate their impacts on the urban heat island effect and thermal comfort within the canyon.

The urban environment is widely recognized for its substantial impact on microclimatic conditions, particularly air temperature ((Lachir, 2022); (Lachir & Nia, 2023)). The results of this study reinforce this understanding, showing that the UHI effect significantly increases nighttime air temperatures while having a comparatively smaller, slightly cooling, effect during the daytime. This diurnal temperature pattern is attributed to the heat storage capacity of buildings and the shadowing effect from surrounding buildings, which reduces daytime solar gains. Conversely, the intensified nighttime UHI effect is primarily explained by the release of stored heat from buildings, along with the reduced sky view factor and lower wind speeds in urban canyons, which limit the radiative and convective cooling of the area (Svensson, 2004) (Yang et al., 2017).

Incorporating UHI effects into the estimation of the UTCI offers a more accurate and context-specific evaluation of outdoor thermal comfort. This study reveals that UHI exacerbates thermal discomfort for outdoor occupants, a finding consistent with other studies conducted in Rome (Battista et al., 2016) and Kuala Lumpur (Fong et al., 2023). Multiple studies suggest that effective mitigation strategies, such as increasing urban greenery, applying cool materials, and promoting urban designs that improve ventilation and shading, can reduce UHI intensity and enhance outdoor thermal comfort ((Evola et al., 2017);(Jia & Wang, 2022); (Morini et al., 2016); (Wang et al., 2016)).

In this study, we focused on three key geometrical features of urban form to assess their impact on both UHI intensity and outdoor thermal comfort. The analysis of urban canyon geometry reveals that wider canyons, with a higher sky view factor, can mitigate UHI intensity by facilitating more efficient radiative cooling at night. In contrast, deep urban canyons trap heat within the canopy, reducing convective cooling due to lower wind speeds in more compact areas (Kolokotsa et al., 2022) (Theeuwes et al., 2014). However, the impact of the street aspect ratio on UTCI indicates that compact urban canyons contribute to improved thermal comfort during the day, particularly in spring and summer, by reducing solar exposure and increasing shading (Salleh, 2007). Thermal comfort is also enhanced during colder nights in winter and spring, as the reduced sky view factor in deep canyons limits heat loss from the human body to the sky. Overall, in the context of Marrakech, a compact urban canyon design proves to be more advantageous for thermal comfort. The average UTCI throughout the day suggests enhanced comfort in winter and spring, with minimal impact during summer. A similar case study in Fez, Morocco, revealed that while deep canyons provide thermal comfort during the summer, shallow canyons are more comfortable during winter (Johansson, 2006).

Street orientation emerges as a significant factor influencing thermal comfort, primarily due to its effect on solar access, which is a critical determinant of outdoor thermal comfort. For instance, a study in Liverpool, Australia, highlighted street orientation as the most influential factor affecting outdoor thermal comfort (Abdollahzadeh & Biloria, 2021). In the current study, we found that northeast-southwest (NE-SW) orientations are particularly favorable for enhancing outdoor thermal comfort. However, optimal orientations are likely to vary depending on the climate context and their combination with the street aspect ratio (Abdelhafez et al., 2022) (Elkhayat et al., 2024).

Urbanization and climate change have amplified the Urban Heat Island effect, making outdoor thermal comfort a critical aspect of urban design. In Marrakech, Morocco, our study shows that pedestrians may experience slight to moderate cold stress during winter nights, while heat stress is prevalent at most other times, especially during summer and spring, where strong to extreme heat stress can occur even at air temperatures around 22°C due to significant solar exposure. Among the tested urban design features, street aspect ratios (H/W) have a greater impact on UHI compared to building heights and orientation, but strategies to reduce UHI don't always align with those that enhance thermal comfort. While compact urban canyons tend to increase UHI, they are effective in improving outdoor thermal comfort, particularly during winter cold nights and hot days. Conversely, open urban canyons offer better comfort during summer nights and cold winter days. Street orientation, particularly a northeast-southwest (NE-SW) alignment, proves most favorable for outdoor thermal comfort across all seasons, providing optimal solar exposure in winter and reduced exposure in summer. Ultimately, the best urban design strategies should be adapted to the intended use of outdoor spaces, while also considering other factors like the impact of building energy efficiency for a more sustainable urban environment.

The abstract of this paper was presented at the Urban Planning & Architectural Design for Sustainable Development (UPADSD) Conference – 9th Edition which was held on the 22^nd - 24^th of October 2024.