Τhis study explores the climatic characteristics and variability of the annual and seasonal precipitation height (PH) and the number of wet days (WD) over the greater Mediterranean region, as derived for the period 1950–2023, by the 5th ECMWF reanalysis (ERA5) hourly precipitation dataset. The characteristics of the mean daily precipitation intensity (PI) inferred from the combined evolution of PH and WD are also investigated. Three oscillatory modes, at about 65 year, 35–50 year, and 4–8 year, found to be the most important components of PH and WD variability. Combined, these modes modulated periods of high PH and WD during the 1960s and 1970s across the Mediterranean domain, followed by drier conditions until the 2000s, and recovering trends over the subsequent two decades. Averaged over the Mediterranean Sea, the annual PH was found to decrease at a mean rate of −11 mm/decade, and WD at −1.9 days/decade. However, the decreasing signal in annual WD appears more robust and widespread than in PH, as significant negative trends in WD dominate most of the Mediterranean basin, central Black Sea, and northwestern Africa, while significant negative trends in PH are confined to five distinct marine or coastal areas. Because WD has decreased more rapidly than PH over marine areas, significant positive trends prevail in PI, especially over the Mediterranean Sea. On the other hand, significant positive trends in the PH and WD fields prevail only over land areas, particularly along the major mountain ranges in southern Europe, Anatolia, and parts of the Middle East.

The study of hot extremes has traditionally been grounded in the analysis of maximum air temperature, often overlooking the effect of minimum air temperature but also the combined effect of daytime and nighttime thermal conditions (compound hot extremes). The present study examines climatic indices related to nighttime conditions like tropical nights (TRN), compound hot extreme days (CHD) but also three types of heat waves (HWs) (nighttime, daytime, compound) at large cities of the eastern Mediterranean, over the period 1960–2022. Three sub-periods (1960–1980, 1981–2001, 2002–2022) are also examined in the study. To this end, long-term observational data from one meteorological station with sufficiently long records in each city were used. The analysis reveals increasing trends in the number of TRN (+ 2.8 to + 6.2 tropical nights/decade) but also in their intensity (+ 0.11 to + 0.31 °C/decade), more prominent at the most highly urbanized cities. The frequency of CHD over the last two decades is almost twofold to threefold times the 1980s and 1990s levels, in all cities. Daytime HWs are more frequent than nighttime HWs in all cities except in the city with the largest urban area (Athens), where nighttime HWs dominate. The number of HWs during the last sub-period 2002–2002 is much higher compared to the previous one 1981–2001 (1.8–3.7 times higher for nighttime and 1.4–2.6 times higher for daytime HWs, depending on the city). The coupling between intensity and duration of daytime HWs is strong and apparent in all cities, indicating that the longer the duration of daytime HWs, the higher is their intensity on average, while the coupling for nighttime HWs seems to be weaker.

Climate change and rising global temperature values lead to a cascade of effects on human health and well-being. Methodologies for assessing thermal conditions and identifying areas with increased thermal stress are important for enhancing the quality of life in urban environments. This study is aimed at developing a methodology that combines high-resolution simulation data with surface meteorological observations for application in urban thermal stress assessment. Eleven urban public sites within the metropolitan area of Athens, Greece (i.e., squares and parks) were simulated using the three-dimensional microclimate model ENVI-met. The model was validated using micrometeorological data from field campaigns conducted in summer, autumn and winter. The validation results confirmed that ENVI-met showed satisfactory performance for further research analysis. Subsequently, Physiologically Equivalent Temperature (PET) and Universal Thermal Climate Index (UTCI) were calculated using data from weather stations operated by the National Observatory of Athens and the Hellenic National Meteorological Service. PET and UTCI were then spatially interpolated using a mixed modeling and kriging method, with parameters optimized based on statistical validation metrics derived from the ENVI-met simulations. Finally, seasonal bioclimatic maps were produced to identify areas experiencing unfavorable thermal conditions. The spatial analysis revealed distinct seasonal patterns in the distribution of unfavorable thermal conditions across the Athens metropolitan area.

The Mediterranean faces frequent heavy precipitation, deadly heatwaves, and wildfires fueled by its climate. Greece, with its complex topography, experiences severe and extreme weather events that have escalated in recent years and are projected to continue rising under future climate conditions. This paper analyzes severe weather events and trends in Greece from 2010 to 2023, leveraging data from an expanded network of weather stations spanning across Greece, as well as long-term meteorological data from the reference weather station in the center of Athens. The focus includes analysis of heat waves, intense rainfall and droughts, thunderstorms, hail, tornadoes, and fire weather conditions. The societal impact of severe weather events is also discussed. The paper aims to provide both long-term (1901–2023) and recent year analyses (2010–2023). The main results show that between 2010 and 2023, Greece experienced: nearly one heatwave per summer; heavy rainfall events, most common in winter and autumn, showing a significant increase, particularly in the eastern Aegean and western continental Greece; dry spells, which are longest in southern Greece; thunderstorm and hail events peaking in spring and summer; fire weather conditions and risk peaking in southern Greece. Finally, societal impacts from weather hazards have increased in Greece over the past 14 years, with flash floods being the most frequent and damaging events, while public preparedness and effective risk communication remain low.

Extreme weather events and rising temperatures pose significant risks, not only in urban areas but also in metropolitan forests, that affect the well-being of the people who visit them. City forests are considered one of the best bets for mitigating high temperatures within civic areas. Such areas modulate microclimates in contemporary cities, offering environmental, social, and economic advantages. Therefore, comprehending the intricate relationships between municipal forests and the climatic changes of various destinations is crucial for attaining healthier and more sustainable city environments for people. In this research, the thermal comfort index (Modified Temperature–Humidity Index (MTHI)) has been analysed using Landsat images of six urban forests in London during July 2022, when the area first experienced record-breaking temperatures of over 40 °C. Our results show a significant growth in the MTHI that goes from 2.5 (slightly hot) under normal conditions to 3.4 (hot) during the heat wave period. This situation intensifies the environmental discomfort for visitors and highlights the necessity to enhance their adaptability to future temperature increases. In turn, it was found that the places most affected by heat waves are those that have grass cover or that have small associated buildings. Conversely, forested regions or those with lakes and/or ponds exhibit lower temperatures, which results in enhanced resilience. These findings are noteworthy in their concentration on one of the UK’s most severe heat waves and illustrate the efficacy of integrating spectral measurements with statistical analyses to formulate customized regional initiatives. Therefore, the results reported will allow the implementation of new planning and adaptation policies such as incorporating thermal comfort into planning processes, improving green and blue amenities, increasing tree densities that are resilient to rising temperatures, and increasing environmental comfort conditions in metropolitan forests. Finally, the applicability of this approach in similar urban contexts is highlighted.

Over the past decades, extreme weather phenomena like hot extremes and heat waves (HWs) stand out as a major threat for humans and ecosystems. Compound extremes are understood as simultaneous, concurrent or sequential extreme events, taking place at a single or different locations. Compound extreme events may exacerbate the risk and increase associated adverse impacts, compared to individual events.In the study, we examined the occurrence of compound hot extremes at an urban site of the eastern Mediterranean over a century-long period, using the historical climatic records of the National Observatory of Athens (NOA, 1897-2023). Compound hot extremes are defined as concurrent daytime and nighttime hot extremes, namely cases when both, daily maximum (Tmax) and daily minimum (Tmin) air temperatures are above a predefined threshold value. The threshold values for Tmax and Tmin were set equal to 36.7 oC and 25.9 oC respectively, corresponding to the 90th percentile of the summer Tmax and Tmin distributions at NOA, over the reference period 1981-2010. Likewise, we examined compound heat waves, defined as sequences of at least 3 consecutive days when both Tmax and Tmin exceed the predefined thresholds. Analysis has shown that 60% of the total number of compound hot extremes and compound heat waves in Athens (NOA) was observed from 2000 onwards. Besides, 57% of the daytime HWs over the whole study period constitute also compound HWs, while this percentage increases to 72% after the 2000s, indicating an increase in nighttime HWs, very likely related to the urban heat island effect. In addition to the hot extremes based on air temperature, we have also estimated compound daytime and nighttime extremes related to human thermal comfort, using the bioclimatic index UTCI (Universal Thermal Climate Index), accounting also for relative humidity, solar radiation and wind speed conditions. Compound hot extremes based on UTCI were defined as the cases when the daily maximum UTCI value was above the index threshold indicating ‘at least very strong heat stress’ (UTCI > 38), and simultaneously, the daily minimum UTCI value was above the index threshold indicating ‘at least moderate heat stress’ conditions (UTCI > 26). The analysis detected 45 compound hot extremes based on UTCI from 1960-2023, with 34 of them occurring after the 2000s, suggesting a dramatic increase in the frequency of cases with heat-related thermal discomfort throughout the whole day and night. The higher frequency of compound hot events was observed during the extreme years 2007, 2021 and 2023.

There is a close relationship between tourism and climate, the latter being one of the most important factors influencing the choice of destination. Today, rising temperatures and extreme weather events pose significant risks to the tourism sector by affecting the safety and well-being of visitors. Urban tourism is particularly vulnerable due to the additive effect of the urban heat islands which exacerbate heat-related risk in cities. This research aims to examine the spatiotemporal variation of thermal conditions at Seville, a popular tourist destination in Spain, where the tourism sector represents 25 % of the gross domestic product. To this end, the Universal Thermal Climate Index (UTCI) and Landsat 8 images have been used, corresponding to August 2017, when the city experienced two heat waves and decreased number of visitors. Our results showed high variability of the UTCI between 28 and 39 degrees C corresponding to strong thermal stress that increased in the afternoon hours. During heat waves, this variability intensified by 9.77 %, reaching values between 32 and 42 degrees C corresponding to very strong thermal stress. Our findings show that adverse thermal conditions negatively affect tourist arrivals, which could lead to significant economic repercussions. Also, our results point to the urgent need for mitigation and resilience measures including the use of Blue Infrastructure (BI), new green areas, naturalizing streets, and use of green facades and roofs. These results will allow the development of adaptation and urban planning policies together with the development of resilience measures that improve the environmental comfort conditions of the historic center and therefore the visitors' experience.

The study explores long-term changes in the maximum number of consecutive hours per day of heat-related discomfort in Athens over the period 1960–2024, using the Universal Thermal Climate Index (UTCI). This index includes a four-category scale to represent heat stress intensity, ranging from ‘moderate’ to ‘extreme’, as part of its broader multi-category classification system. The analysis indicated a clear increase in the frequency of days with a large number of consecutive discomfort hours over the past decades. Almost 70% of the total number of days with 11 consecutive hours under at least ‘strong heat stress’ and 7 consecutive hours under at least ‘very strong heat stress’ were detected after the year 2000.

The Mediterranean, and particularly its Eastern basin, is a crossroad of air masses advected from Europe, Asia and Africa. Anthropogenic emissions from its megacities meet over the Eastern Mediterranean, with natural emissions from the Saharan and Middle East deserts, smoke from frequent forest fires, background marine and pollen particles emitted from ocean and vegetation, respectively. This mixture of natural aerosols and gaseous precursors (Short-Lived Climate Forcers—SLCFs in IPCC has short atmospheric residence times but strongly affects radiation and cloud formation, contributing the largest uncertainty to estimates and interpretations of the changing cloud and precipitation patterns across the basin. The SLCFs’ global forcing is comparable in magnitude to that of the long-lived greenhouse gases; however, the local forcing by SLCFs can far exceed those of the long-lived gases, according to the Intergovernmental Panel on Climate Change (IPCC). Monitoring the spatiotemporal distribution of SLCFs using remote sensing techniques is important for understanding their properties along with aging processes and impacts on radiation, clouds, weather and climate. This article reviews the current state of scientific know-how on the properties and trends of SLCFs in the Eastern Mediterranean along with their regional interactions and impacts, depicted by ground- and space-based remote sensing techniques.

Diurnal Temperature Range (DTR), defined as the difference between the daily maximum (Tmax) and daily minimum (Tmin) air temperature, has received considerable attention as an important indicator of climate change. In the present study, we analyse long-term highly homogenized DTR data from 51 Greek stations and investigate their spatiotemporal changes. The long-term temporal changes of DTR revealed mixed patterns with both increasing and decreasing trends over the study period and distinct seasonal differentiations. DTR pattern in Athens has fluctuated since the beginning of the twentieth century, generally following warming and cooling air temperature trends. After the mid-1980s, DTR showed a pronounced decreasing trend at a rate of 0.47 °C/decade in summer (p < 0.01) due to higher warming rates of Tmin, suggesting the combined effects of regional warming and urbanization levels.

The study presents results of the research project ASPIRE (atmospheric parameters affecting spectral solar irradiance and solar energy). A new dataset has been created for Athens, Greece, for one year (December 2020–November 2021), consisting of detailed measurements of spectral solar irradiance (SSI) from 300 to 1020 nm and its influencing parameters (clouds, aerosols, ozone, nitrogen dioxide, sulfur dioxide, water vapor). The new dataset is explored, and major results are presented. The datasets are available to scientists from interdisciplinary scientific communities for research and education purposes. Here, we present an overview of different studies within ASPIRE, dealing with effects of different aerosol types on solar measurements and PV-related outputs and also the evaluation of a nowcasting solar model under different atmospheric conditions.

In August 2021, a historic heatwave was recorded in Greece which resulted in extreme wildfire events that strongly affected the air quality over the city of Athens. Saharan dust was also transferred over Greece on certain days of the same period due to the prevailing southern winds. The impact of these events on air quality and surface solar radiation is investigated in this study. Event characterization based on active and passive remote sensing instrumentation has been performed. The study shows that significantly increased levels of air pollution were recorded from the end of July to the first week of August. The smoke led to unusually high aerosol optical depth (AOD) values (up to 3.6 at 500 nm), high Ångström exponent (AE) (up to 2.4 at 440–870 nm), and a strong and negative dependence of single-scattering albedo (SSA) on wavelength that was observed to decrease from 0.93 at 440 nm to 0.86 at 1020 nm, while the dust event led to high AOD (up to 0.7 at 500 nm), low AE (up to 0.9 at 440–870 nm), and a positive dependence of SSA on wavelength that was observed to increase from 0.89 at 440 nm to 0.95 at 1020. Furthermore, the smoke plume was also detected over the PANhellenic GEophysical observatory of Antikythera on 7 August, which is about 240 km away from Athens. Increased AOD values (up to ∼ 0.90 at 500 nm) associated with a high fine-mode AOD (up to ∼ 0.85 at 500 nm) and decrease in SSA with wavelength suggested the dominance of fine biomass burning aerosols. The impact of dust and smoke on solar irradiance revealed significant differences in the spectral dependence of the attenuation caused by the two different aerosol types. The attenuation of solar irradiance in the ultraviolet (UV-B) spectrum was found to be much lower in the case of dust compared to smoke for similar AOD500 values. Differences were less pronounced in the near-infrared and visible spectral regions. The large AODs during the wildfires resulted in a decrease in the noon UV index by up to 53 %, as well as in the daily effective doses for the production of vitamin D (up to 50 %), in the daily photosynthetically active radiation (up to 21 %) and in the daily global horizontal irradiance (up to 17 %), with serious implications for health, agriculture, and energy. This study highlights the wider impacts of wildfires that are part of the wider problem for Mediterranean countries, whose frequency is predicted to increase in view of the projected increasing occurrence of summer heatwaves.

&lt;p&gt;Many regions across the globe have been witnessing changes in both mean climate and climatic extremes during the last decades. Cities in particular, where the concentration of urban population is high, have been in the spotlight of scientific research seeking to address climate-related issues. At the same time, climate change is expected to largely impacts cities until the end of 21&lt;sup&gt;st&lt;/sup&gt; century. However, changes in air temperature levels, as an essential element of the climate, are probably not uniform nor of the same rate across different areas of the globe. An important indicator of the global climate change is the diurnal temperature range (DTR), defined as the difference between the daily maximum and daily minimum air temperature, thus reflecting the temperature variation within a day.&lt;/p&gt; &lt;p&gt;This study analyses the distribution and long-term trends in DTR in seventeen European cities of different base climate. The response of DTR under exceptionally hot weather has also been investigated. The study uses observed and projected data of daily maximum and minimum air temperature over the periods 1961-2019 and 1971-2100, respectively. The projected data, over the studied cities for the closest land grid point to the stations&amp;#8217; location, were derived from the Regional Climate Model (RCM) RCA4 of the Swedish Meteorological and Hydrological Institute driven by the Max Planck Institute for Meteorology model MPI-ESM-LR, with the simulations carried out in the framework of the EURO-CORDEX modeling experiment. The projected data were bias adjusted applying the empirical quantile mapping technique. Future simulations were based on two climate scenarios, the Representative Concentration Pathways (RCPs) 4.5 and 8.5.&lt;/p&gt; &lt;p&gt;The distribution of the DTR frequency based on observations shows a similar pattern in cities that share the same background climate, while is clearly differentiated at diverse climate types. The mean DTR for normal summer days (maximum air temperature lower than 95 percentile) ranges between 8.0 and 10.5 &lt;sup&gt;o&lt;/sup&gt;C for all cities, with the exception of Nice that shows lower mean DTR and Nicosia, Madrid and Bucharest that present higher. The change of the mean DTR between summer normal days and hot days (maximum air temperature higher than 95 percentile) is greater for cities in higher latitudes, while it is smaller for cities in lower latitudes. According to the projected data for the period from 1971 to 2100 under the RCP4.5 scenario, a statistically significant decreasing trend in mean DTR/yr is projected for the cities in the highest latitudes (Oslo, Stockholm and Helsinki), suggesting higher increasing rates in the minimum air temperature compared to the maximum air temperature. At the same time, the opposite result is expected in Madrid (statistically significant increasing trend), while no statistically significant trends in mean DTR are projected for the rest of the cities.&lt;/p&gt;

An important indicator of climate change is the diurnal temperature range (DTR), defined as the difference between the daily maximum and daily minimum air temperature. This study aims to investigate the DTR distribution in European cities of different background climates in relation to the season of the year, climate class and latitude, as well as its response to exceptionally hot weather. The analysis is based on long-term observational records (1961–2019) coupled with Regional Climate Model (RCM) data in order to detect any projected DTR trends by the end of the 21st century under intermediate and high emission greenhouse gases (GHGs) scenarios. The analysis reveals marked variations in the magnitude of DTR values between the cities, on the one hand, and distinct patterns of the DTR distribution according to the climate class of each city, on the other. The results also indicate strong seasonal variability in most of the cities, except for the Mediterranean coastal ones. DTR is found to increase during hot days and heat wave (HW) days compared to summer normal days. High latitude cities experience higher increases (3.7 °C to 5.7 °C for hot days, 3.1 °C to 5.7 °C for HW days) compared to low latitude cities (1.3 °C to 3.6 °C for hot days, 0.5 °C to 3.4 °C for HW days). The DTR is projected to significantly decrease in northernmost cities (Helsinki, Stockholm, Oslo), while it is expected to significantly increase in Madrid by the end of the 21st century under both the intermediate- and high-emission scenarios, due to the asymmetric temperature change. The asymmetrical response of global warming is more pronounced under the high-emission scenario where more cities at higher latitudes (Warsaw, Berlin, Rotterdam) are added to those with a statistically significant decrease in DTR, while others (Bucharest, Nicosia, Zurich) are added to those with an increase in DTR.

The ASPIRE project aims to contribute to the scientific knowledge of interdisciplinary aspects that are related with solar radiation by investigating the effect of various atmospheric parameters such as clouds, aerosols, water vapor and absorbing trace gases to the Spectral Solar Irradiance (SSI) reaching the Earth’s surface. Such aspects deal with solar energy research and technology (e.g. Photovoltaic Systems, PV), impact on health (melanoma, skin cancer and Vitamin D efficiency), agriculture (photosynthetically active radiation, PAR, and crop production) and the complexity of the SSI determination through an atmosphere with various spectral absorbing, scattering and reflecting atmospheric constituents. The project has four specific objectives: (a) To investigate the effect of atmospheric composition in different solar spectral regions, (b) To assess the impact of atmospheric composition on UV Index, Vitamin D and PAR, (c) To improve PV efficiency based on spectral solar data for various atmospheric composition cases, (d) To evaluate the performance of the Solar Energy Nowcasting SystEm (SENSE) using real solar spectra. The means to fulfil these objectives is a sophisticated atmospheric field experiment that has been held in the city of Athens, Greece, with a unique set of instrumentation and a synergistic approach on the retrieved datasets. Atmospheric composition and solar radiation related measurements and models are coordinated in ASPIRE in order to contribute to the following scientific advancements: * Accuracy of public awareness solar radiation related factors, such as erythemal (UV Index), eye cataract, DNA damage, Vitamin D production and agricultural (PAR) related indexes, could be essentially improved using the proposed measurements’ dataset. * PV efficiency will be assessed and improvements will be suggested, with the use of detailed spectral information that will be available. In addition, atmospheric variables that attenuate SSI will be linked with possible deviations of the PV output. ASPIRE will provide to PV module users and constructors the know-how to link individual atmospheric solar attenuators (e.g. aerosols and trace gases) to such deviations. * Solar energy nowcasting is a crucial aspect for national and private power transmission and distribution system operators worldwide. Detailed evaluation and improvement of such a developed system for Greece is an important aspect of the project, given the fact that the proposed SENSE system has been used by the national energy transmission operator.

The thermal conditions that prevail in cities pose a number of challenges to urban residents and policy makers related to quality of life, health and welfare as well as to sustainable urban development. However, the changes in thermal stress due to climate change are probably not uniform among cities with different background climates. In this work, a comparative analysis of observed and projected thermal stress (cold stress, heat stress, no thermal stress) across four European cities (Helsinki, Rotterdam, Vienna, and Athens), which are representative of different geographical and climatic regions of the continent, for a recent period (1975 - 2004) and two future periods (2029 - 2058, 2069 - 2098) has been conducted. Applying a rational thermal index (Universal Thermal Climate Index) and considering two models of the EURO-CORDEX experiment (RCA4-MOHC, RCA4-MPI) under two Representative Concentration Pathways (RCP4.5, RCP8.5), the projected future changes in thermal conditions are inspected. The distribution of thermal stress in the current climate varies greatly between the cities, reflecting their climatic and urban heterogeneity. In the future climate, a reduction in the frequency of cold stress is expected across all cities, ranging between - 2.9% and - 16.2%. The projected increase in the frequency of optimal thermal conditions increases with increasing latitude, while the projected increase in the frequency of heat stress (ranging from + 0.2 to + 14.6%) decreases with increasing latitudes. Asymmetrical changes in cold- and heat-related stress between cities were found to affect the annual percentage of optimal (no thermal stress) conditions in future. Although future projections are expected to partly bridge the gap between the less-privileged cities (with respect to annual frequency of optimal thermal conditions) like Helsinki and Rotterdam and the more privileged ones like Athens, the former will still lag behind on an annual basis.

Heat waves (HWs) rank among the most dangerous weather phenomena, with catastrophic impacts on societies and ecosystems. Since the beginning of the 21th century, many regions worldwide have been experiencing unprecedented extreme heat episodes. The Mediterranean countries in particular, are very prominent and vulnerable to climate change and heat-related risk. During summer 2021, Greece faced one of the worst HWs in its modern history, with exceptionally high temperatures prevailing from July 28th to August 6th. The special characteristics and the rarity of this event have been highlighted and evaluated through the historical climatic record of the National Observatory of Athens (NOA), dating since the mid 19th century. The study analysed daily maximum (Tmax), mean (Tmean) and minimum (Tmin) air temperatures of the historical record, and estimated several indices of all HW episodes detected during the study period. The analysis showed that the HW of 2021 (HW2021) exceeded all previous records in a number of indices concerning the persistence, amplitude, mean intensity of HWs (based on Tmean and Tmin thresholds), but also ‘cumulative heat’ (an index combining both intensity and duration of a HW). Specifically, HW2021 was found to be the longest HW ever recorded at NOA (since the mid 19th century), with a total duration of 10 days. The amplitude of HW2021 (maximum temperature of the hottest day) was 43.9 0 C, representing the second highest temperature ever recorded at NOA, following the absolute record value of 44.8 0 C observed on June 26th 2007. The most prominent features of HW2021 include the maintenance of very high temperatures throughout the whole 24-hour period and especially the elevated nighttime temperatures, inherent to the additive effect of the urban heat island in the city of Athens. The values of 31.6 and 36.5 0C for the daily minimum and mean temperatures respectively, represent the highest values ever recorded at NOA. National all-time temperature records were observed in other Greek stations, with maximum temperatures reaching up to 47 0C. The prolonged hot and dry conditions triggered the ignition of catastrophic wildfires in Greece, with dramatic environmental and economic loss.

The Mediterranean has been identified as a 'climate change hot spot', already experiencing faster warming rates than the global average, along with an increased occurrence of heat waves (HWs), prolonged droughts, and forest fires. During summer 2021, the Mediterranean faced prolonged and severe HWs, triggering hundreds of wildfires across the region. Greece, in particular, was hit by one of the most intense HWs in its modern history, with national all-time record temperatures being observed from 28 July to 6 August 2021. The HW was associated with extreme wildfires in many parts of the country, with catastrophic environmental and societal consequences. The study accentuated the rarity and special characteristics of this HW (HW2021) through the analysis of the historical climate record of the National Observatory of Athens (NOA) on a centennial time scale and comparison with previous HWs. The findings showed that HW2021 was ranked first in terms of persistence (with a total duration of 10 days) and highest observed nighttime temperatures, as well as 'cumulative heat', accounting for both the duration and intensity of the event. Exceptionally hot conditions during nighttime were intensified by the urban heat island effect in the city of Athens. Human exposure to heat-related stress during the event was further assessed by the use of bioclimatic indices such as the Universal Thermal Climate Index (UTCI). The study points to the interconnected climate risks in the area and especially to the increased exposure of urban populations to conditions of heat stress, due to the additive urban effect.

Heat Waves (HWs) have received outstanding scientific attention over the past decades, inherent to their devastating societal and environmental effects. Observed and projected trends in HWs characteristics differ between geographical areas and studies, due to varying regional warming rates but also the adoption of different methods to assign a HW. The present study investigates and compares long-term statistics and trends in HWs characteristics (occurrence, duration and timing) from 1900 to 2019, at a very sensitive to climate change area, the eastern Mediterranean, derived from 15 HW definitions. In addition to broadly used definitions based on air temperature thresholds, the study introduces new definitions based on bioclimatic indices, like the UTCI (Universal Thermal Climate Index) and PET (Physiologically Equivalent Temperature). The analysis revealed coherent results, especially between definitions of the same type, but also marked deviations across different definitions, with respect to the magnitude of observed trends. Statistically significant trends (p < 0.01) in the number of HWs events and frequency of HWs days have been found in the area, irrespective of HWs definition. Changes in the timing of HWs - consistent across definitions- have been also found, resulting in the lengthening of HWs season by up to 7 days/decade since the 1960s.