RESEARCH ARTICLE A comprehensive analysis of hail events in Portugal: Climatology and consistency with atmospheric circulation João A. Santos 1 | Margarida Belo-Pereira 2 1 Centre for the Research and Technology of Agro- Environmental and Biological Sciences, CITAB, Universidade de Trás-os-Montes e Alto Douro, UTAD, Vila Real, Portugal 2 Instituto Português do Mar e da Atmosfera, Rua C do Aeroporto, Lisbon, Portugal Correspondence João A. Santos, Departamento de Física, Escola de Ciências e Tecnologia, Quinta de Prados, 5000-801 Vila Real, Portugal. Email: jsantos@utad.pt Funding information FEDER/COMPETE/POCI, Grant/Award Number: POCI-01-0145-FEDER-006958; Fundação para a Ciência e a Tecnologia, Grant/Award Number: UID/AGR/04033/2013; INNOVINE&WINE – Vineyard and Wine Innovation Platform, Grant/ Award Number: NORTE-01-0145-FEDER- 000038 A first hail climatology in mainland Portugal is provided, based on a network of 15 meteorological stations and over the period of 1971–2009 (39 years). Three hail sub-classes based on the SYNOP codes were considered (small hail, hail without thunder and hail with thunder). Furthermore, hail occurrences were also compared with thunderstorm, convection and precipitation days. Hail events are more fre- quent in northern Portugal during winter and spring (peaking in April). Both thun- derstorm and convection events present a maximum in April–May and a secondary peak in autumn (October). This secondary peak is frequently associated with con- vective storms without hailfall at the ground, as no clear autumnal maximum is recorded in the hail frequency, particularly in the case of small hail and hail with- out thunder. The typically higher temperatures in October than in February–April may lead to excessively high-freezing level heights that hamper hailfall at the sur- face. The dynamical mechanisms underlying hail occurrence in mainland Portugal were also analysed using eight three-dimensional weather types. The atmospheric conditions associated with three weather types: Western European Trough, Block- ing and Scandinavian Trough were responsible for ca. 75% of the hail events throughout Portugal. Overall, hail events are favoured either by extra-tropical depressions, with cold front passages over Portugal, or by upper-level troughs/lows over western Iberia, but preferably occurring in the period from December to May. These dynamical drivers promote instability conditions, which were also diagnosed by the convective available potential energy and total-totals index. KEYWORDS convection, hail, k-means, lightning, Portugal, small hail, thunderstorm, weather types 1 | INTRODUCTION Hail is a widely known meteorological hazard, potentially causing severe damages to property, such as buildings, struc- tures and automobiles (Brown et al., 2015), crops (Sánchez et al., 1996), aircrafts (Das et al., 2009), wind turbines (Macdonald et al., 2016), among others. Furthermore, hail- related economic losses have been increasing in different regions worldwide, for example, being recently estimated to be over $1 billion US dollars annually for the United States (e.g., Changnon, 1999; Cintineo et al., 2012). In Europe, the insurance costs of the hailstorms occurred on June 8–10, 2014 were estimated to be of 2.3 billion EUR in France, Bel- gium and Germany (Punge and Kunz, 2016). By definition, hail is the precipitation of transparent, partly or completely opaque particles of ice (hailstones), usu- ally spherical, conical, or irregular in form, with a diameter generally greater than 5 mm (WMO, 2012). Smaller particles of similar origin may also be classified as small hail or ice pellets, but other frozen hydrometeors are often misinter- preted as hail. Hail commonly occurs in the form of showers and is typically observed during heavy thunderstorms Received: 10 February 2018 Revised: 28 June 2018 Accepted: 3 July 2018 Published on: 23 August 2018 DOI: 10.1002/joc.5794 188 © 2018 Royal Meteorological Society wileyonlinelibrary.com/journal/joc Int J Climatol. 2019;39:188–205.