A hailstone the size of a golf ball or larger is not a by-product of an ordinary thunderstorm. It needs one specific combination of updraft strength and cloud depth that occurs only in the most powerful storm cells, the ones known as supercells.
The lift that lets the ice grow
It starts with a small ice nucleus, often no bigger than a grain of sand, forming in the cold upper part of the cloud. In an ordinary thundercloud that nucleus falls fairly quickly and melts into rain on the way down. Inside a supercell, however, there is an exceptionally strong and sustained updraft that carries the nucleus back up again and again rather than letting it fall.
On each pass through the supercooled water droplets in the cloud, another layer of ice is deposited on the stone. The longer the updraft keeps the stone aloft, the more layers accumulate, much like an onion. A hailstone can run that cycle several times before it finally becomes heavy enough that even the strongest updraft can no longer hold it.
A hailstone grows by riding repeatedly up and down inside the updraft of a storm cell, adding another layer of ice on each pass, much like the layers of an onion.
Why the Po Valley and southern Germany are hit hardest
Fist-sized hail takes updrafts often above 150 kilometres per hour (93 mph), a wind speed reached only in well-organised supercells. Those cells form preferentially where warm, humid air meets pronounced wind shear, meaning a change in wind direction and speed with height. Exactly those conditions occur frequently in late summer over northern Italy and southern Germany, where humid Mediterranean air meets cooler air masses arriving from the north.
Proximity to the Alps amplifies the effect further, because the mountains lift the rising air still higher and lay the groundwork for even stronger updrafts.
Why hail is not the same size everywhere inside the cell
Inside a supercell, the largest hail is not spread evenly. The heaviest stones usually fall in a relatively narrow strip, often right beside or behind the strongest part of the updraft, an area meteorologists call the hail corridor. That explains why two places only a few kilometres apart can have completely different experiences of the same storm: one is hit by golf-ball sized stones, while the other gets only heavy rain with no hail worth mentioning.
That tight spatial limit is also why even a precise regional severe weather warning cannot say which individual town inside the warned area will actually take the largest hail.
What the size of a hailstone really tells you
The size of a fallen hailstone is, after the fact, a direct indicator of the strength of the updraft that carried it. Golf-ball sized stones point to updrafts of roughly 100 kilometres per hour (62 mph), tennis-ball sized stones to considerably more. That information is valuable to meteorologists because it shows how intense a given cell really was, regardless of how it looked from the outside.
RainViewer shows the core of a storm cell live on the radar, including the strongest reflectivity values, which typically go hand in hand with large hail.
Track the cell, do not just watch the sky
A dark sky on its own says little about whether a storm will produce hail. The decisive information is in the structure of the cell itself, which is best followed on radar , long before the first hailstone actually falls.
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