Humid heatwaves in the UK: Meteorological drivers and health impacts

Heatwaves in the UK are projected to become more frequent and intense due to climate change. While the health risks of extreme heat are well documented, less is known about the effects of compound weather hazards, specifically the co-occurrence of high temperatures and elevated humidity. High humidity can substantially increase heat stress, yet its role in modifying health outcomes in the UK remains underexplored. This study addresses this research gap by investigating how and why humidity varies across UK heatwaves from a meteorological perspective for the first time and quantifying its impact on health. First, we identify historical heatwaves over the past four decades using a spatially extended reanalysis dataset. Second, we stratify these events into more humid and less humid categories and identify the key weather patterns and meteorological drivers leading to humid heatwaves. Third, using daily all-cause mortality and hospital admissions data, we conduct heat episode analyses to assess differential health impacts associated with isolated heatwaves versus those co-occurring with high humidity. Preliminary results suggest that humid heatwaves in the UK have distinct meteorological features which, through compensatory mechanisms, may moderate their impact on health outcomes. On average, more humid heatwaves exhibit higher wet-bulb temperatures and nighttime dry-bulb temperatures, but lower daytime dry-bulb temperatures and downward surface solar radiation, suggesting higher cloud cover. Furthermore, humid heatwave events tend to occur later in the summertime, when populations may be more acclimatised to heat. This work will provide new insights into the drivers of compound humid-heat events and their health implications in the UK, which can help to inform public health preparedness and climate resilience strategies.