It's time we got used to hailstorms

Hailstorms occur more often in certain parts of the world due to the morphology of the land. They are most likely in areas with high mountain landscapes, which affect the circulation of air masses in the atmosphere. In Italy, which has two important mountain chains - the Alps and the Apennines - hailstorms are more common than in other countries, and are particularly prevalent in the Po Valley.

But there are other factors that also affect the probability of hailstorms, such as atmospheric conditions, and the phenomenon is more common at certain times of year than others. In Italy, hailstorms are traditionally seen in the period from April to October, but in recent years they have also occurred in the subsequent months.

There is still relatively little understanding of the phenomenon, due to the challenges in gathering data. Until a few decades ago, studies into hailstorms were very limited because of the short nature of the events and small windows of opportunity to monitor them. At the same time, effective measurement tools were not widespread. However, from the start of the new millennium this changed, with the use of satellites and meteorological radar equipment enabling detection of hailstones in the clouds across a wide area, even over the seas and oceans.

These tools do not track the hailstones that fall to the ground but provide data on the probability of whether hail will fall. Climate scientists are then able to gather information helping them to understand the frequency of hail events in a specific geographic region.

From 1999 to 2021, various studies have demonstrated an increase in hailstorms in the Mediterranean basin. This applies to both “traditional” hailstorms (those with hailstones that measure 2–10 cm) and “extreme” hailstorms (hailstones with a diameter greater than 10 cm). The use of satellites has also made it possible to demonstrate that in the Mediterranean, in the last twenty years alone, this meteorological phenomenon has increased in frequency by 30 percentage points. In fact, the surface temperature of the Mediterranean sea, which influences the probability of hailstorms, has long been rising due to climate change.

Together with other factors linked to global warming, the conditions required for hailstorms are increasingly common, and this upward trend is unlikely to alter going forward.

The issue should not be underestimated, because hailstorms can cause serious damage to crops, vehicles, homes and infrastructure, amongst other problems. The associated costs can be very high, particularly in regions where agriculture is vital to the economy. In summer 2023, Friuli Venezia Giulia, historically one of worst-affected Italian regions, set a new European record for the hailstone with the largest diameter, at 19 cm.

Various experimental techniques exist or are currently under development to prevent or mitigate the effects of hailstorms. These include use of “hail cannons” to interrupt the formation of hail or the use of chemicals to modify clouds. The progress made possible by weather-forecasting technology has also enabled development of alert systems to warn of approaching hailstorms.