Successive Hours of Heat Stress in Athens Based on the Universal Thermal Climate Index (1960–2024) ^†

Abstract

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.

1. Introduction

Escalating global warming poses serious risk to humans and ecosystems, with many parts of the world experiencing extreme heat. Urban areas face increased risk as a consequence of the urban heat island (UHI) effect, which exacerbates the effect of global/regional warming. At the same time, the increased frequency, duration and intensity of heat waves, and their synergies with UHIs, further amplify heat stress and aggravate health risks for urban residents [1].

The study examines long-term changes in the outdoor thermal environment in the city of Athens, using the Universal Thermal Climate Index (UTCI) and a long record of hourly climatic data spanning from 1960 to 2024. The UTCI describes the human physiological response to specific meteorological conditions and provides a quantitative assessment of thermal stress [2]. Not only the magnitude of thermal discomfort but also the duration of continuous exposure to such conditions is vital for human health. Consequently, the study focuses on the maximum number of consecutive hours per day under different heat stress categories. Successive nighttime hours of thermal discomfort have also been examined. Recent research studies point to the growing role of nighttime warming on heat-related health effects, including its association with increasing burden and mortality risks, as well as the degradation of sleep quality [3,4].

2. Materials and Methods

The Universal Thermal Climate Index (UTCI) was used to assess the effect of the thermal environment on human thermal stress levels. The UTCI is a state-of-the-art thermal stress index based on a multi-node thermophysiological model with an integrated adaptive clothing model [2].

Table 1. Selected categories from the UTCI scale representing heat-related stress and no thermal stress [2].

UTCI (°C)	Stress Category
>46	Extreme heat stress
38–46	Very strong heat stress
32–38	Strong heat stress
26–32	Moderate heat stress
9–26	No thermal stress

presents selected categories of the UTCI scale, representing the intensity levels of heat-related stress and the category indicating ‘no thermal stress’. UTCI calculations were performed using hourly data on air temperature, water vapor pressure, wind speed, and mean radiant temperature, the latter was computed from global solar radiation. These hourly data were obtained from the long-term climate records of the National Observatory of Athens (NOA), located on a hill in central Athens, covering the period 1960–2024. Nighttime hours are defined as the period from 21:00 to 05:00 Local Standard Time (LST), and this definition is applied throughout the subsequent analyses.

3. Results and Discussion

The maximum duration (consecutive hours) under different heat stress categories based on the UTCI scale was calculated for each day over the study period. Only sporadic days with consecutive hours under ‘extreme heat stress’ (UTCI > 46 °C) were detected over the study period, with the highest duration of six consecutive hours being observed during the extreme heat wave of 2021 [5]. The analysis showed increasing frequency of days with persisting heat stress conditions lasting for several consecutive hours. Specifically, almost 70% of the total number of days with 11 consecutive hours under at least ‘strong heat stress’ (UTCI > 32 °C) were detected after 2000.

Figure 1 shows the annual frequency of days experiencing consecutive hours under at least ‘very strong heat stress’ (UTCI > 38 °C) for varying maximum duration values over the study period. While days with two to three consecutive hours under at least ‘very strong heat stress’ are common throughout the entire study period, the frequency of days with more than three consecutive hours in this category increases considerably after the early 2000s. Approximately 70% of the total number of days with seven consecutive hours under at least ‘very strong heat stress’ were detected after 2000.

Figure 2 presents results similar to those in Figure 1, but for nighttime hours (21:00 to 05:00 LST) under at least ‘moderate heat stress’ conditions (UTCI > 26 °C). The maximum number of consecutive heat stress hours at night has escalated over the past two decades. Nearly 70% of the days with five successive hours under at least ‘moderate heat stress’ conditions during nighttime were recorded after 2000. This share rises to 85% for days with eight successive hours of such conditions.

Additional analysis was conducted to examine long-term trends in the frequency of nights with uninterrupted thermal comfort throughout the whole night (21:00 to 05:00 LST) in summer (Figure 3). A striking decline in the number of ‘no thermal stress’ nights is observed over the past two to three decades. Specifically, the average number of ‘no thermal stress’ nights decreased by approximately 30% after the 2000s (from 70 to 50 nights). Notably, only 17 nights with uninterrupted ‘no thermal stress’ conditions were detected during summer 2024, a year assigned as the warmest year on record globally and in Athens, according to the historical climatic record of NOA [6].

4. Conclusions

Our results underpin the degradation of the urban thermal environment in the city of Athens and the increased risks to human health from continuous exposure to heat-related stress, especially after the 2000s. The increasing duration of exposure to uninterrupted heat stress conditions applies for both daytime and nighttime conditions. The striking decline in the frequency of nights with uninterrupted thermal comfort undermines sleep quality and duration, with serious impacts on human health, both physically and mentally.