Reflectance and Thermal Micrometeorological Characteristics of an Urban Green Space in the Mediterranean During July’s 2023 Heatwave

Abstract

The thermal and optical behavior of different elements in the urban environment is critical for urban climate regulation and planning. This study investigates the micrometeorological conditions prevailing in an urban green space (UGS) in Greece, during the heatwave of July 2023, addressing the effects of various surface materials on thermal dynamics and the urban heat island (UHI) phenomenon. The research is based on ground surface temperature and albedo measurements on different materials in the UGS, in the morning and at noon, showing great temperature differences between the different types of materials. The findings highlight the complex interaction between high-albedo surfaces and surface temperature values, suggesting that the proper selection of materials can highly affect the optical and thermal behavior of the urban environment. Artificial materials absorb more heat compared to natural vegetation, leading to high surface temperature values, reaching at noon, for example, 58.9 °C for asphalt. For the natural surfaces, dry bare soil presents similar thermal behavior (64.1 °C at noon), while green surfaces had much lower temperatures (e.g., 38.3 °C for grass). Thermal comfort indices revealed that July 2023 experienced extensive “very hot” conditions, imposing the urgent need for strategic urban planning to mitigate heat impacts. The study highlights that in order to create climate-resilient environments and improve thermal comfort, it is crucial to include suitable materials and a variety of vegetation in urban design. Such insights into the complex nature of urban microclimate indicates also the issue of the careful selection of materials and plant species in urban greening initiatives to help cities face the UHI phenomenon.

1. Introduction

Urbanization—and especially dense built environments—is the main cause forming the urban heat island (UHI) effect, mainly due to the use of artificial materials that seal soil and form complex surfaces with increased roughness and enhanced ability to store thermal energy. This may result in a rapid increase in urban temperatures and in less favorable thermal comfort conditions for the citizens, especially in the Mediterranean region, which is characterized by warm summers and frequent heatwaves and droughts [1,2]. Incorporating green elements into the urban design can increase the resilience of the urban landscape to climate change and extreme weather episodes [3,4] and regulate the urban climate [5], highly affecting the thermal and optical behavior of the urban environment and also providing benefits to the local communities and positive impacts on citizen welfare [6,7,8].

The use of materials with high albedo, i.e., the increased reflectivity of solar radiation, results in decreased radiation quantities that are accumulated and absorbed by the natural and artificial surfaces of the urban green area and large quantities that are reflected to the atmosphere. This is beneficial for the thermal behavior of the urban green space; however, it may have adverse effects on the local thermal comfort conditions since these reflected amounts of energy are partly absorbed by the surrounding to the urban green space buildings or adjacent surfaces, resulting in increased surface and air temperatures in densely built areas [9,10,11,12]. Taleghani [11] found that increasing surface albedo in an outdoor university garden raised surface temperature by 0.4 °C and mean radiant temperature by 1.2 °C with each 0.01 rise in albedo. In their study, air temperature spanned between 21.2 and 23.7 °C, with the higher values measured around asphalt covered sections of the garden.

In all cases, the addition of green elements in the urban environments instead of artificial ones benefits local environments resulting in decreased temperature values. Song and Park [13] demonstrated that in Changwon City, South Korea, regions with a high proportion of artificial surfaces benefited significantly from the addition of green spaces, resulting in substantial temperature drops. Conversely, areas with already substantial vegetation saw minimal additional benefits. Similarly, Taleghani [11] noted that the vegetation cover resulted in a reduction in the Physiological Equivalent Temperature (PET) within a university campus and reported higher PET values in the parking lot of the campus by 11 °C higher compared to grass-covered surfaces. Additionally, Salata et al. [9] analyzed the effects of vegetation and high-albedo materials on the urban microclimate in Rome, Italy. They revealed that vegetation showed remarkably improved thermal comfort during the hot summer periods, while high-albedo coating had increased air and radiant temperatures owing to small sky-view factors, which prevented reflected radiation from being returned to the atmosphere.

The internal features of the urban green spaces and their morphology are identified as a major issue for green infrastructures effectiveness as a means to control ecological crises [14], along with the UGS’s ecological connectivity [15], shape, and density [16]. The qualitative characteristics of the green elements highly affect the effectiveness of vegetation to regulate urban climates. The different vegetation types may perform differently, causing larger or smaller temperature drops in the cities. Also challenging is the proper combination of artificial and green elements to enhance the efficiency of the urban green space to mitigate the urban heat island effect [17]. Karimi et al. [17] investigated the thermal status of a medium-sized urban park and found that altering the type of vegetation significantly influenced local climatic conditions. According to their study, pine trees, for example, were found to have lowered air temperature by 0.3 °C on average, while plane trees caused a slight increase of 0.1 °C. Such differences highlight the importance of choosing specific plant species that can help in achieving better thermal comfort conditions. The integration of artificial surfaces with high albedo in conjunction with the proper placement of vegetation can improve urban thermal conditions. Karimi et al. [17] found that among other alternatives, combining high-albedo surfaces with pine vegetation produced the highest improvement in thermal comfort. This hybrid strategy combines the benefits of reflective surfaces and vegetation to further enhance urban resistance to heat stress. The reflective characteristics of artificial urban materials are largely composition- and age-based. According to Sweeney et al. [18], the albedo value for black paint is about 0.05, while for white acrylic paint, it may be as high as 0.80, whereas large differences were identified between old and new concrete. Taha [19] noted that urban areas with higher vegetation cover and water bodies tend to have higher albedo values and lower temperatures, regulating the UHI.

The artificial materials used in urban green spaces have a major impact on the surrounding air temperature. Concrete, paving, or even dry soil heat up quicker and retain heat compared to vegetation, and their ability to store heat results in higher surface temperatures, hence enhancing the intensity of the urban heat islands. In contrast, vegetation with grasses, trees, and shrubs dominates the features in areas that are relatively low in heat absorption and that act positively to potentially lower the ambient temperature through various means such as evapotranspiration and shading.

The aim of this work is to evaluate the performance of different urban materials, either artificial or natural, regarding their thermal and optical properties in an urban green space in Amaroussion (Central Greece) during extremely warm conditions in July 2023 [20]. The study is based on ground measurements taken during the peak of the heatwave and, to our knowledge, is among the few research studies assessing the impact of a strong heatwave on the optical and thermal behavior of urban materials. The results underline the positive effect of the green infrastructure on the urban climate and could be a useful tool for the local stakeholders and researchers in the environmental planning of cities and urban design to mitigate the UHI effect in the Mediterranean cities.

2. Materials and Methods

The study site is located in Amaroussion (Attica) Greece (38.04° N, 23.80° E, alt.: 190 m) and is an urban green area of 0.91 ha covered with trees, herbaceous plants, grasses, and bare soil. Artificial materials also occupy parts of the area including asphalt, pavements, paved corridors, and concrete. The map of the urban green area, along with the depiction of different materials, is presented in Figure 1.

The broader area of the site is characterized by a semi-arid climate [21] according to UNEP’s [22] aridity classification system, presenting also a significant trend to more arid conditions during the last decades [23]. According to Tsiros et al. [24], the broader area receives an annual precipitation of 430 mm but has a quite high evapotranspiration water demand (891 mm), resulting in increased summer water deficits (460 mm) and almost zero water surpluses in winter, whereas the climatic value of the aridity index is 0.73, corresponding to a semi-arid climate. A detailed description of the site is also provided in Proutsos et al. [25,26].

The present study was carried out in a single-day campaign in 26 July 2023 when one of the most strong and lengthy heatwaves ever recorded hit Europe [27,28], introducing a new record of extreme heat stress days across Europe, according to the United Nations [20]. In general, 2023 was identified as one of the warmest years ever recorded according to WMO, increasing the heat-related mortality by 30% in the last 20 years in most of the European regions [20]. In our site, 2023 was a year with low annual precipitation (406 mm) compared to the above-mentioned climatic average, whereas its annual air temperature was 18.9 °C. On a seasonal basis, notable was the decreased summer precipitation (40.8 mm), which was negligible in July (0.6 mm) and absolute zero in August, whereas the average air temperature during July was at its maximum, reaching on average 30.1 °C.

In situ, direct measurements at 185 points from representative parts of the urban area were taken twice during the campaign date: in the morning and during the noon. The set of measurements included surface temperature, reflectance (albedo) of global solar radiation, and air temperature and relative humidity. Surface temperature was measured using an MI-210 infrared radiometer (Apogee Electronics, Santa Monica, CA, USA) placed 40 cm above the surface, facing downward. Reflectance was determined with a portable LP 471 RAD radiation sensor (Delta OHM, Caselle di Selvazzano, Italy) with cosine correction, which measured incoming and reflected solar radiation flux densities in the spectral range of 400 to 1050 nm. Air temperature and relative humidity were recorded using a portable HD 2301.0 handheld thermo-hygrometer equipped with a Pt100 humidity/temperature sensor.

The first set of radiation and surface temperature measurements was collected in the morning in a two-hour time window from 08:30 to 10:30 a.m. This time slot is characterized by relative low radiation fluxes and temperatures and was selected to avoid inconsistences in the radiation measurements introduced at low sun inclination angles [29,30]. The second test of measurements was collected in the noon, from 13:00 to 15:00 p.m., and was characterized by the maximum values of air temperature and solar radiation. All measurements were collected by portable devices facing north and scanning the area from east to west and recording radiation fluxes and surface temperatures above the different materials. By that way, the measurements of a specific material were taken multiple times within the two-hour window.

Data were analyzed in association with the recordings of a continuously monitoring station installed in September 2019. The station was located above a wild-grass-covered surface in the center of the urban green space and was equipped with high-accuracy sensors. These sensors measured global solar radiation (SP-Lite, ADCON Telemetry, Klosterneuburg, Austria, with a sensitivity change of 2% per year), air temperature, relative humidity (E08, E+E, Engerwitzdorf, Austria), wind speed and direction (Small Wind Transmitter, THIES CLIMA, Göttingen, Germany), soil heat flux (HFP01-05, HUKSFLUX, Delft, the Netherlands), soil temperature and moisture (5TM, Campbell Scientific, Garbutt QLD, Australia), precipitation (PROFESSIONAL, PRONAMIC, Skjern, Denmark), and photosynthetically active radiation (QSO-S, APOGEE, Santa Monica, CA, USA, with ±5% accuracy). Data were recorded in 10 min timesteps and transmitted in real time using a telemetry unit (A753 addWAVE GSM/GPRS, ADCON Telemetry, Klosterneuburg, Austria).

For the purpose of the present work, spatial patterns of the urban green area surface temperatures and albedo were produced by employing the Surfer^® ver. 13 software [31], using the Point Kriging geostatistical gridding method, which is widely used to generate reliable gridded values across a spatial domain [32], and producing accurate grid values for each irregularly spaced point dataset at all points across a well-defined spatial domain by using weighted averages of all known values around each grid point [32].

Thermal bioclimatic indices were also employed in this work. Specifically, the Predicted Mean Vote (PMV, [33]) and the Physiological Equivalent Temperature (PET, [34]) were estimated using data from the meteorological station, following the standard guidelines and classifications of thermal comfort groups described in Matzarakis and Mayer [35], and by employing the RayMan Pro © software [36,37]. Both indices quantify and classify the persistent meteorological conditions in terms of human thermal comfort, using hourly meteorological data for their estimation. The indices’ values quantify the thermal sensation of an average human with specific characteristics (age, height, weight, clothing, etc.) under the micrometeorological conditions of the urban green site. In the present work, the default values for an average human from Central Europe were employed as proposed by Matzarakis and Mayer [35]. More specifically, the person is an average male, with an age of 35 years old, height 1.75 m and weight 75.0 kg, standing in the urban green area, with internal heat production due to activity of 80 W and a clothing index of 0.9 clo (0: no clothing, 1: winter clothing).

The thermal images, presented in Appendix A for this work, depict the surface temperature ranges of the different types of materials and were photographed by a HIKMICRO handheld thermography camera (model: HM-TP42-3AQF/W-Pocket2, Hangzhou Microimage Software Co., Ltd., Hangzhou, China, https://www.hikmicrotech.com/en/, accessed on 18 January 2025).

3. Results

3.1. Thermal Comfort and Prevailing Meteorological Conditions

In the study site of Amaroussion, the prevailing micrometeorological conditions have been continuously monitored since September 2019. In general, 2023 was the warmer year in the recording period (2020–2023), presenting an average annual temperature of 19.0 °C, whereas in all other years, it has not exceeded 18.4 °C. The monthly values of the Predicted Mean Vote (PMV) and the Physiological Equivalent Temperature (PET), which are two widely used thermal bioclimatic indices to evaluate human comfort, are presented in

Table 1. Monthly average values of the Predicted Mean Vote (PMV) in the study site of Amaroussion. The cells of the table are shadowed according to the classes described in the legend.

Month	2019	2020	2021	2022	2023		PMV
1		−2.9	−2.4	−3.0	−2.2		+3 hot
2		−2.3	−2.2	−2.3	−2.6		+2 warm
3		−1.8	−2.1	−2.8	−1.6		+1 slightly warm
4		−1.3	−1.1	−0.8	−1.0		0 neutral
5		0.4	0.6	0.3	−0.2		−1 slightly cool
6		1.1	1.3	1.4	1.1		−2 cool
7		1.7	2.0	1.7	2.3		−3 cold
8		1.7	2.0	1.6	1.9
9	0.7	1.0	0.6	0.8	0.9
10	0.0	0.0	−0.9	−0.4	0.2
11	−0.8	−1.7	−1.5	−1.3	−0.7
12	−2.2	−2.0	−2.5	−1.7	−1.6

and

Table 2. Monthly average values of the Physiological Equivalent Temperature (PET) in the study site of Amaroussion. The cells of the table are shadowed according to the classes described in the legend.

Month	2019	2020	2021	2022	2023		PET (°C)	Thermal Perception	Grade of Physical Stress
1		5.3	7.7	4.6	8.4		>41	Very hot	Extreme heat stress
2		8.3	8.9	8.0	7.0		35–41	Hot	Strong heat stress
3		11.2	9.6	6.3	12.2		29–35	Warm	Moderate heat stress
4		14.5	15.2	16.7	15.3		23–29	Slightly warm	Slight heat stress
5		23.4	24.2	23.0	19.5		18–23	Comfortable	No thermal stress
6		27.2	28.1	28.6	26.7		13–18	Slightly cool	Slight cold stress
7		30.5	32.1	30.1	33.7		8–13	Cool	Moderate cold stress
8		30.1	31.7	29.4	30.8		4–8	Cold	Strong cold stress
9	24.3	26.3	24.1	25.0	25.2		≤4	Very cold	Extreme cold stress
10	20.6	20.6	15.6	18.3	21.8
11	15.5	11.2	12.6	13.3	16.4
12	8.3	9.5	7.2	10.9	11.5

.

On an annual basis, PMV showed an average value of −0.45, ranging between years from −0.27 in 2023 to −0.53 in 2022, with intermediate values in the rest of the studied years, i.e., −0.49 in 2020 and −0.50 in 2021. The lower value in 2023 indicates cooler thermal sense compared to all previous years. However, PMV presents high seasonal variability, with, overall, negative values in winter (−2.3), spring (−0.8), and autumn (−0.3), indicating generally cool conditions, and positive values in summer (+1.7, i.e., warmer conditions), which is typical for the Mediterranean climate. PMV showed highest values in the summers of 2021 and 2023 (+1.8) and lower in 2020 (+1.5) and in 2022 (+1.6).

The above pattern is also in line with the PET values for the period 2020–2023, presenting an average annual value of 18.4 °C, ranging by year from 17.9 °C in 2022 to 19.3 °C in 2023. On a seasonal basis, PET showed, in general, lower average value in winter (8.3 °C) and highest values (29.9 °C) in summer, with intermediate values during the transitional seasons of spring and autumn (16.7 °C and 19.1 °C, respectively). Based on the results of PET, which defines nine thermal comfort categories, the number of hours with specific conditions per year and hour of day was also assessed. The number of the annual “very hot” hours showed a steady decreasing trend from 613 and 665 in 2020 and 2021 to 555 and 587 in 2022 and 2023, respectively. Overall, the “warm”, “hot”, and “very hot” conditions are recorded during the midday; however, in the years 2021 and 2023, high numbers of hours with “slightly warm” conditions were also recorded during nighttime hours compared to all other years.

The annual frequency distribution of the number of hours according to the human comfort level for 2023 is depicted in Figure 2, along with the respective distribution for July 2023. Based on the results of all years (2020–2023), about 50% of the annual hours in Amaroussion were characterized by cool (“very cold”, “cold”, “cool”, and “slightly cool”) conditions, whereas 37% were warm (either “slightly warm”, “warm”, “hot”, or “very hot”) and 13% were “comfortable”. This general pattern presents year-to-year variations. For example, the “very cold” hours represent 7% of the total annual but range between 10% (in 2020 and 2021) to 12% (in 2022 and 2023). Similarly, the “very hot” hours were about 6% of the total annual but varied from 6% in 2022 to 8% in 2021.

Table 1 and Table 2 and Figure 2 confirm that July 2023 was a very warm July with average PMV and PET values of 2.3 and 33.7 °C, respectively. The diurnal variations presented in Figure 2 also confirm that 2023 was a warm year and only about 15% of its hours categorized as “comfortable”, whereas 31% were warm (either “slightly warm”, “warm”, “hot”, or “very hot”). These percentages differed in July 2023, when 23% of the month’s total hours were “comfortable”, and 75% were “warm” with a high percentage (34%) of “very hot” hours with PET greater than 41 °C.

The diurnal changes of the site’s main meteorological attributes during the campaign date, alongside the average weather conditions of July 2023 and the hourly July averages for the years 2020–2023, are shown in Figure 3.

The campaign date (26 July 2023) was characterized by extreme heat conditions for the region. The last rain event (0.6 mm) was recorded 23 days before, whereas the average daily air temperature was higher than 30 °C for the previous 14 consecutive days. During this period, the maximum air temperature ranged from 34.4 °C to 42.5 °C, with the highest value to be recorded during the campaign day and also at 23 July 2023. In addition, during the campaign date, the soil temperature was high (39.1 °C daily average) and the relative humidity (39.3%), soil moisture (7.4% p.v.), and windspeed (1.03 m/s) were decreased. In the morning (08:30–10:30 local time) of the specific day, the average air temperature reached 33.4 °C, ranging from 31.6 to 35.9 °C with calm SW winds (windspeeds ranging from 0.0 to 0.7 m/s) and relative humidity (37.1%). At noon (13:00 to 15:00 local time), the average air temperature increased to 40.6 °C, presenting a range between 39.2 and 42.5 °C, with its maximum value recorded at 14:40 local time. During noon, the winds remained calm and SW, with windspeeds ranging from 1 to 2.1 m/s, slightly higher compared to the morning. Relative humidity was very low, presenting an average value of 20.3%, though being even lower (16.1%) at 14:40 local time. The average soil temperature, solar radiation flux density, and soil heat flux were maximized (47.4 °C, 840 W m^–2, and 92.5 W m^–2, respectively).

3.2. Solar Radiation Reflectance

The reflectance of the natural and artificial surfaces to global solar radiation (albedo) was measured above the different cover types inside and around the urban green space, and the results are presented in Figure 4 and

Table 3. Surface temperature (Tc) and differences between Tc and air temperature (Tair) measured in the morning (08:30–10:30) and at noon (13:00–15:00) of the campaign date on different types of materials in the urban green space of Amaroussion under shade and unshaded conditions. The respective albedo values at noon are also presented.

Type of Material	Morning (08:30–10:30)				Noon (13:00–15:00)				Albedo ρ
	Unshaded		Shaded		Unshaded		Shaded
	Tc (°C)	Τc − Tair (°C)	Tc (°C)	Τc − Tair (°C)	Τc (°C)	Τc − Tair (°C)	Τc (°C)	Τc − Tair (°C)
1. Asphalt	43.0	9.0	33.9	−1.0	58.9	18.7	39.2	−2.6	0.195
2. Paved road	38.3	4.0	31.2	−2.8	52.2	11.5	36.8	−3.8	0.207
3. Old concrete	37.7	3.8	32.8	−1.5	58.2	17.9	42.2	0.3	0.283
4. Paved corridors	36.0	2.0	29.4	−4.1	51.5	10.6	37.8	−2.9	0.234
5. Pavement	39.8	5.8	31.7	−2.7	53.1	12.4	38.5	−2.4	0.240
6. Dry soil	44.5	10.2	36.0	2.2	64.1	23.9	42.6	0.9	0.148
7. Shrubs	35.2	1.4			42.5	2.1			0.154
8. Wild grass	33.8	−0.6			40.8	0.5			0.146
9. Grass	30.7	−3.4			38.3	−2.8			0.156
Cars	50.8	16.8			60.6	19.9			0.177

. Higher albedo values indicate a higher reflectance of radiation and thus a lower absorbance of energy by the surfaces. The absorbed solar energy in the cases of the artificial materials or the soil is generally used to heat the surfaces, highly contributing thus to the urban heat island effect.

The results in our case study show the overall higher reflectance of the artificial materials at noon ranging from 0.195 on asphalt to 0.283 on old concrete, with intermediate values for the rest of the examined materials (0.207 on paved road, 0.234 on the internal paved corridors, and 0.240 on the external pavements around the green space). On the contrary, the green elements of the park showed decreased albedo between 0.146 on the wild grass and 0.156 on the lawn-covered surfaces, with quite similar values among the other natural materials (0.154 on shrubs and 0.148 on dry bare soil).

Notable are the differences in albedo between the different species, which is highly associated mainly by the color and the architecture of the plant leaves and branches. For example, the dark-colored Berberis thunbergii presented lower albedo (0.119), compared to the light-green-leaf-colored Artemisia absinthium (0.155), suggesting diverse properties of the selected urban green elements, regarding their ability to absorb and reflect solar energy. Accordingly, the relatively higher and not dense Vitex agnus-castus shrubs had an even higher albedo (0.160), and this can be partly attributed to the canopy architecture, which for higher plants with sparser foliage favors solar light absorbance against reflectance. On the other hand, the herbaceous Solanum nigrum L., which is much shorter than the Vitex agnus-castus, presented even higher albedo (0.211) attributed to its light-green-colored leaves and dense low height foliage that forms a relatively uniform and less rough surface, which favors radiation reflectance. In addition, the albedo of other shrub species was 0.122 for Lavandula angustifolia and 0.154 for Rosa sp.

The above results indicate that the selection of the appropriate plant species to be established in an urban green space along with their planting design should incorporate the knowledge of the species’ individual optical properties to mitigate the UHI effect since these properties are associated with the thermal behavior of the green area.

It is worth mentioning that the optical properties of the surroundings to the green space element are also critical for the effectiveness of urban vegetation to mitigate UHI impacts. The buildings’ optical behavior and specifically low albedo values can enhance absorbance and trap solar radiation, resulting in temperature increases. In addition, in our work, we found that parked cars around the urban green area are extreme heat sources attributed to their high ability to absorb solar radiation. The optical properties of these metallic elements are mainly associated with their color, resulting in either very high (e.g., 0.490 for white-colored cars) to extremely low (e.g., 0.138 for dark grey, 0.123 for dark red, and 0.07 for black cars) albedo values. The presence of cars around the green space highly influences the amount of radiation reflected back to the green space from the cars, altering the energy budget.

3.3. Surface Temperature

The above-described optical behavior of the green space highly impacts the distribution of energy between the different elements inside the green space, which affects also the thermal regime in and around the small park. The measured surface temperatures of the materials inside the urban green area along with the respective differences with the air temperature during the morning and the noon of the last and warmer day of July’s heatwave in 2023 are shown in Figure 5 and Figure 6.

The average surface temperatures (Tc) present high variation between the different types of materials both in the morning (08:30–10:30) and at noon (13:00–15:00). More specifically, during morning hours, Tc ranged from 30.8 °C at the grass-covered surfaces of the park to 44.5 and 43.0 °C at the dry soil and the asphalt parts, respectively, when the average air temperature was 33.4 °C. The surface temperature values appear to be cooler compared to air temperature (Tair) at the covered with wild grass and grass part of the park and only slightly (by 1.4 °C) warmer at those parts covered by shrubs. Very high Tc–Tair differences are recorded even in the morning at the unshaded surfaces covered with artificial materials. The parts of the park occupied by pavements, old concrete, paved roads, and asphalt were by 2.0, 3.8, 4.0, and 9.0 °C warmer compared to Tair, respectively. The warmest surface was recorded at the dry soil part of the park, at which the Tc–Tair differences reached 10.2 °C in the morning. Further, it is worth mentioning that the shadow cooling effect is significant even in the morning. The Tc–Tair differences at the shadowed artificial surfaces indicate cooler conditions that range between −1.0 °C at the shadowed asphalt part to −4.1 °C at the shadowed paved corridors inside the park. The respective difference for the bare dry soil, however, is positive, suggesting that the shadowed dry soil is +2.0 °C warmer compared to the air, probably due to both the high Tc values and the high amounts of stored energy in the soil.

The above-described pattern of high-temperature values enhances at noon, and Tc values are much higher on all surfaces ranging from 38.3 °C on the grass to 64.1 and 58.9 °C, respectively, on the dry soil and the asphalt, whereas the average Tair is 40.6 °C. The Tc–Tair differences are much lower at the green surfaces, and the surface temperatures are only +2.1 and +0.5 °C warmer at the shrub and wild grass or even cooler by −2.8 °C at the grass-covered surfaces of the park. This pattern is the opposite for the dry soil part or the parts of the park that are covered with artificial materials. In these cases, the Tc–Tair differences are positive, suggesting that the surface temperatures of the artificial materials are much higher (from 10.6 °C at the paved corridors to 18.7 °C at the asphalt part) compared to the air temperature values and even warmer (by 23.9 °C) at the dry bare soil part. The shadow effect at noon is more profound for the artificial materials and indicate that, under shadow, the surface temperatures are cooler than the air in most cases by at least 2.4 °C, with the exception of the shadowed old concrete, which presented almost similar Tc values (average Tc only +0.3 °C higher than the air temperature). Similarly, the shadowed dry soil also presented Tc values 0.9 °C warmer compared to the air.

Notable are also the extremely high temperature values of the cars parked around the green area. In the morning, their average Tc was recorded on average at 50.8 °C and reached 60.6 °C at noon, highly contributing to the formation of the urban heat island. Their positioning is also quite important since a thermal “wall” is formed around the park that is very likely to make adverse the positive impact of the green elements to regulate the local conditions even in short distances from the park.

The differences of Tc between the different types of materials are clearly depicted in the thermal photographs presented in Appendix A.

4. Discussion and Concluding Remarks

The densely built environment significantly contributes to the formation of urban heat islands (UHIs) in cities, resulting in increased temperatures that impact citizens’ thermal comfort, especially during the warm summers and heatwaves that are typical for the Mediterranean. This research was conducted under the extreme heat conditions prevailing during the strongest, in recent history, heatwave that had occurred in Europe and in the Mediterranean in July 2023 [20,28,38,39]. The findings concerning the behavior of urban materials (artificial and natural) under such conditions are also in line with the results presented by Cao et al. [40], proposing that urban parks may mitigate the urban heat island (UHI) and decrease cooling energy demand in summer. The study on thermal comfort in Amaroussion provides valuable insights into the UHI phenomenon, regarding the role of different surface materials and vegetation under extremely warm meteorological conditions and aligning with findings from other studies. For example, Mohajerani et al. [41] studied the contribution of the different materials used in urban areas to the UHI phenomenon, and, according to their study, asphalt concrete largely influences the thermal conditions of the urban environment, boosting the UHI mainly due to its high heat capacity. Our results confirm the significant role of asphalt and concrete in exacerbating UHI effects, corroborating the findings of Mohajerani et al. [41]. In our case study, asphalt surface temperature values at noon reached as high as 58.9 °C and are in line with results by Pomerantz et al. [42], who noted that fresh asphalt can absorb up to 95% of solar radiation, contributing to peak surface temperature well above 60 °C on hot summer days. Similarly, Đekić et al. [43] monitored an average maximum temperature (for the warm period from mid-July to mid-August) well above 50 °C for all artificial surfaces, while the grass-covered areas more or less matched the temperature of the ambient environment.

The high surface-to-air temperature differences of urban materials relative to vegetation documented in this study reinforce Grimmond’s [44] observation regarding the strong contrast in thermal properties between artificial and natural surfaces. It is important, as our analysis underlines, that surface temperatures for urban artificial materials are consistently several degrees Celsius above the air temperature, emphasizing the need for mitigation strategies.

The findings by Edmondson et al. [45], that the high cooling effect provided by woody plants compared to herbaceous cover, correspond also to our results that grass surfaces (and especially wild grass and grass) indeed presented lower surface temperature values than the surrounding artificial materials. Further, the surface temperature differences of 18.1 and 20.6 °C between asphalt and grass surfaces (wild grass and grass, respectively) during peak heat events found in our study are also in line with other studies [43] and strongly indicate that high temperature differences between vegetation and asphalt can drastically affect local thermal comfort. More specifically, Đekić et al. [43] found a distinct temperature difference between the grass surface (coolest) and the dark asphalt ranging from 9 °C to 19.7 °C, whereas the difference between the asphalt’s temperature and that of the air was between 10 and24.5 °C, which is also in line with our findings (18.7 °C).

It is worth mentioning that the qualitative characteristics of vegetation impact highly their thermal behavior. In our study, grass-covered surfaces were more effective in producing a cooling effect compared to shrubs characterized by much lower surface temperatures, both in the morning and at noon. This effect is critical for the selection of plants when designing a UGS; however, additional issues should be also taken into consideration regarding the maintenance requirements along with the water consumption for irrigation to preserve each species. Similarly, when a UGS is occupied by bare soil instead of artificial materials, the thermal energy budget of the surface generally worsens, considering the soil’s thermal conductivity properties. In this case, the bare dry soil surface temperatures were found to be similar to the artificial materials both in the morning and at noon. However, their selection in urban open spaces should not be excluded considering their enhanced ability to infiltrate water and prevent flooding [26].

The positive effect of shading is identified in our study and is also in line with other studies [43]. Even partly shaded asphalt surfaces were found to maintain lower temperatures than their counterparts that were fully exposed to sunlight. In most cases, the artificial surfaces’ temperatures (shaded by trees) were similar or even cooler compared to the air, even during noon. However, the presence of trees in cities can highly contribute to the increase in the roughness of the urban surface, increasing heat absorbance, preventing the free circulation of air, and finally trapping heat in the city, especially during night hours as also discussed by Armson et al. [46] and Wu et al. [46].

The green-covered surfaces in the urban green space of Amaroussion had much higher surface temperatures and showed smaller differences between surface and air temperatures compared to the previous year [26]. This can be attributed to the much warmer conditions prevailing during the prolonged heatwave of July 2023 that resulted in increased heat storage in the park. Under extreme heat conditions prevailing during the heatwave, even the green surfaces were less effective in providing cooling in the urban area.

Artificial materials in the urban green site of Amaroussion presented higher reflectivity to global solar radiation (albedo) compared to natural elements, either green or dry soil. Đekić et al. [43] reported albedo values of some common urban artificial surfaces as well as those of green areas in order to accentuate their different influence on the city’s thermal balance. They mentioned an asphalt albedo between 0.04 and 0.15 (slightly lower than our case study, 0.198), a respective range of 0.10–0.35 for concrete (similar to this study, 0.283), 0.25–0.30 for loans, and 0.15–0.18 for arbor (0.156 for grass, 0.146 for wild grass, and 0.154 for shrubs in our urban green site).

The mitigation measures of increasing albedo through surface treatments discussed in the previous literature, including recommendations to coat dark pavements with lighter materials, present practical solutions for urban planners. Salata et al. [9] reported significant temperature reductions from the installation of vegetation and high-reflective materials; however, our study cautions that simply applying high-albedo materials could yield unintended consequences in dense urban settings by increasing local radiant load, as noted by Yang et al. [12]. In areas with a limited sky view, reflective materials will increase the heating of surrounding surfaces and can even create a feedback loop, which exacerbates the UHI rather than alleviates it.

In conclusion, this study highlights the multifaceted nature of urban thermal dynamics. The results support the impact of urbanization in densely built areas on the thermal comfort in Mediterranean cities, a region characterized by warm summers and frequent heatwaves. In such environments, the incorporation of green infrastructure proved to be essential for providing urban cooling to mitigate climate change and the UHI since vegetation elements absorb high quantities of the radiative energy (reflect solar radiation to a less degree comparted to artificial materials) without storing it as heat. Our results indicate also that the thermal and optical properties of specific plant species can yield to varying effects on the urban green surfaces thermal and energy budgets, altering the effectiveness of the UGSs to mitigate the UHI, particularly during strong heatwaves as in July 2023. This suggests the critical role for the selection of proper plant species and artificial materials. Elements in the UGSs with high albedo are generally beneficial for reducing the solar energy absorption; however, they may raise temperatures in the surrounding areas considering that the reflective energy may be absorbed by the adjacent to the UGS surfaces. In all cases, the cooling effectiveness of the green infrastructure is limited by strong and prolonged heatwaves, which are likely to be increased in the future. Therefore, increasing surface albedo and improving vegetation type and cover can be considered as effective measures to counter the UHI, imposing the need for a multifaceted approach to urban design by combining high-albedo materials with proper vegetation species and distribution to counter the UHI and improve urban thermal comfort. The results point out that careful planning and the appropriate selection of artificial materials, in conjunction with the integration of green infrastructures, can alter the cities’ thermal comfort status. Further study is required to precisely quantify the relationships between artificial materials, vegetation, and local climate in order to improve and adopt appropriate urban design strategies to effectively deal with constantly rising urban temperatures.