In recent days the Marche coast between Fano and San Benedetto del Tronto was hit by a violent wave of bad weather, characterized by hailstorms of exceptional dimensions. Dozens of people were injured, and the hail caused extensive damage to homes, vehicles and agricultural crops, prompting the administrations of the municipalities involved to urge the declaration of a state of emergency. However, the Marche region is not an isolated case: similar events have been repeated several times in recent years, especially in Northern Italy. And many are now wondering whether climate change is making these extreme weather events more and more common, and whether giant hail will be our new normal in the future. Let’s see how things are.
How hail is born
To understand the phenomena that lead to the formation of hailstones of extraordinary dimensions it is necessary to analyze the conditions that characterize the interior of storm clouds. In particular cumulonimbus clouds, enormous clouds that can exceed 10 kilometers in height. The process from which the hailstones are born originates with the drops of water coming from the lower and warmer layers of the cloud, which are pushed upwards by powerful upward currents, thus encountering the graupel – or condensation nuclei – ice crystals present in the upper layers of the cumulonimbus, where temperatures are well below zero.
This encounter leads the graupels to become covered in droplets of water which freeze upon contact, and therefore begin an up and down movement inside the cloud under the push of the winds and gravity, which gradually increase their size and weight by incorporating other drops of liquid water, until the mass of the ice grain overcomes the resistance of the ascending currents, and falls to the ground in the form of hail. Under normal conditions the grains that form measure a few millimetres.
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But what determines the final size of the grain is the strength of the upward currents: in the most intense storm systems these currents can exceed 100 kilometers per hour, and in similar conditions the ice grains can remain suspended in the cloud for a prolonged time, making continuous ups and downs between areas with different temperatures and humidity concentrations, continuing to increase in size. In events like the one that hit the Marche, the grains easily reach several centimeters in diameter, with dimensions similar to those of a golf or even tennis ball. However, it is not unusual for even greater masses to be reached, with grains which, as happened in Northern Italy in 2023, can weigh up to half a kilo or more.
Climate changes
In recent years, the progressive warming of the Earth’s atmosphere has accelerated the frequency and severity of extreme weather phenomena in every corner of the planet. Triggering increasingly violent and unpredictable events, from exceptional heat waves to torrential rainfall. Hailstorms are no exception: storms capable of spilling hailstones of devastating dimensions to the ground no longer represent a rare exception, but an increasingly recurring threat to deal with.
How do temperatures affect the size of hailstones? It’s easy to say: if on the one hand the rise in temperature can cause the smallest ice fragments to melt during the fall, transforming hail into simple rain, on the other hand it provides an enormous quantity of energy and water vapor which can fuel more intense storms. This energy surplus allows updrafts to support the growth of colossal hailstones, making giant hailstones increasingly frequent and dangerous for infrastructure, agriculture and communities. The result? The more traditional hailstorms, with small grains, may become increasingly rare, and those with colossal grains more and more common.
This is confirmed by a study published in Nature last May, which examined the relationship between the progressive increase in average global temperatures and the atmosphere’s ability to generate destructive hailstorms. The research, carried out by researchers from Peking University and Central Michigan University, highlights how global warming increases the amount of water vapor present in the lower layers of the atmosphere and the potential energy available. Using data from fourteen thousand hailstorms recorded in the past, the team of scholars estimated that the economic damage caused by hailstorms worldwide will increase between 36 and 42 percent by the end of the century, based on different greenhouse gas emission scenarios.
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The research also confirms the phenomenon we were discussing earlier: the increase in water vapor and heat in the lower layers of the atmosphere will reduce hailstorms with stones less than three centimeters in diameter by 12 percent, due to the complete fusion of the hail during the fall, while the frequency of those with giant stones, equal to or greater than three centimeters, will increase drastically, with increases between 37 and 52 percent.
Another fundamental element that emerged from the investigation concerns the geographical distribution of this threat, which will not affect the planet uniformly. The increase in risk will be concentrated mainly in medium and high latitudes, directly affecting areas such as Europe and North America. In contrast, an overall decline in the risk of hailstorms is expected in tropical and equatorial regions. For us Italians, however, the danger will become more and more frequent: in an increasingly hotter world, the Mediterranean is one of the areas most exposed to the effects of climate change, and giant and destructive hailstorms will unfortunately be a phenomenon we will have to learn to live with.