Spatial Variation of Thermal Conditions in Athens Metropolitan Area, Greece †

: Climate change raises concerns over the sustainability of the outdoor thermal environment and its impacts on societies. This study aims to examine the thermal conditions in Athens metropolitan area, Greece. Hourly data from 15 stations of the Automatic Weather Station Network of the National Observatory of Athens (2010–2021) were used to estimate the Net Effective Temperature (NET) and the Universal Thermal Climate Index (UTCI). The NET and UTCI were higher within the Athens basin (NET: mean ± sd = 15.3 ± 7.1 ◦ C; UTCI: mean ± sd = 18.9 ± 10.4 ◦ C) than in the surrounding area (NET: mean ± sd = 11.2 ± 9.3 ◦ C; UTCI: mean ± sd = 13.4 ± 13.4 ◦ C; p < 0.001). The highest numbers of hours with unfavorable warm conditions were found in Faliro (28.6%, NET; 27.7%, UTCI), Neos Kosmos (28.6%, NET; 27.4%, UTCI) and Patissia (28.6%, NET; 27.2%, UTCI). With respect to unfavorable cool conditions, the highest number was in Spata (65.1%, NET; 28.4%, UTCI).


Introduction
Urban areas are key contributors to global warming and climate change because industries, transport and waste disposal are major sources of greenhouses gases [1].On the other hand, global warming increases extreme weather events, making populations in cities vulnerable to several direct and indirect effects [2].Heatwaves, droughts and storms change natural and social resources suddenly and unexpectedly and impose important costs on economies and societies.Water supplies, infrastructure and properties are seriously affected and often damaged, and human health is at great risk as morbidity and mortality increase [3].
The outdoor thermal environment is identified as a significant threat to well-being.It is the combined effect of several meteorological factors, which, however, may vary greatly between areas within an urban setting.These meteorological factors are frequently combined into a single kind of value, namely, a thermal index [4].Thermal indices assess the thermal environment by quantifying human thermal perception.They assign their estimations (often measured in degrees Celsius) to a category of an assessment scale expressing the degree of human thermal sensation (e.g., very cold, cold, cool, neutral, warm, hot, very hot), discomfort (e.g., no, noticeable, evident, great, dangerous discomfort) or stress (e.g., slight, moderate, strong, very, extreme cold/warm stress).
The aim of this study was to examine the spatial variation of two thermal indices in the metropolitan area of Athens, Greece, in order to identify areas with unfavorable thermal conditions.In view of climate change, and given the rising concern over the sustainability of the thermal environment, the findings of this study could contribute to the development of strategies with which to address environmental impacts.

Data and Methodology
The data included hourly values of air temperature (Tair, • C), relative humidity (Rh, %) and wind speed (WS, m/s) recorded from 2010 to 2021 in 15 weather stations (8 within Athens basin and 7 in the surrounding area) of the Automatic Weather Station Network of the National Observatory of Athens (NOAAN).Hourly data of global solar radiation (SR, W/m 2 ) were recorded in three stations (Ampelokipoi, Markopoulo and Spata).The stations were classified into two regions (Figure 1) with respect to their location, i.e., within Athens basin-the area bounded by the four large mountains of Athens metropolitan area: Mount Egaleo to the west, Mount Parnitha to the north, Mount Penteli to the northeast and Mount Hymettus to the east-and in the surrounding area of the Athens basin.

Data and Methodology
The data included hourly values of air temperature (Tair, °C), relative humidity (Rh %) and wind speed (WS, m/s) recorded from 2010 to 2021 in 15 weather stations (8 within Athens basin and 7 in the surrounding area) of the Automatic Weather Station Network of the National Observatory of Athens (NOAAN).Hourly data of global solar radiation (SR, W/m 2 ) were recorded in three stations (Ampelokipoi, Markopoulo and Spata).Th stations were classified into two regions (Figure 1) with respect to their location, i.e within Athens basin-the area bounded by the four large mountains of Athens metropol itan area: Mount Egaleo to the west, Mount Parnitha to the north, Mount Penteli to th northeast and Mount Hymettus to the east-and in the surrounding area of the Athen basin.The thermal conditions were assessed using the Net Effective Temperature (NET, °C and the Universal Thermal Climate Index (UTCI, °C).The NET and UTCI are calculated in degrees Celsius and can be assigned to a category of an assessment scale expressing th estimated degree of human thermal sensation or stress.The NET [5] is estimated by simple statistical formula involving Tair, Rh and WS, and it assesses thermal sensation based on a 7-point scale ranging from 'very cold' (−3) to 'very hot' (+3) (Table 1).The UTC [6] is a thermophysiological index calculated via Tair, Rh, WS and SR.The UTCI assesse the thermal stress using a 10-point scale ranging from 'extreme cold stress' (−5) to 'extrem heat stress' (+4) (Table 1).The thermal conditions were assessed using the Net Effective Temperature (NET, • C) and the Universal Thermal Climate Index (UTCI, • C).The NET and UTCI are calculated in degrees Celsius and can be assigned to a category of an assessment scale expressing the estimated degree of human thermal sensation or stress.The NET [5] is estimated by a simple statistical formula involving Tair, Rh and WS, and it assesses thermal sensation based on a 7-point scale ranging from 'very cold' (−3) to 'very hot' (+3) (Table 1).The UTCI [6] is a thermophysiological index calculated via Tair, Rh, WS and SR.The UTCI assesses the thermal stress using a 10-point scale ranging from 'extreme cold stress' (−5) to 'extreme heat stress' (+4) (Table 1).The NET and UTCI were calculated using hourly data and independently for each station.In lack of SR monitoring, data were used from the closest available station of NOAAN.In order to facilitate comparisons between indices' estimations and the interpretation of results, the estimated category of perception (i.e., thermal sensation or stress) was assigned to a common numerical scale for both indices, ranging from −5 to +4 (i.e., the General Category column in Table 1).Additionally, the original assessment scales of the NET and UTCI were collapsed into three categories: 'cool', 'neutral' and 'warm' (three-point scales; Table 1).

Statistical Analysis
Statistics including mean, median, standard deviation, interquartile, maximum and minimum values were used to summarize the data and indices' estimations.The frequency of indices' categorical estimations, the hours of the day and the number of days in which warm/cold thermal conditions prevail were estimated in order to compare the spatial variation of the NET and UTCI.The number of days with unfavorable thermal conditions were considered using the mean values of the indices and when the corresponded perception was classified to the 'cool' or 'warm' categories of the three-point assessment scale.
The equality of means was examined using the t-test.The two-sample test of proportions was used to test the equality of proportions.

Results
The summary statistics of the hourly data showed that Tair ranged between −12 • C and 43.4 • C (mean ± sd = 18.1 ± 7.8 • C, median = 17.6 • C; Table 2), with the mean Tair to be higher in stations within the Athens basin (19.2 • C) than in the surrounding area (16.9 • C; p < 0.001).Similarly, the mean NET was estimated to be 15.3 • C in stations within the Athens basin and 11.2 • C in the surrounding area, both corresponding to the 'cool' (−1) category of the original NET assessment scale.The mean UTCI was 18.9 • C in the Athens basin and 13.4 • C in the surrounding area, corresponding to the 'no thermal stress' category (0).In summer, the hourly mean Tair reached 31.5 • C at 14:00 (Local Standard Time-LST) in the Athens basin, while in the surrounding area, the maximum hourly mean was 28.5 • C at 13:00 and 14:00 LST (Figure 2).In the Athens basin, the 'warm' thermal conditions (three-point scales; i.e., NET > 21 • C and UTCI > 26 • C) prevail during the entire day according to the NET and from 7:00 to 19:00 LST according to the UTCI.In the surrounding area, 'cool' conditions (three-point scales; NET < 17 • C and UTCI < 9 • C) were found during the entire day according to the NET and from 16:00 to 9:00 according to the UTCI.The highest mean Tair over the years was found in the stations of Faliro (19.8 ° Neos Kosmos (19.7 °C) and Patissia (19.6 °C) across the Athens basin and of Lavrio (1 °C) in the surrounding area (Figure 3).The highest mean of the indices was found in stations of Faliro (NET, 16.2 °C; UTCI, 19.9 °C) and Patissia (NET, 16.1 °C; UTCI, 19.9 ° In the surrounding area of Athens, the highest means of NET (14.7 °C) and UTCI (18.3 were found in Kantza station (Figure 3).The NET and UTCI estimations extend over the entire range of their assessment sc (Figure 4).The highest frequency of NET estimations in categories 1 (10.4%) and 2 (16.5 was found in Faliro, and in category 3 in Neos Kosmos (3.1%).Similarly, UTCI estimatio in categories 1 (15%) and 2 (9.6%) were more frequent in Faliro, whereas those in catego 3 were more frequent in Patissia (4.5%).The highest frequency of NET estimations in c egories −2 (20.6%) and −3 (5.1%) was found in Maroussi, whereas according to the UT the Maroussi station showed the highest frequency of estimations in categories −1 (18 and −2 (4.4%).
Focusing on the three-point assessment scale of thermal perception, NET classifi  The highest mean Tair over the years was found in the stations of Fa Neos Kosmos (19.7 °C) and Patissia (19.6 °C) across the Athens basin and o °C) in the surrounding area (Figure 3).The highest mean of the indices was stations of Faliro (NET, 16.2 °C; UTCI, 19.9 °C) and Patissia (NET, 16.1 °C; U In the surrounding area of Athens, the highest means of NET (14.7 °C) and U were found in Kantza station (Figure 3).The NET and UTCI estimations extend over the entire range of their ass (Figure 4).The highest frequency of NET estimations in categories 1 (10.4%) was found in Faliro, and in category 3 in Neos Kosmos (3.1%).Similarly, UTC in categories 1 (15%) and 2 (9.6%) were more frequent in Faliro, whereas tho 3 were more frequent in Patissia (4.5%).The highest frequency of NET estim egories −2 (20.6%) and −3 (5.1%) was found in Maroussi, whereas according the Maroussi station showed the highest frequency of estimations in catego and −2 (4.4%).
Focusing on the three-point assessment scale of thermal perception, N The NET and UTCI estimations extend over the entire range of their assessment scale (Figure 4).The highest frequency of NET estimations in categories 1 (10.4%) and 2 (16.5%) was found in Faliro, and in category 3 in Neos Kosmos (3.1%).Similarly, UTCI estimations in categories 1 (15%) and 2 (9.6%) were more frequent in Faliro, whereas those in category 3 were more frequent in Patissia (4.5%).The highest frequency of NET estimations in categories −2 (20.6%) and −3 (5.1%) was found in Maroussi, whereas according to the UTCI, the Maroussi station showed the highest frequency of estimations in categories −1 (18%) and −2 (4.4%).

Conclusions
This study assessed the thermal conditions in the metropolitan area of Athens in order to examine variations in thermal load and to identify areas with unfavorable conditions.Warmer thermal conditions were identified within the Athens basin compared to the surrounding area.Unfavorable warm conditions were the most frequent in Faliro, Neos Kosmos and Patissia within the Athens basin and in Kantza in the surrounding area.Unfavorable cool conditions were most often estimated in Maroussi within the Athens basin and in Spata in the surrounding area.
The thermal environment is an important environmental hazard that, combined with other environmental variables such as air pollution, may affect human health and quality of life.Assessing urban thermal conditions could assist in selecting useful policies for health, tourism, urban planning and energy efficiency.

Figure 1 .
Figure 1.Stations of the Automatic Weather Station Network of the National Observatory of Athen (NOAAN) in the Athens metropolitan area.

Figure 1 .
Figure 1.Stations of the Automatic Weather Station Network of the National Observatory of Athens (NOAAN) in the Athens metropolitan area.

Figure 2 .
Figure 2. Mean air temperature (Tair), Net Effective Temperature (NET) and Universal Therm Climate Index in each hour of a day (LST) in summer and winter for the stations (a) within Athens basin and (b) in the surrounding area.

Figure 3 .
Figure 3. Mean air temperature (Tair), Net Effective Temperature (NET) and Universal Therm Climate Index over the period of study for the 15 stations of the Automatic Weather Station Netw of the National Observatory of Athens (NOAAN) in the Athens metropolitan area.

Figure 2 .
Figure 2. Mean air temperature (Tair), Net Effective Temperature (NET) and Universal Thermal Climate Index in each hour of a day (LST) in summer and winter for the stations (a) within the Athens basin and (b) in the surrounding area.The highest mean Tair over the years was found in the stations of Faliro (19.8 • C), Neos Kosmos (19.7 • C) and Patissia (19.6 • C) across the Athens basin and of Lavrio (19.1 • C) in the surrounding area (Figure3).The highest mean of the indices was found in the stations of Faliro (NET, 16.2 • C; UTCI, 19.9 • C) and Patissia (NET, 16.1 • C; UTCI, 19.9 • C).In the surrounding area of Athens, the highest means of NET (14.7 • C) and UTCI (18.3 • C) were found in Kantza station (Figure3).

Figure 2 .
Figure 2. Mean air temperature (Tair), Net Effective Temperature (NET) and Univ Climate Index in each hour of a day (LST) in summer and winter for the stations Athens basin and (b) in the surrounding area.

Figure 3 .
Figure 3. Mean air temperature (Tair), Net Effective Temperature (NET) and Univ Climate Index over the period of study for the 15 stations of the Automatic Weather S of the National Observatory of Athens (NOAAN) in the Athens metropolitan area.

Figure 3 .
Figure 3. Mean air temperature (Tair), Net Effective Temperature (NET) and Universal Thermal Climate Index over the period of study for the 15 stations of the Automatic Weather Station Network the National Observatory of Athens (NOAAN) in the Athens metropolitan area.

Figure 4 .
Figure 4. Frequency of hours classified into the categories of thermal perception according to the Net Effective Temperature (NET) and Universal Thermal Climate Index (UTCI) in 15 stations (a) within Athens basin and (b) in the surrounding area of Athens.

Figure 5 .
Figure 5. Distribution of hours classified in the 'cool', 'neutral' or 'warm' categories of the threepoint assessment scale according to the Net Effective Temperature (NET) and Universal Thermal Climate Index (UTCI) (a) within Athens basin and (b) in the surrounding area of Athens.

Figure 4 .
Figure 4. Frequency of hours classified into the categories of thermal perception according to the Net Effective Temperature (NET) and Universal Thermal Climate Index (UTCI) in 15 stations (a) within Athens basin and (b) in the surrounding area of Athens.

Figure 4 .
Figure 4. Frequency of hours classified into the categories of thermal perception according to the Net Effective Temperature (NET) and Universal Thermal Climate Index (UTCI) in 15 stations (a) within Athens basin and (b) in the surrounding area of Athens.

Figure 5 .
Figure 5. Distribution of hours classified in the 'cool', 'neutral' or 'warm' categories of the threepoint assessment scale according to the Net Effective Temperature (NET) and Universal Thermal Climate Index (UTCI) (a) within Athens basin and (b) in the surrounding area of Athens.

Figure 5 .
Figure 5. Distribution of hours classified in the 'cool', 'neutral' or 'warm' categories of the three-point assessment scale according to the Net Effective Temperature (NET) and Universal Thermal Climate Index (UTCI) (a) within Athens basin and (b) in the surrounding area of Athens.

7 Figure 6 .
Figure 6.Distribution of days with the Net Effective Temperature (NET) and Universal Thermal Climate Index (UTCI) estimations classified in categories 'cool' and 'warm' according to the threepoint assessment scale.

Figure 6 .
Figure 6.Distribution of days with the Net Effective Temperature (NET) and Universal Thermal Climate Index (UTCI) estimations classified in categories 'cool' and 'warm' according to the threepoint assessment scale.

Table 1 .
Assessment scales of the Net Effective Temperature (NET, °C) and Universal Thermal Cli mate Index (UTCI, °C).

Table 1 .
Assessment scales of the Net Effective Temperature (NET, • C) and Universal Thermal Climate Index (UTCI, • C).

Table 2 .
Summary statistics of air temperature (Tair), thermal indices and thermal indices' estimations according to their assessment scales.