Quantifying Missed Heat Stress by Temperature-Only Definitions over Europe

Heatwave phenomena are associated with excess mortality. Their occurrence and frequency are projected to increase in future years. Therefore, it becomes urgent to develop tools and methods to better understand the association between heatwaves and their impacts on human health. This association relies greatly on the chosen definition of heatwave and the specific health condition under investigation.

Heatwave days are often defined using temperature-based quantile thresholds. However, other meteorological factors play a crucial role in shaping the physiological response of the human body during extreme heat. Multivariate model-based indicators such as the Universal Thermal Climate Index (UTCI) can be used to accurately estimate heat stress under specific hypotheses. Yet even these indices are frequently applied only to days pre‑selected by a temperature‑only criterion. This raises questions on the robustness of the results to changes in the heatwave definition, and it is unclear to what extent high heat stress days coincide with heatwave days.

The mismatch between extreme heat stress events according to UTCI and those identified by conventional definitions based on the simpler apparent temperature (AT) and two‑metre temperature (T2) is presented and discussed. The in-depth analysis has been based on available UTCI datasets (ERA5-Heat¹ and HiGTS²) for the period 2000-2023 over Europe and the Mediterranean region.

It has been found that around one quarter of the extreme UTCI days are not covered by extreme T2. In addition, from the study it emerges the presence of several strong heat stress hotspots in Southern Europe, e.g., in Spain and in the Po Valley. These have been identified as areas where each of the tested conventional heatwave definitions misses more than 10% of strong heat stress days. The use of AT in place of T2 mitigates this disagreement and may offer a low-cost alternative when UTCI is not available.

The days with the strongest physiological impact do not necessarily correspond to the hottest days in a season. This may impact current studies in the areas most affected by the disagreement, leading to an underestimation of the impacts. These findings also support the need to revisit extreme‑heat alerts, as well as the criteria that trigger financing mechanisms for heat-related losses.

¹ https://doi.org/10.1002/gdj3.102
² https://doi.org/10.1038/s41597-024-03966-x