Hail
CSENHow hail forms
Why one storm drops rain and the next drops ice — and what decides how big the stones get.
Hail needs two things an ordinary shower does not have: a storm cloud with real vertical depth, and above all a strong enough updraft. That updraft is the only thing capable of holding a stone up long enough for it to grow at all.
High in the cloud sits a great deal of supercooled water — droplets below freezing that have not yet frozen because they have nothing to freeze onto. The moment they meet a small ice embryo they freeze onto it instantly. A hailstone therefore does not grow by rolling up snow; it grows by collecting droplet after droplet, each one freezing on contact.
How fast that freezing happens is what gives a stone its layers. Where water freezes instantly, air is trapped in the ice and the layer comes out opaque and milky — dry growth. Where it freezes more slowly, it spreads into a smooth film and the layer is clear — wet growth. The rings in a sliced hailstone are thus a record of the conditions it passed through, not a tally of trips up and down, as the story usually goes.
It starts falling the moment it is heavy enough that the updraft can no longer hold it. Hence the rule that makes hail such a good gauge of a storm: the stronger the updraft, the larger the stone it can keep aloft. A big hailstone is direct evidence of an exceptionally strong updraft, not a fluke.
It may not survive the trip down intact. Below the freezing level, where temperatures are above zero, the stone begins to melt, and smaller ones vanish entirely before reaching the ground. That is why only the biggest make it down on a warm day — and why the same storm can drop ice on a hilltop and nothing but water a few kilometres lower.
Hailstones are commonly between 5 mm and 5 cm across; anything smaller than 5 mm counts as graupel rather than hail. The record stone fell in South Dakota in 2018 and measured 23.6 centimetres.
On radar, a core capable of hail shows as very high reflectivity — the violet and white end of the scale. It is not proof, because radar measures reflectivity rather than ice itself, but the higher the value, the likelier it is ice rather than rain alone.
FAQ
Why is hail large one day and small the next?
The strength of the updraft decides. A stone stays in the cloud for as long as the updraft can carry it, so a stronger updraft holds a heavier stone and gives it more time to grow. Large hail therefore comes from storms with the strongest updrafts, typically supercells.
Why do hailstones have layers like an onion?
The layers come from alternating dry and wet growth. Water that freezes instantly traps air and makes a milky layer; water that freezes slowly makes a clear one. They record the conditions the stone passed through, not the number of round trips it made.
What is the difference between hail and graupel?
Size, with the boundary at 5 mm. Anything smaller counts as graupel, which tends to be softer and can fall outside thunderstorms — from spring showers, for instance.
Can hail fall in winter?
True hail only rarely, because it needs the strong convection typical of the warm half of the year. The small white pellets that fall in winter are usually graupel rather than hail.
Can you see hail coming on radar?
Roughly. Cores at the top of the reflectivity scale — violet through white — contain hail far more often than weaker ones. But radar measures reflectivity, not ice, so it is no guarantee, and an official ČHMÚ warning takes precedence.