Temporal Discomfort Variation in the Eastern Mediterranean city of Athens, Greece. Part 1: Analysis of High Resolution Measurements Since the Beginning of the 20th century (1901-2024).

Human health is directly influenced by environmental conditions associated with temperature, relative humidity, wind and incoming solar radiation. Among these, temperature and humidity exert the greatest influence on thermal comfort and human well-being. To assess thermal stress, various bioclimatic indices have been developed to simplify the complex interactions between atmospheric variables and human perception, enabling meaningful comparisons across different climatic regions. Thom’s Discomfort Index (TDI), introduced in 1957, is the most widely accepted and the first physiological index of its kind, primarily designed to describe discomfort at the population level (e.g. what percentage of people feel stressed), has been analyzed across the globe in areas with different climatology.

Athens has undergone continuous urbanization since the 1950s, characterized by rapid expansion and increased building density. This development has intensified the urban heat island effect, contributing to local warming and altering the climatic characteristics of the Athens basin, especially during summer, which are characterized by long periods of sunshine, high temperatures and varying levels of humidity. Concurrently, the city has experienced significant regional climate change since the mid-1980s, futher amplifying thermal stress conditions.

This study investigates the temporal evolution of thermal discomfort in the historic center of Athens, using one of the oldest, largest and most homogeneous urban meteorological datasets in the Mediterranean region, recorded at the National Observatory of Athens (Thissio station). The dataset spans more than a century (1901-2024) and is available at hourly resolution, allowing for a robust assessment of both long-term trends and short-term extremes. This high temporal resolution provides a clear advantage over studies based solely on daily observations. Within the Mediterranean climatic context of Athens, TDI index is particularly suitable for evaluating long-term variability and changes in thermal stress conditions.

This comprehensive analysis reveals a clear shift toward more frequent, intense and prolonged heat stress conditions since the 1980s, with a marked acceleration after 2000. In earlier decades, high discomfort conditions were sporadic and largely confined to July and August. After 1980, however, they became more systematic and extended from June to September, occasionally reaching October. Extreme discomfort levels (TDI≥32°C), which were virtually absent before 1980, have emerged during the last two decades. Additionally, the duration of the high-risk period has increased substantially, exceeding 90 days in some recent years, compared to fewer than 40 days during the early 20th century.

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.