Hail: an extreme phenomenon at the centre of meteorological research

In recent days, several areas of Italy have been hit by violent hailstorms, with exceptionally large hailstones causing damage to crops, vehicles, and infrastructure. Forecasts for the coming hours still indicate favourable conditions for the development of intense convective phenomena, typical of the spring season, which is increasingly marked by extreme atmospheric instability.  But how exactly does hail form? What risks does it pose? And what tools does modern meteorology have to monitor and predict it? 

From cloud to ground: how hail forms and why it matters more and more 

Hail is a form of solid precipitation consisting of ice particles (so-called hailstones) that develop within towering convective systems, typically cumulonimbus clouds. Hailstones form through complex accretion processes, in the presence of supercooled water and strong updrafts that repeatedly carry them through the cloud. This continuous up-and-down motion allows them to accumulate layers of ice until they reach a size large enough to fall to the ground. 

The conditions that favour hail formation include: 

• Strong atmospheric instability 

• High humidity in the lower layers 

• Updrafts exceeding 15 m/s 

• Presence of supercooled water down to -40 °C in the upper part of the cloud 

Although hail events are generally localized and short-lived, they can be extremely intense and cause significant damage. They typically occur during the summer season, when conditions for convective development are more favourable. The increasing frequency and intensity of such phenomena is indirectly linked to the effects of climate change, which amplifies extreme atmospheric variability. 

Hail damage affects several sectors: 

• Agriculture: even small hailstones can compromise entire crops 

• Infrastructure: roofs, vehicles, and facilities are vulnerable to large hailstones 

• Public safety: the sudden intensity of the phenomenon can pose a danger to exposed individuals lacking shelter 

Forecasting hail: between current technologies and future innovations 

Hail forecasting is one of the most complex challenges for operational meteorology, due to the intricacy of the parameters involved and the small spatial and temporal scale on which these events occur. Currently, the most effective monitoring and forecasting tools include: 

• Weather radars, capable of detecting high reflectivity areas compatible with the presence of hailstones 

• Weather satellites, enabling large-scale observation of the vertical structure of convective clouds 

• Nowcasting algorithms and predictive models based on machine learning techniques, which integrate real-time data to improve forecast accuracy 

Looking to the future, upcoming European satellite missions — such as Meteosat Third Generation (MTG) — will provide even more advanced tools for observing thunderstorm systems, including: 

• Lightning Imager, for real-time detection of atmospheric electrical discharges 

• Sub-millimetre microwave sensors, capable of analysing thin ice clouds and deep convective systems 

In parallel, the use of low-cost micro-satellites (CubeSats) will allow for more frequent data acquisition and higher spatial resolution, enhancing the ability to identify hail-prone areas. 

In this context, CIMA Research Foundation is actively engaged in operational research, using a weather model that assimilates radar data and the development of nowcasting algorithms capable of providing frequently updated forecasts. A concrete example is the MAGDA Project – recently concluded – which demonstrated the effectiveness of these technologies in more accurately predicting the formation of potentially hail-producing storm cells. Learn more about the project's results: