Variation in Discomfort Indices in Athens, Greece, for the Period from 1901 to 2024 ^†

Abstract

This study investigates long-term thermal discomfort trends in central Athens, from 1901 to 2024, using hourly data from Thissio station. Two widely used indices, Thom’s Discomfort Index and Humidex, assess human thermal sensation. Results indicate a significant increase in thermal discomfort, extending from June to September, especially during recent decades. Since 1980, days with severe discomfort have more than doubled compared to early 20th century years. In July and August, such days often exceed 20–25, especially after 2000. These findings highlight growing heat-related stress in Athens’ urban environment and underscore the need for climate adaptation strategies in Mediterranean cities.

1. Introduction

Climate change is one of the most pressing global challenges of the twenty-first century, with profound implications for environmental conditions and human health. Driven by the increasing frequency and intensity of extreme weather events and a consistent rise in global temperatures, the climate crisis is influencing numerous parameters that directly affect thermal comfort and human wellbeing. While air temperature remains a principal factor, thermal comfort is a multivariate phenomenon shaped by relative humidity, solar radiation, wind speed, clothing, and metabolic activity. However, numerous studies [1,2] highlight that temperature and humidity are the most spatially stable and impactful factors for large-scale assessments of thermal discomfort.

Thermal comfort indices serve as essential tools for quantifying the effects of weather on human perception and health. Among the most commonly used are Thom’s Discomfort Index (TDI) [3], the first physiological index of this kind presented in 1959, and the Humidex (HMDX) [4], developed at Canada’s Atmospheric Environment Service (now the Meteorological Service of Canada) in 1979. These indices are based on air temperature and relative humidity, albeit with differing assumptions and computational approaches.

Several studies have examined thermal discomfort in Greece using different indices. Angouridakis and Makrogiannis [5] assessed the environmental thermal sensation for summer in Thessaloniki by analyzing air temperature and relative humidity data in the period 1950–1957. Giles et al. [6] examined the heatwaves that affected Athens and Thessaloniki in July 1987 and July 1988, using TDI and the “relative strain” index. Paliatsos and Nastos [7] compared the thermal effect during air pollution episodes in Athens during the summers of 1993–1995 using TDI and found that it was related to the ozone concentration. Tselepidaki et al. [8] examined the summer thermal perception in Athens for cooling purposes using TDI. Stathopoulou et al. [9], incorporating TDI, calculated the thermal effect from satellite-derived meteorological data and compared it with that from in situ measurements. Kambezidis et al. [10] investigated the future thermal sensation of the Greek population, using TDI, for 33 locations in Greece, based on Typical Meteorological Years developed for each location. Pantavou et al. [11] used various thermal comfort indices in Athens (see Table 2 in [11]). Katavoutas and Founda [12], using hourly values of air temperature and relative humidity for Athens, examined the smaller period of 1960–2017 and found an increasing risk of heat stress using two simple and two rational indices.

This study investigates the long-term variation in thermal discomfort in Athens, Greece, using hourly data from 1901 to 2024. Utilizing air temperature and relative humidity data from the National Observatory of Athens station in the Thissio area at the center of the city, we evaluate the temporal dynamics of two thermal discomfort indices. The findings provide insight into historical and recent trends in human thermal stress, contributing to our understanding of climate impacts on urban populations.

Athens, the capital and largest city of Greece, is located within the Attica Basin. The city experiences a hot-summer Mediterranean climate, characterized by long, dry summers and mild, relatively wet winters [13]. Surrounded by mountains and open to the sea in the south, Athens is heavily influenced by both natural and anthropogenic climatic factors. Its urban expansion, particularly after the Second World War (1950s), has led to significant environmental pressures, such as increased traffic, reduced green spaces due to urbanization pressures, peri-urban forest fires, and the intensification of the urban heat island effect. These conditions make Athens an exemplary case for studying the intersection of urbanization, climate change, and human thermal comfort.

2. Data and Methodology

2.1. Data Sources

The present analysis was based on meteorological data records from the Thissio meteorological station, maintained by the National Observatory of Athens, Greece (37° 58′ N, 23° 43′ E). The station is located on a small hill near the Acropolis at an elevation of 107 m above sea level. The distance to the coastline is about 5 km. Although the station lies near the center of Athens, it is isolated from heavy traffic and densely built areas. Measurements of air temperature and relative humidity at a 1 h time step were collected and analyzed, covering a period of 124 years, from 1901 to 2024. Both parameters are necessary for the calculation of thermal discomfort indices due to their dominant role in determining human heat perception. The long-term air temperature time series was assessed for its homogeneity and accuracy [14]. Based on input parameter availability, a total of 1,079,938 calculated indices were analyzed, with missing values corresponding to less than 0.65%.

2.2. Thermal Discomfort Indices

Two simple thermal discomfort indices, studied worldwide in areas with different climatology, were selected to assess human thermal perception for the examined 1901–2024 period in Athens: the Thom’s Discomfort Index (TDI) [3] and the well-known Humidex (HMDX) [4], calculated using the following representative formulas:

TDI = Ta − (0.55 − 0.0055 × RH) × (Ta − 14.5)

(1)

HMDX = Ta + (5/9) × (e − 10)

(2)

where Ta is the air temperature in °C, RH is the relative humidity in % and e is the vapor pressure given by the expression:

e = 6.112 × 10^{(7.5 × Ta/(237.7 + Ta))} × RH/100

(3)

These indices combine only air temperature and relative humidity, measurable parameters that are readily available at any meteorological station worldwide, but in different ways to reflect human-perceived temperature and discomfort levels. TDI is a simpler index that correlates discomfort with a temperature–humidity relationship. HMDX is basically an equivalent temperature representing what the average person would feel. It is designed for humid environments and incorporates the dew point to express perceived heat. In this study, a simplified formula (Equations (2) and (3)) that takes air temperature and relative humidity as inputs for the HMDX calculation was used. Discomfort levels for each index are categorized in

Table 1. Thom’s Discomfort Index classes, ranges, and descriptions.

Range (°C)	Discomfort Level
TDI < 21	No discomfort
21 ≤ TDI < 24	Under 50% of the population feels discomfort
24 ≤ TDI < 27	Over 50% of the population feels discomfort
27 ≤ TDI < 29	Most of the population suffers discomfort
29 ≤ TDI < 32	Everyone feels severe stress
TDI ≥ 32	State of medical emergency

and

Table 2. HUMIDEX classes, ranges, and description.

Range (°C)	Discomfort Level
HMDX < 20	No discomfort
20 ≤ HMDX < 30	Little to no discomfort
30 ≤ HMDX < 38	Some discomfort
38 ≤ HMDX < 45	Great discomfort, avoid exertion
HMDX ≥ 45	Dangerous, heat stroke quite possible

.

In this analysis, the threshold of 38 °C was used between the second and third discomfort levels of the Humidex (instead of the originally proposed 40 °C), following the proposal of [15]. This value was chosen in order to identify days with significant thermal discomfort, as it marks a level where human health may be adversely affected, especially during prolonged outdoor exposure. This threshold aligns with guidelines from Environment Canada [2] and is widely adopted in climate–health impact assessments and extreme heat studies.

Both indices, TDI and HMDX, are calculated at a 1 h time step, allowing for a detailed temporal analysis of thermal discomfort conditions in Athens, Greece.

3. Analysis and Results

The analysis involved computing each of the two indices across the entire time span of the dataset, incorporating a FORTRAN code especially developed for this purpose. The results were then grouped by decade to detect long-term trends in thermal discomfort levels.

In Figure 1, box-and-whisker plots of air temperature, relative humidity, TDI, and HMDX values for the period 1901–2024 are presented both for the whole period from 1901 to 2024 and also by decade. In

Table 3. Main statistical values for each parameter for the examined period of 1901–2024.

Statistical Parameter	Ta	RH	TDI	HMDX
Number of values	1,083,876	1,082,598	1,079,938	1,079,938
Average	17.97	61.6	16.76	19.00
Standard deviation	7.57	17.1	5.69	8.35
Minimum	−5.4	8	−5.4	−5.4
1st quartile (25%)	12.1	49	12.5	12.3
Median (50%)	17.5	63	16.9	18.4
3rd quartile (75%)	23.8	75	21.5	25.7
Maximum	43.8	100	32.7	49.0
90th percentile	28.2	84	24.0	30.3
99th percentile	34.1	94	27.0	36.4

, the main statistical values for each input parameter and calculated index, for the whole examined period from 1901 to 2024, are presented. The median values and interquartile ranges (IQR) increase over time, indicating both rising central tendencies and greater variability in thermal discomfort.

The IQR of air temperature presents a gradual increase over the decades, starting from 11.4 in the first decade and rising to 12.1–12.5 in the last. This corresponds to an overall increase from 6.14% to 9.65%, suggesting the intermediate temperatures (between the 25th and 75th percentiles) are increasingly different. The distance between the adjusted trend lines show a steady mean increase of 0.78% per decade, reflecting a gradual but steady increase in variability. The IQR of TDI started at 8.7, reached 9.7 during the decade of 1991–2000, and dropped to 9.0 during the last decade. This increase shows that TDI values have greater dispersion over the decades. This implies that the range of discomfort is widening, and possibly more days are reaching values closer to thresholds of severe discomfort. The regression values demonstrate a steady mean increase in the IQR at a rate of +0.74% per decade, confirming the increasing trend. The IQR variation over time for HMDX reveals a clear trend of increasing intra-decadal variance. HMDX shows an overall increase in the IQR as high as 14.94% from 1991 to 2010, with a constant rate of +0.96% per decade. These rises suggest that thermal discomfort conditions are becoming increasingly unstable. The widening of the mean values reinforces the need to adopt adaptive strategies to ensure the protection of public health.

In order to investigate the appearance of outlier cases for both TDI and HMDX indices, the total number of days per month (not necessarily consecutive) with great to extreme discomfort (TDI ≥ 27, HMDX ≥ 38) are introduced in Figure 2 for each year of the examined period of 1901–2024.

The analysis of thermal discomfort outliers for both TDI and HMDX revealed clear trends in the evolution of perceived heat stress in Athens from 1901 to 2024. A clear increase in high-discomfort days is observed, especially during the summer period (JJA), with expansion into May and September. Days with TDI ≥ 27 and HMDX ≥ 38 have become notably more frequent since the 1990s, while high (TDI ≥ 29, HMDX ≥ 40) and extreme categories (TDI ≥ 32, HMDX ≥ 45) now occur more often and across more months than in previous decades. High-stress days often cluster during consecutive hot months, identifying heatwave impacts.

Finally, in order to investigate in more detail the hot period from June to September, also including the neighboring months of May and October, available 1 h index calculations were classified according to the categories in Table 1 and Table 2, using 20-years intervals. The frequency distribution per month and period was normalized using the number of days per month (30 or 31), allowing the results to be expressed as the number of days per class. The results for each index are presented in Figure 3 and Figure 4. The discomfort indices increase from May, peak in midsummer, and decline by October.

The analysis clearly shows an increase in thermal discomfort, especially after 1980, with the peak occurring after 2000. In the first decades of the 20th century, the overwhelming majority of summer days were classified in the categories of low or moderate discomfort, while high discomfort categories were almost never observed. From 1941 to 1980, there was an increase of up to 34% on these days compared to previous years, with this growth rising to 38% in July and August alone, which are the most dangerous months. After the 1980s, the number of these days has continued to increase gradually. During the period from 1981 to 2000, the number of peak summer days when more than half of the population felt discomfort was almost equal to the number of days during the previous forty years. In the last two decades, these days have become more numerous than low-risk ones and have increased by more than 100% compared to the previous two decades. More specifically, both discomfort indices show an increase in these days of 175% and 219%, respectively, compared to the period of 1981–2000. Also, looking at the TDI and HMDX charts (Figure 3), it is clear that there are now more “dangerous” days, when everyone feels intense discomfort, indicating a clear climate deterioration and increased heat stress conditions. This significant change reflects the growing impact of climatic change, which has intensified the frequency, duration, and severity of heatwaves in recent decades. The sharp increases in the thermal discomfort indices correspond to observed global warmings trends, highlighting the direct impact of climatic change to thermal stress.

4. Conclusions

The findings of the study highlight the following:

• Both indices, TDI and HMDX, indicate a long-term increasing trend in thermal discomfort and its variability in Athens, with a prominent rise from the 1980s onwards, especially during the two most recent decades.
• From 2001 to 2024, the number of uncomfortable days during the peak of summer increased by more than 200% compared to the first half of the 20th century and by more than 150% compared to the 40-year period from 1941 to 1980. This number also rose in comparison to the 20-year period from 1981 to 2000.
• During this time, more than 20 days per month fall into the category of discomfort, while there were also months where every day was risky, showing a transition to heat-alert conditions.
• The median values and interquartile ranges of both indices have increased over time in Athens, indicating both rising central tendencies and greater variability in thermal discomfort.
• High-stress days often cluster during consecutive hot months, identifying heatwave impacts.
• Moreover, thermal discomfort has tended to extend to May and September during the two most recent decades.