Climatology and convective mode of severe hail in the United Kingdom

Abstract

Severe or large hail, with diameter ≥ 20 mm, is a hazard associated with severe convective storms (SCS) that can cause significant damage. In the UK, the rarity and small footprint of severe hail events makes obtaining well-documented hail reports difficult, and the reports are spread across multiple databases. In this study, three databases of UK severe hail reports are merged for the first time. The combined dataset set (1979–2022), comprising >800 reports, is used to investigate interannual variability and the seasonal, spatial and size distributions of severe hail. The seasonal cycle peaks in early–mid summer, and the peak month has shifted from June to July since around 2005. The distribution of reported hail size is exponential, with a slower decay (larger hail) during summer. The time of day, basic convective mode (isolated, clustered or linear), and presence or absence of supercellular characteristics are assessed for 274 of the reports since 2006, using composite radar rainrate data. The diurnal cycle is strong year-round, peaking during the late afternoon (1500–1800 UTC). 53% of severe hail events are associated with isolated cells, 33% with clusters, and 14% with linear storms. Around 35% of severe hail-producing storms are probable supercells, increasing to 70% for storms producing ≥40 mm hail. This demonstrates that the prevalence of supercells producing very large hail extends to temperate maritime climates. These results may be of relevance in other regions with a relatively low incidence of severe hail in the present climate. This comprehensive analysis of severe and potentially impactful hail in the UK provides novel insight into its characteristics, enabling improved assessment of climate risk from this hazard.