Hailstones probably don't get their onion layers by bouncing up and down
Slice a big hailstone and you find rings like an onion. The old explanation said stones yo-yo repeatedly through the storm, gaining a coat each trip. Field studies and theory now show a single journey through patches of wetter and drier cloud is usually enough to build every layer.
Hail is solid precipitation made of balls or lumps of ice, distinct from the smaller, translucent ice pellets that Americans call sleet. A stone counts as hail from 5 millimetres across, and the biggest reach 15 centimetres and weigh over half a kilogram. It grows inside cumulonimbus clouds with fierce updrafts, lots of liquid water, great height and a thick layer below freezing, conditions that can also signal a tornado.
It starts as droplets carried up past the freezing level, where they become supercooled and freeze onto tiny particles. Updrafts of up to 180 kilometres per hour keep lifting the growing stone. Where it meets many supercooled droplets it gains a clear layer; where there is mostly vapour it picks up opaque white ice full of tiny trapped air bubbles. In so-called wet growth, released heat keeps the surface liquid and sticky, so stones can merge into irregular clumps. Only in multicell storms, when a stone is thrown from one cell into a stronger neighbour, does a second trip really occur, and even that is exceptional.
A stone keeps rising until the updraft can no longer hold it, which can take 30 minutes or more in storms whose tops exceed 10 kilometres, then keeps growing as it falls and starts melting in warmer air. Growth almost stops below minus 30 degrees Celsius, because supercooled drops become scarce. Stones can fall up to about 3.7 kilometres from the parent storm, sometimes out of apparently clear air beneath the anvil.
Geography matters. Hail favours continental interiors in the mid-latitudes, where the freezing level sits low, and it is rarer in the tropics despite far more thunderstorms, because warm air extends much higher there. Mountains push winds upward and shorten the fall, so ranges are hotspots; northern India suffered one of the deadliest hail events on record in 1888. In North America the corner where Colorado, Nebraska and Wyoming meet is called Hail Alley, and Cheyenne averages nine to ten hailstorms a season. Farmers' crops take the most frequent damage.
Source: Hail
