High-Resolution Temporal Variation of Thermal Discomfort Indices in the Eastern Mediterranean City of Athens, Greece, Since the Beginning of the 20th Century (1901–2024)

This study analyzes more than a century of hourly meteorological data (1901–2024) from the Thissio station in central Athens, Greece, to assess the long-term changes in human thermal discomfort. Three simple and widely used bioclimatic indices, Thom’s Discomfort Index (TDI), Humidex (HMDX), and Heat Index (HI), were calculated to capture the combined effects of air temperature and humidity. The results show a marked increase in the frequency, intensity, and duration of thermal discomfort since the 1980s, with a strong acceleration after 2000. The number of days with severe or dangerous heat stress has more than doubled compared with the mid-20th century, and periods of high discomfort now extend from June to September. The maximum values of HMDX and HI have exceeded critical health thresholds, highlighting increasing risks for the urban population. These findings demonstrate how rising temperature and humidity amplify heat stress in a Mediterranean city and emphasize the need for adaptation strategies in urban planning and public health to reduce vulnerability to extreme heat.