Evaluating UHI Mitigation and Outdoor Comfort in a Heritage Context: A Microclimate Simulation Study of Florence’s Historic Center

Abstract

This paper evaluates Urban Heat Island (UHI) mitigation strategies in Florence’s historical centre, characterized by relevant cultural heritage value and significant tourist fluxes but increasingly susceptible to heatwaves. The research work focused on the evaluation of both current microclimate conditions and mitigation solutions for UHI-related issues, using ENVI-met microclimate modelling software as a simulation tool. Different models, featuring a 2 m grid resolution and detailed material properties, were produced to assess outdoor air temperature (Ta), mean radiant temperature (MRT), and Universal Thermal Climate Index (UTCI), chosen as reference parameters for human thermal sensation. Diversified conditions induced by the peculiarities of the urban layout were highlighted, with current Ta up to 32 °C and MRT exceeding 55 °C in paved open areas. Site-specific measures and their expected effectiveness were hence analyzed. De-paving and greening yield modest local cooling (Ta reduction up to −0.25 °C, MRT up to −1.75 °C), while tree installation ensures that MRT decreases by −7.50 °C to −12.00 °C. Most effectively, suspended shading fabrics preventing direct radiation can act on Ta (−0.09 °C to −0.25 °C) and provide substantial MRT reductions (−7.50 °C to −17.00 °C), significantly improving thermal comfort. The findings emphasize the potentialities of site-specific, reversible interventions in historic centres to combine climate adaptation and heritage preservation.

1. Introduction

The Urban Heat Island (UHI) effect poses significant environmental and public health challenges in cities worldwide, leading to elevated temperatures and diminished thermal comfort. These threats are moreover emphasized by ongoing climate change [1]: the most recent heatwaves that struck Europe led to extreme consequences for human health, with heat-related deaths tripling during June 2025 [2]. Moreover, as reported in the literature, UHI also causes significant issues in terms of increased energy consumption following consistent rises in cooling loads to be satisfied [3].

A growing interest in the topic was registered. At the institutional level, the wellbeing and sustainability of urban settlements was targeted by the United Nations among the Sustainable Development Goals (SDGs), while the European Union developed a dedicated initiative to promote knowledge and innovative actions [4,5].

At the same time, a growing body of scientific research is being produced in these fields and research efforts identify crucial parameters influencing urban microclimate. According to the available studies, such as the ones of Sharston et al. and Sharifi et al., both building-related factors (e.g., Floor Area Ratio, Building Density and Height) and urban shape can significantly affect the summer outdoor air temperature in cities and the resilience of built settlements, and their relationship with currently used comfort-evaluation systems needs to be investigated [6,7]. Following this trend, several authors tried to define optimized configurations to enhance liveability during urban planning design stages [8]. For instance, Reitberger et al. applied multi-objective optimization to mainly investigate the relationship between outdoor thermal comfort in urban spaces and Global Warming Potential, discovering that trees and PV are key design interventions in urban environments [9].

Research in recent years has largely benefited from the development of refined modelling tools and approaches [10,11] as well as from satellite-based data [12]. Multi-disciplinary satellite-based analyses have been performed, for example, by Moreno et al. for Seville and Almeria historic centres, highlighting the insurgence of UHI hazards in districts with poor vegetation coverage [13]. By analyzing Land Surface Temperature (LST) inputs, Guerri et al. pointed out the urban area of Florence as particularly prone to experiencing UHI-related issues, identifying diurnal summer hotspots both in peripherical and central neighbourhoods [14]. This aspect is particularly relevant for cities located in Mediterranean regions, as demonstrated in studies addressing Spanish historical centres such as Malaga [15] and Valencia [16].

Parallel to studies focusing on the assessment of local discomfort conditions, several contributions dealing with the evaluation of the effectiveness of various mitigation strategies to be applied in urban spaces can be retrieved in the literature. Green infrastructures are widely recognized for their mitigation benefits at both urban and building level through evapotranspiration, thermal mass and shading effect. For instance, Cuce et al., following an extensive study, emphasized that they can reduce the surface temperature up to 2 °C within the urban environment, improving the outdoor thermal comfort [17].

A reduction of 2 °C in LST was proven by Sanchez et. in case of green roof installation in Granada [18]. Aligned with the previously cited contributions, Lopes et al. demonstrated the effectiveness of greening solutions in mitigating UHI and at the same time ameliorating outdoor air quality through onsite monitoring in Porto [19]. Moreover, the role of urban green infrastructures has been evaluated in the literature, specifically targeting outdoor pedestrian comfort in different climate contexts [20].

Concurrently, material-based interventions, including the use of high-albedo (“cool”) pavements and the installation of shading structures, have proven effective in reducing surface temperatures and improving thermal comfort [21]. Considering the latter, Gai et al. analyzed the positive effect of various urban shades (trees, buildings, shades) on the value of Physiological Equivalent Temperature (PET) [22].

UHI and urban resilience are nowadays relevant topics at a worldwide scale, with studies targeting regions characterized by different urban configurations, socio-demographic contexts, and climate conditions, such as Teheran [23], Osaka [24] or Turin, where Ellena et al. propose a methodological approach to derive UHI-related thematic risk maps [25].

However, the proposed solutions in the current literature often fail to account for the unique constraints of historical urban environments. While a growing body of literature addresses UHI mitigation, a significant gap remains in developing and validating strategies suitable for heritage-sensitive, UNESCO-protected urban cores, and similar studies are not available for the Florentine context. UHI-related issues are in fact particularly relevant for heritage and monumental historical contexts, which need to balance the needs of cultural preservation and improving resilience to climate change. Such environments impose strict limitations that render conventional solutions like extensive green roofs or intervening in buildings’ finishing materials not feasible. Within this perspective, this contribution aims to evaluate the current comfort conditions experienced by pedestrians in the historical centre of Florence, performing dedicated environmental simulations using the ENVI-met software. The core novelty of this research lies in its focused application of microclimate modelling to evaluate context-specific, context-sensitive, and reversible UHI mitigation strategies within a globally significant, historically preserved, and pedestrian-dense urban environment.

In a city centre that functions as an open-air museum, the research directly addresses the public health risk to a highly transient and vulnerable population (tourists, who may not be acclimatized) during peak heatwaves and tourism seasons.

Hence, by focusing on the historic centre of Florence, this study aims at providing a quantitative assessment of deployable, mobile and reversible solutions aimed at enhancing pedestrian thermal comfort without compromising the site’s unique identity.

2. Methods

The overall research workflow is illustrated in this section and graphically synthesized in Figure 1. Each procedural step will be detailed to introduce the software used, the settings chosen during the modelling phase and the different assumptions made.

A portion of the historical centre of Florence was selected as reference case study to investigate the comfort conditions for pedestrians under reference summer conditions. Given the exceptional historical and artistic value of the area, more details about the urban layout and characteristics are reported in Section 2.1, Case Study Area, as they represent crucial design factors when defining mitigation strategies.

Data useful for the investigation of the area were retrieved by accessing free open data repositories provided by the local regional institution: GIS-based databases and aerial orthophoto views were collected to be used as solid knowledge base through the Geoscopio portal of Tuscany Region [26].

2.1. Case Study Area

Florence has a warm to hot climate, with humid summers and mild winters and with rainfall occurring year-round, classified as Cfa according to Koppen classification and as Climate zone D following the Italian regulation. Detailed average climate data are reported in

Table 1. Climate data for Florence. HDD means Heating Degree Days, Gh stands for Global Horizontal Radiation, Dh means Diffuse Radiation, Bn means Direct Normal radiation, Ta stands for air temperature, Td stands for Dewpoint Temperature and Ws means wind speed.

Latitude	Longitude	HDD [K/d]	Gh [kWh/m2a]	Dh [kWh/m2a]	Bn [kWh/m2a]	Ta [°C]	Td [°C]	Ws [m/s]
43.34° N	11.52° E	2041	1447	629	1496	15	7.9	2.8

.

During summer periods, the average maximum and minimum temperatures registered are about 30 °C and 18 °C, respectively, with peak values on the hottest days reaching up to 34 °C. In particular, July represents a critical month for summer heatwaves, with 23 days with temperatures above 28 °C and being affected by a lower amount of cumulated rainfall (<25 mm).

The case study urban area covers a surface of approximately 0.9 km² in the heart of the historic centre of Florence and it is currently an almost entirely pedestrian zone. A brief description of the key features of the area will now be provided, as they represent key factors influencing the development of potential mitigation strategies.

This portion of the city centre, highlighted in Figure 2, is one of the oldest areas of urban settlement in Florence and it is included in the area under UNESCO heritage recognition [27]. It is delineated by Via Calimala to the west, Via del Proconsolo to the east, Via del Corso to the north, and Piazza della Signoria, with the adjacent Via Vacchereccia and Via della Ninna, to the south. The case study district still preserves its medieval urban layout, characterized by narrow, uneven streets that only approximately follow an orthogonal grid. Two main squares are located within the district: Piazza della Signoria and Piazza San Firenze. Historically, these squares have served as the heart of the city’s political life, housing the most relevant administrative institutions [28], and remain today a vibrant, bustling hub that serves as both a meeting point and a compelling open-air museum of Renaissance art. The analyzed part of the city also encompasses Via Calzaiuoli, the main pedestrian street in central Florence, serving as the vital artery connecting the two most important squares: Piazza del Duomo and Piazza della Signoria. This street is a bustling commercial thoroughfare, constantly filled with a steady flow of locals and tourists, lined with numerous shops, and offering direct views of major landmarks, serving as a dynamic main axis within the city centre.

This district was selected for its particular significance, given its historical and artistic relevance and the extremely high pedestrian traffic it experiences. Moreover, the unique combination of local climate conditions during summer months, the dense medieval urban layout and the prevalence of heat-absorbing historical building materials makes this area suitable for the purposes of the study. Furthermore, the presence of key arteries and central squares makes UHI mitigation here crucial for public health, comfort, and the sustained vitality of a world-renowned cultural heritage site.

Studying this historically significant and densely populated area can offer invaluable insights into how UHI mitigation measures can be adapted and implemented in existing, architecturally sensitive urban landscapes, providing transferable knowledge for similar heritage cities worldwide.

2.2. ENVI-Met Modelling

The subsequent phase involved creating an urban environment model using ENVI-met [29]. ENVI-met is a microclimate modelling software that simulates complex multi-factorial interactions within urban environments, considering factors like building characteristics, vegetation, soil, and materials. Due to its detailed analysis capabilities for outdoor thermal comfort, air quality, and the impact of urban design strategies, it has become a widely adopted computational tool for scientific research in these fields in recent years [30,31]. For the specific application presented in this paper, the core study area, measuring approximately 310 m × 340 m, was recreated in the ENVI-met model. It was established with dimensions of 196 grid cells in the x-direction and 215 in the y-direction, extending vertically to 24 grid cells. Given the inherent complexity of the building morphologies, their representation within the model needed a volumetric simplification that was achieved by discretizing their footprints onto the modelling grid. To retain sufficient detail for the microclimatic simulation, a fine horizontal grid resolution of 2 m × 2 m was employed. An initial vertical resolution of 1 m was adopted even if, to optimize computational resources while maintaining accuracy for atmospheric processes higher above the ground, a telescopic factor of 20% was applied, increasing the cells’ height progressively starting from 5 m level. Buffer zones, proportional to the maximum building height within the model, were left at the domain boundaries, meant for minimizing edge effects. The geometries of buildings are complex and intricate.

Material properties for building envelopes and pavements were defined using ENVI-met’s Database Manager. Given the architectural heterogeneity of the historic centre, two primary vertical wall stratigraphy were selected to represent the dominant solutions for facades based on the findings of literature review [32,33]:

• A cavity wall construction (traditionally termed ‘muratura a sacco’) featuring an outer layer of exposed Pietraforte sandstone;
• A similar ‘muratura a sacco’ constructed still with Pietraforte sandstone blocks but finished with an external plaster layer.

This wall construction system was frequently employed in historic Italian buildings and defensive structures [34]. It characteristically includes two parallel masonry wythes (stone or brick) acting as formwork, with the intervening cavity filled with a mixture of rubble, construction debris, and mortar, lending significant thermal mass and robustness.

On the other hand, for roofing, uniform construction was attributed to all buildings, comprising a timber structure and decking, covered with clay tiles. For the outdoor pavements of roads and open spaces, two distinct surface types were included in the model, stone paving and bituminous asphalt, limited in the sections regularly open to car traffic. The outer buffer areas were left empty, and the default loamy soil was assigned. The selection of this specific wall and roof stratigraphy was informed by the available information, on-site visual surveys and the scientific literature on the theme [35]. The detailed thermal characteristics and input parameters for the building materials and components in the stratigraphy were derived from existing research studies, inventories and reglementary references [36,37,38]. Further developments of the study could involve in-field data acquisition to better characterize the built surfaces. The settings adopted in the ENVI-Met model are provided in

Table 2. Façade stratigraphy: masonry cavity wall.

Material	Thick. [m]	Solar Absorpt.	Solar Reflect.	Emissivity	Specific Heat [J/kgK]	Thermal Conduct. [W/mK]	Density [kg/m3]
Pietraforte stone	0.20	0.60	0.40	0.90	880	3.00	2700
Inert infill	0.40	0.70	0.30	0.90	880	2.30	2400
Pietraforte stone	0.20	0.60	0.40	0.90	880	3.00	2700

,

Table 3. Façade stratigraphy: masonry cavity wall with plastered external finishing.

Material	Thick. [m]	Solar Absorpt.	Solar Reflect.	Emissivity	Specific Heat [J/kgK]	Thermal Conduct. [W/mK]	Density [kg/m3]
Mortar–calcium plaster	0.04	0.40	0.60	0.60	1000	0.9	1800
Pietraforte stone	0.20	0.60	0.40	0.90	880	3.00	2700
Inert infill	0.40	0.70	0.30	0.90	880	2.30	2400
Pietraforte stone	0.20	0.60	0.40	0.90	880	3.00	2700

,

Table 4. Roofing stratigraphy.

Material	Thick. [m]	Solar Absorpt.	Solar Reflect.	Emissivity	Specific Heat [J/kgK]	Thermal Conduct. [W/mK]	Density [kg/m3]
Terracotta roof tiles	0.01	0.70	0.30	0.90	840	1	2000
Pianelle tiles	0.03	0.60	0.40	0.90	800	1	2000

and

Table 5. Outdoor paving materials.

Material	Volumetric Thermal Capacity [J/m3K]	Thermal Conductivity [W/mK]	Albedo
Stone paving	2.35	3.00	0.30
Asphalt	2.25	0.70	0.12

.

Input meteorological data for the environmental and microclimate simulations were sourced from the Peretola airport weather station, situated 6 km from the area of interest. These data, downloaded from EnergyPlus databases, were then input into the ENVI-met model. Simulations were performed for 21 June, 21 July and 21 September, covering the 4 h interval from 11:00 to 15:00; 21 July was selected to analyze performance during the period of the highest historical temperatures in Florence; 21 June, the summer solstice, was selected to model the period of maximum solar radiation, while 21 September was chosen to represent the shoulder season, characterized by less intense solar radiation and more moderate ambient temperatures.

The microclimate conditions within the area of interest were evaluated through dedicated simulations, which addressed both key microclimate parameters (outdoor air temperature, mean radiant temperature (MRT)) and recognized human thermal comfort indicators (Universal Thermal Climate Index (UTCI)). In particular, the UTCI serves as an advanced thermo-physiological index, representing the thermal stress experienced by the human body by calculating an equivalent temperature based on the combined influence of meteorological variables (air temperature, mean radiant temperature, humidity, and wind velocity).

A preliminary numerical simulation was undertaken to assess the current state of outdoor thermal comfort within the historic downtown district. This initial investigation served as a propaedeutic analysis, designed to identify key thermal vulnerabilities and critical areas influenced by local microclimate and existing built environment characteristics. The insights gained were crucial for informing the subsequent evaluation of potential UHI mitigation strategies, with the goal of ameliorating summer outdoor conditions and reducing heat stress.

2.3. Mitigation Strategies

The monumental nature of the historic city centre and its profound historical–artistic significance placed considerable constraints on the choice of UHI mitigation strategies, needing to balance the technical effectiveness against their compatibility with the area’s cultural heritage. Therefore, the primary aim was to avoid significative alterations of the integrity and authenticity of the built environment. Following these inputs, the selection process prioritized interventions aimed at minimizing their visual impact on wall surfaces and ensuring their proper and respectful integration with the existing architectures. Due to their invasive nature, extensive green roofs or vegetated walls, while beneficial for energy efficiency and surface temperature reduction, were not considered for potential implementation measures.

The chosen strategies, specifically targeted at critical areas identified by preceding microclimate assessments, included

• Selective greening of pedestrian areas and squares;
• Implementation of trees, carefully planned in their layout arrangement;
• Installation of removable shading fabrics canopies over streets;

Specifically, both trees and shading fabrics were analyzed in view of their combination with the installation of green flowerbeds. Shading fabrics were considered to be coupled with actions on paving surface, making it impossible to deploy overhanging shaders in the heritage context of Piazza della Signoria due to both spatial and visual constraints.

Details about each solution will be briefly provided to introduce the simulation settings used and the rationale behind them.

As for the flowerbeds, their implementation was foreseen in the northern sectors of Piazza della Signoria, with a layout designed to preserve access to the streets on that side of the square. Similarly, de-paving interventions and substitution with greening were considered for San Firenze Square, right behind the former. Here, two different green portions were assumed, to both preserve road viability and ensure adequate distance from cultural and monumental heritage buildings. The parameters used for environmental simulations are reported in

Table 6. Greening flowerbed settings in the ENVI-met model.

Greening	Albedo	Emissivity	Plant Height [m]	Roots Depth [m]	Leaf Area Density (LAD) [m2/m3]
Grass	2.35	0.95	0.10	0.20	0.30

.

Bitter orange (Citrus × aurantium), an evergreen species belonging to the Rutaceae family, was instead considered for tree implementation as a heat mitigation strategy. This species, a hybrid of pomelo and mandarin originating from Southeast Asia, is well-suited for urban environments, characterized by a medium-sized vegetated crown, thriving in temperate climates and proving drought tolerance. Citrus × aurantium offers significant thermal advantages due to its high Leaf Area Density (LAD), which leads to greater solar radiation absorption and promotes evapotranspiration. These processes contribute to reducing ambient temperatures and improving thermal comfort in surrounding areas. Beyond its microclimatic benefits, including providing shade, the tree’s ornamental appearance and fragrant flowers can enhance urban aesthetics. Starting from 2024, the same species has been installed in Via Cavour in Florence within the context of a broader urban redevelopment intervention and greening effort [39]. Also to adhere to this strategic choice, bitter orange trees were selected for the purposes of the study discussed here. In this case, Via Calimala and Via dei Calzaiuoli were addressed as suitable contexts for the deployment of plants, as well as the greening sectors previously introduced in Piazza della Signoria and Piazza San Firenze. Here, the effectiveness of combined measures was hence evaluated. As for the plants’ arrangement, these were introduced in the model with about 8 m spacing between specimens to promote correct growth and optimal distribution of branches. Notably, to ensure the complete reversibility of this solution, trees could be directly rooted in the ground or installed in movable vases to allow, for instance, for their removal during the winter season or in case of events, maintenance work or other needs. The ENVI-met settings used are reported in

Table 7. Vegetation species settings in ENVI-met model.

Species	ENVI-Met Model Geometry	Leaf Albedo at Low-Length Waves	Leaf Transmissivity at Low-Length Waves	Leaf Weight [g/m2]	LAD [m2/m3]
Citrus × aurantium		0.95	0.20	0.20	1

.

In addition, an extra simulation has been performed to include the possibility of installing a public ornamental fountain in the renovated Piazza San Firenze, to promote both aesthetic quality and environmental conditions for pedestrians. In this case, a water spray height of 2.5 m and a flux of water 10 g/s were modelled.

High-albedo shading fabrics installed over the main roads were also modelled to assess their effectiveness. These fabrics, made from various materials like permeable textiles or special anti-UV nets, are designed to reduce direct solar radiation exposure by reflecting a significant portion of incident radiation, thereby lowering the absorption and transmission of heat to the surfaces below, such as streets and sidewalks. This mechanism helps to decrease surface temperatures and consequently lowers ambient temperatures, creating cooler and more comfortable urban spaces for pedestrians. To allow sufficient natural light for visual comfort, both seasonal variability in design and the use of photochromatic materials that adapt to changing light conditions could be explored. While effective, implementing shading fabrics in historical cities like Florence requires careful consideration to preserve the urban landscape’s aesthetic and historical character, necessitating their strategic placement to avoid visual obstruction of monumental buildings and to focus on areas most susceptible to overheating, to reduce heat stress without compromising the visual harmony of the historic district. The proposed strategy for the historical centre of Florence includes installing these high-albedo overhead shade sails along Via Calimala, Via dei Calzaiuoli, Via del Corso, and Via del Proconsolo at a height of about 20 m, hung with tensioned metal wire systems placed below water drainage gutters. These roads were selected in view of their broader section, currently preventing them from enjoying the beneficial effects of the shade cast by the flanking buildings, and the higher number of pedestrians usually walking there. The shading fabrics have been introduced into the ENVI-met model coupled with grass flowerbeds in the main squares of the district, assuming PVC sails with high reflectivity (0.7).

In Figure 3, the mitigation scenarios evaluated are synthesized and illustrated with reference to the real aerial views to locate the interventions and to the ENVI-met models.

3. Results

As previously mentioned, the simulations carried out were first meant to assess the current state conditions and comfort levels registered in the area of interest, and later on to evaluate the effectiveness of the mitigation interventions proposed. The same approach will be adopted also by presenting the results obtained. All of the numerical findings illustrated in this section were derived considering 21 July at 1 p.m. and at 1.5 m above ground level.

In Figure 4, the current values of outdoor air temperature and MRT are illustrated. As shown, the potential air temperature distribution is relatively uniform across the study area, with a total maximum temperature differential of approximately 1.5 °C. The lowest air temperatures (below 30.60 °C) are concentrated in the narrow street canyons and enclosed courtyards, likely due to the combined effects of shading and potentially trapped, cooler air. In contrast, the open areas of Piazza della Signoria and Piazza San Firenze to the south and west exhibit the highest air temperatures, reaching up to about 32 °C.

On the other hand, the distribution of MRT displays extreme spatial variability, with values ranging from below 29.00 °C to above 55.00 °C. This distribution is strongly influenced by the presence of direct solar radiation or shaded areas. The latter, particularly adjacent to building facades, register the lowest MRT values, comparable to or even lower than the external air temperature. Conversely, surfaces directly exposed to the sun, such as the centre of streets and open plazas, experience exceptionally high radiant temperatures, also due to the paving materials used.

This trend highlights that MRT can be a crucial indicator of the instantaneous thermal stress induced by solar exposure in a complex urban geometry, as it is directly influenced by the percentage of built areas and the thermal and visual properties of the materials used. This becomes more evident when analyzing the results of the UTCI simulations, reported in Figure 5.

To translate the environmental parameters into a direct measure of human physiological experience, UTCI was calculated for all pedestrian-accessible areas. The resulting UTCI maps visualize a thermally heterogeneous environment, with a perceived temperatures varying from less than 30.50 °C in the most shaded locations to over 39.50 °C in the sun-exposed ones. This spatial distribution shows a significative correlation with the MRT trend, highlighting how the radiant heat load can be considered in this case as the driving factor in influencing human thermal sensation. Moreover, such a heterogeneous scenario can induce drastic changes in the perceived heat that can further alter the physiological stress of the pedestrians involved.

These UTCI values were contextualized using standard heat stress classifications (Figure 2b), according to the recognized assessment scale. The majority of the open public space, including the main squares and the wider streets, imposes strong to very strong heat stress for pedestrians. These areas, especially the ones characterized by the highest severity level, must be hence considered as thermally hazardous zones during peak sun hours, potentially inducing significant heat strains and posing considerable health risks.

On the other hand, the network of shaded spaces in the narrow medieval streets created by urban morphology represents a zone of relative thermal safety and comfort, being characterized by only moderate heat stress, with UTCI ranging approximately from 28.3 °C to 31.2 °C. However, the existence of these safer areas is exclusively dependent on building orientation and massing. This preliminary analysis assesses the need for mitigation initiatives to reduce the urban heat stress registered at the pedestrian level. As suggested by the findings obtained, interventions were defined to ensure, more than a broad-scale air temperature reduction, a strategic design of shaded areas and the amelioration of radiative properties of paved surfaces.

In the following, the impact of the mitigation solutions considered is evaluated with reference to both air temperature and MRT.

Figure 6 offers a comprehensive analysis of the efficacy of the UHI amelioration solutions. The spatial representations clearly delineate the localized and broader cooling effects of the proposed interventions. The introduction of “Flowerbeds and greening” (top row in Figure 6a,b) starts a modest, albeit localized, cooling effect, with some areas experiencing reductions of up to −0.25 °C for air temperature and up to −1.75 °C for MRT.

The “Flowerbeds + Trees installation” scenario (middle row in Figure 6a,b) demonstrates a more pronounced and spatially distributed cooling benefit. For air temperature (Figure 6a), a significant portion of the study area shows reductions ranging from −0.09 to −0.25 °C, with some specific segments exceeding −0.25 °C. The impact on MRT (Figure 6b) is even more striking, with widespread areas experiencing substantial cooling benefits, primarily due to the direct shading provided by tree canopies. Localized reductions in MRT mainly fall within the −7.50 to −12.00 °C range, with some zones indicating drops greater than −12.00 °C, underscoring the effectiveness of trees in reducing direct solar load on surfaces and people.

However, the “Flowerbeds + Shading fabrics” strategy (bottom row in Figure 6a,b) emerges as particularly suitable in mitigating both outdoor air temperature and MRT. The introduction of suspended fabrics induces temperature drops at least comparable to those achieved by tree installation, consistently showing cooling within the −0.09 to −0.25 °C range. Crucially, the MRT reductions under shaders are extremely high, with areas experiencing cooling of −7.50 °C to over −17.00 °C (Figure 6b), highlighting their superior ability to intercept solar radiation before it reaches the urban surfaces and occupants.

The same outputs were moreover investigated from a quantitative point of view through dedicated statistical analysis of air temperature presented in Figure 7. Compared to the “Current conditions” (black dashed line), all mitigation strategies effectively shift the temperature distribution towards lower values. The “Flowerbeds” and “Flowerbeds + Trees installation” strategies visibly reduce the peak occurrence of higher temperatures and increase the frequency of cooler points. For instance, “Flowerbeds + Trees installation” reduces the share of points in the 31.50–31.75 °C interval from 30% to 15% and increases the share in the 31.25–31.50 °C range from 19% to 35%. However, these greening strategies are characterized by similar effectiveness in terms of average air temperature.

Remarkably, the “Flowerbeds + Shading fabrics” strategy demonstrates the most significant shift towards cooler temperatures, almost entirely eliminating temperatures above 31.75 °C and substantially reducing occurrences in the 31.50–31.75 °C interval, limited to just 5%. This strategy concentrates the highest proportion of points (43%) within the 31.25–31.50 °C temperature range, suggesting its superior capability in flattening extreme temperature peaks and fostering a more consistently comfortable thermal environment across the entire study area. This detailed quantitative evidence reinforces that while urban greening is beneficial, the strategic deployment of high-albedo shading fabrics offers a particularly effective and scalable solution for mitigating urban heat, especially in dense urban fabrics where tree planting might be constrained.

Following the same approach adopted for the analysis of the baseline scenario, the UTCI reduction achieved through the various Urban Heat Island mitigation strategies was also investigated, demonstrating both spatial impacts and quantitative shifts in thermal comfort. Figure 8 provides a graphical visualization of these results. Spatially, the “Flowerbeds + Shading Fabrics” scenario stands out as the most effective, demonstrating extensive UTCI reductions across the delineated area of interest, with reductions extending below −3.20 °C in numerous locations, indicating a broad cooling effect especially across streets. “Flowerbeds + Trees installation” also yields significant localized improvements, particularly evident in the concentrated areas of tree planting, demonstrating the efficacy of vegetation in ameliorating microclimate conditions. In contrast, the “Flowerbeds and greening” strategy, while contributing positively, shows more modest and less pervasive reductions, primarily confined to the vegetated portions.

Quantitatively, the UTCI distribution plots in Figure 9 provide crucial insights into the overall impact of each strategy on thermal comfort across the study area. The implementation of “Flowerbeds” alone results in a minor yet noticeable shift in this distribution towards slightly lower UTCI values with respect to the current state levels, indicating a modest alleviation of thermal stress. Installing trees offers a more distinct improvement, successfully shifting a portion of the distribution away from the highest stress categories, leading to a reduction in the prevalence of “strong” and “very strong” heat stress conditions. However, this type of analysis also indicates overhead shading fabric deployment as the most beneficial measure. This intervention significantly shifts the entire UTCI distribution towards lower temperatures, significantly increasing the prevalence of conditions falling into the moderate thermal stress category, effectively mitigating the most severe heat impacts. This comprehensive analysis unequivocally highlights the superior effectiveness of high-albedo shading fabrics in enhancing urban thermal comfort across a broad range of conditions, making them a powerful tool for climate adaptation in urban environments, especially when combined with other greening initiatives. However, a localized increase in air temperature conditions in the adjacent streets was registered. Large-scale shading structures can sometimes alter local air circulation patterns, potentially redirecting or trapping warmer air in adjacent unshaded zones, contributing to localized temperature increases outside the directly mitigated areas. Despite their proven effectiveness, the application of high-albedo shading fabrics in monumental historical centres necessitates meticulous planning to preserve the aesthetic and historical integrity of the urban landscape. Installations must carefully avoid compromising the visual perception of historic buildings, open spaces, and unique streetscapes, often leading to their strategic deployment only in the most susceptible areas to overheating.

For completeness, the analysis was moreover refined to encompass June and September conditions. The results obtained in these cases, reported in Figure 10, allow us to assess extremely different scenarios. As demonstrated by making reference to the UTCI mapping, on 21 September the heat stress registered is, on average, below the threshold of moderate levels, with the only exceptions being localized in the Piazza della Signoria perimeter. These results allow us to foresee the opportunity to remove the mitigation solutions for this month, as their presence would ensure only marginal gains, considering also that the average air temperature is in this case about 24 °C. On the other hand, the June scenario exhibits the same heat stress trend highlighted in the case of July simulations: once again the open paved squares emerge as particularly critical (Ta ~31 °C, UTCI above 35 °C), while the narrow medieval streets act as climate shelters.

As these conditions suggest the opportunity of deploying mitigation strategies starting from the very beginning of Summer, the effectiveness of the installation of fabric shaders, which proved to be the most promising, was also tested in this case. The results of the simulations carried out are reported in Figure 11, illustrating both the spatial distribution of UTCI reduction and the overall trends in the area of interest. The outputs obtained demonstrate that shaders are even more effective under the June climate conditions, achieving a maximum reduction in UTCI values of about 5 °C and sensibly ameliorating the thermal stress conditions in the main pedestrian axes. Here, the heat stress is reduced to be only moderate instead of strong or very strong, with a generalized reduction in the share of the latter by 5% over the whole district.

4. Discussion

The results obtained from the microclimatic modelling of a portion of Florence’s historic centre confirm that the UHI effect constitutes a major issue for the liveability of public spaces, especially during summer months. Simulations performed with ENVI-met show that open areas, devoid of shading and paved with high-thermal-mass materials such as stone and asphalt, can reach MRT exceeding 55 °C. These conditions translate into UTCI values that indicate strong to very strong thermal stress.

The findings are aligned with the ones of previous on-site measurement campaigns focusing on Florence’s downtown, with an in-field mobile sensing campaign detecting hyperlocal air temperature variations up to 3.3 °C at midday in summer, comparable with the trends emerging from the environmental simulations carried out [40].

The results of simulations are qualitatively comparable with the analysis of historical densely built urban districts carried out by other authors in the literature, such as the one of Rolim et al. in Valencia [16] that performed an LST-based analysis of urban climate conditions. Compared to the other districts, in that case, alongside the compact urban fabric, the presence of heat-retaining materials, narrow streets, and limited vegetation leads to the intensification of the UHI effect in the historical city centre. Coming to the Italian context, Avola historic centre has been addressed by Evola et al. to be modelled within the ENVI-met environment [41]. Similar results were obtained in terms of outdoor air temperature (ranging between 31 °C and 32 °C) and in this case both localized solutions and city-scale urban planning strategies were also evaluated, leading to a preference for cool pavement implementation.

The amelioration strategies proposed in the study for the Florence city centre—green flowerbeds, mobile trees, and high-reflectance tensile shading structures—proved to be effective in significantly lowering both outdoor air temperature and UTCI. Among them, the tensile shading membranes installed over major pedestrian axes yielded the greatest benefits, with MRT reductions up to 17 °C and UTCI improvements exceeding 3.2 °C. This finding significantly expands upon the work of Cortiços et al., who also documented the high efficacy of tensile membranes in reducing thermal stress [21]. The greater magnitude of reduction in Florence’s case is likely attributable to the specific morphology of historic street canyons. In these narrow, high-mass environments, where surfaces are heated by both direct solar radiation and reflected/emitted longwave radiation from adjacent facades, the interception of solar radiation before it reaches the ground level becomes exceptionally impactful.

In contrast, the performance of tree planting, which reduced MRT by a significant but lower margin (7.5 °C to 12.0 °C), highlights a crucial nuance for heritage contexts. While vegetation remains essential for evaporative cooling and environmental quality, as highlighted by Gai et al. [22], in densely built historic centres with no sufficient space to install large canopy trees, the immediate thermal comfort benefit derived from shading fabrics can be superior. Moreover, Karimi et al., by analyzing a district of Madrid, stress that vegetation alone is not sufficient to face extreme heatwave scenarios, whose frequency is expected to increase in the future [42].

From the standpoint of urban morphology, the results of the environmental simulations underscore the morphological paradox of historic centres. The dense and compact medieval fabric of Florence is protective during peak sunlight hours, with narrow E-W-oriented streets characterized by reduced stress conditions. The effect of deep shading in street canyons providing thermal comfort has been emphasized also by Lan and Zhan [43] and Lai et al. [44]. On the other hand, it poses significant challenges to the implementation of large-scale interventions, making it necessary to adopt punctual and reversible solutions that respect the built heritage.

The analysis becomes even more critical when placed in the context of tourism. Florence is one of the most visited cities in Europe, with exceptionally high numbers of daily visitors, especially during the summer, which coincides with the hottest period of the year. Many thousands of people are estimated to traverse or linger in the case study area daily, with numbers potentially surging to tens of thousands during periods of high visitation, such as events, weekends, or peak holiday times. According to 2023 data, approximately 9 million overnight stays were registered in the Metropolitan City of Florence [45]. Given that the majority of visitors typically choose the summer months for their stay in the city, the daily number of people potentially exposed to UHI-induced effects can be exceptionally high. This overlap dramatically increases the number of people exposed to thermal stress in historical districts. As highlighted by León-Cruz et al., extreme weather events are emerging as serious threats to European tourism destinations [46]. Lopes and Nascimento further stress the lack of tailored climate adaptation strategies for tourist-oriented cities, despite the sector’s vulnerability [47]. Tourists—often including the elderly, children, and individuals with chronic conditions—may be particularly sensitive to heat-related health risks.

Additionally, these findings intersect with broader themes of climate justice and urban equity. As emphasized by Maccabiani et al., cities must assess the spatial distribution of “climate shelters” and ensure equitable access to shaded and thermally comfortable public areas [48]. Lopes et al. highlight that areas exposed to high UHI risk are also affected by socio-economic issues, stressing the need to integrate mitigation measures in the urban planning strategies to address at the same time environmental criticalities and atmospheric pollution [19]. In historical centres, space is often limited and heritage constraints are strict, preventing actions on buildings façades as proposed instead by Camporeale et al. for a peripherical district of Malaga with encouraging results [49]. Mitigation strategies must be both spatially distributed and visually discreet. In this regard, portable trees and tensile shades emerge as effective and reversible solutions that can be flexibly deployed without compromising the cultural landscape. Several cities, especially in the Mediterranean climate zone, have already implemented them in dense and historic neighbourhoods: evidence can be found in Figure 12. Bologna recently deployed in-vase trees in many squares of the centre to provide shadows and ameliorate UHI resilience [50].

Finally, the projected increase in the frequency and severity of future heatwaves calls for a proactive, multi-level adaptation strategy [42,51]. While the interventions tested in this study have shown localized effectiveness, they should be incorporated into broader climate action plans that should encompass the combination of different strategies and holistic evaluation indexes. The classification of morphological urban features into Local Climate Zones is an emerging and promising approach to identify the factors that could expose built areas to UHI effects [52], as adopted by Lehnert et al. at the European level [53]. Similarly, priority assessment tools, such as the Heat Adaptation Priority Index (HAPI) developed by Mohammed [51], could represent an effective support tool in decision making to improve urban resilience, even if they should be calibrated on the peculiarities of historical settlements.

In this regard, Koutsanitis et al. adopt a similar approach demonstrating the effectiveness of acting on greenery, paving surface and building retrofit in an Athens historical district [54]. Perlaza et al., in the high-value heritage context of Matera, encourage the adoption of a holistic approach to urban planning, integrating increased vegetation with the management of impervious surfaces and land cover to optimize cooling effects [55].

However, this study presents some limitations induced by both the limited availability of input data and the computational efforts required. As previously specified, the models produced rely on assumptions or approximations of the material properties derived from the literature and volumetric simplification of complex historic buildings imposed by the limited graphical capabilities of the software. On-site investigation campaigns could lead to the obtainment of refined material data and micro-meteorological measurements, to be used as boundary conditions to enhance the model’s accuracy. Moreover, making reference to simulations focused on peak hours prevents us from assessing the cumulative effects of multi-day heatwaves and diurnal trends. At the same time, the reference comfort indexes adopted are based on standardized physiological models and do not account for the dynamic behaviour of pedestrians or the effects induced by gathered groups of individuals. A series of dedicated sensitivity analyses could better highlight the relevance of the different simulation parameters and their impacts on final results.

Following these considerations, the study should be deepened and extended to encompass different portions of the historical centre and to evaluate a combination of the various intervention measures proposed, to also account for the different degrees of integrability within the historical context.

5. Conclusions

This study quantitatively evaluated the effectiveness of non-invasive UHI mitigation strategies within the historical centre of Florence, a particularly vulnerable context given its high pedestrian traffic and monumental character. The microclimate simulations, carried out using ENVI-met software, highlighted that open, paved areas within the study district, such as Piazza della Signoria and Piazza San Firenze, are critical hotspots during the peak summer season. Here pedestrians experience significant thermal stress, with MRT often exceeding 55 °C and a UTCI indicating “very strong” heat stress (above 38 °C), posing a relevant risk to public health. To ameliorate this critical baseline, the proposed mitigation strategies yielded distinct, quantifiable improvements. The introduction of green flowerbeds can offer modest, localized cooling benefits, with MRT reductions limited to approximately −1.75 °C. The strategic implementation of trees, specifically bitter orange (Citrus × aurantium), proved more effective. This species, chosen for its high Leaf Area Density (LAD) and drought tolerance, locally reduced ambient temperatures and improved thermal comfort through direct shading and evapotranspiration. In the portions with trees, the induced shading reduces MRT by −7.5 °C to −12.0 °C and lowers the UTCI by up to 2.8 °C, demonstrating the efficacy of targeted greening. However, the most impactful intervention is the deployment of high-albedo shading fabrics over main pedestrian axes, which demonstrated superior efficacy in reducing both outdoor air temperature and MRT (up to −17 °C), leading to extensive reductions in UTCI values (−3.2 °C) across wider areas.

By focusing on enhancing pedestrian thermal comfort without compromising the site’s invaluable cultural identity, this study offers a practical blueprint for climate adaptation in historically significant and pedestrian-dense urban environments worldwide.

These findings provide critical, evidence-based insights for urban planners in heritage contexts, even if the exact quantitative results may not be directly applicable to other historic centres with different street canyon aspect ratios, orientations, or building materials. This research highlights the potential for designing and implementing effective UHI mitigation and climate adaptation strategies even in highly constrained historical contexts, emphasizing that microclimate-aware design must become an essential component in historical town management to promote climate equity and resilience for both residents and visitors. Policies should prioritize the strategic deployment of high-albedo shading solutions in the most sun-exposed, high-traffic public space: results statistically confirm that this approach provides a significantly greater and more widespread improvement in pedestrian thermal comfort than selective, ground-level greening alone.

This study still presents limitations connected with both methodological choices and data availability limitation, as previously introduced, and it is not meant as a preliminary design of a real intervention to be implemented but only as an assessment of the potentiality for UHI mitigation in the context of Florence’s historical centre. For this reason, the simulated interventions have been modelled in an idealized form, being aware of real-world limitations of implementing these solutions, such as installation and maintenance costs, public acceptance, aesthetic impact, and the legislative or administrative challenges. While this study focused on technical performance, future research must assess the socio-economic dimensions, including public perception and acceptance, cost–benefit analyses, and the development of governance models for implementing these projects in complex, multi-stakeholder heritage environments.