Hail

Hail is ice that grew up inside a thunderstorm. Not frozen rain — a hailstone is built, layer by layer, on an assembly line powered by the storm's updraft.

>The assembly line

A small ice pellet high in the storm starts to fall, collecting a coat of supercooled water (liquid droplets colder than 0 °C, waiting for any excuse to freeze). Then the updraft catches it and hurls it back up, where the wet coat freezes solid. Fall, coat, rise, freeze — each loop adds a ring, like a tree trunk. Cut a large hailstone in half and you can count its trips.

The stone falls for good only when it's too heavy for the updraft to lift. That's the key insight: hail size is a scale for updraft strength.

• 1 cm (pea) — an ordinary storm, ~60 km/h updraft.
• 2–3 cm (walnut) — a strong storm; car dents begin.
• 4–5 cm (golf ball) — severe; almost always a rotating storm.
• 7 cm+ (tennis ball and up) — supercell territory; updrafts over 150 km/h. The European record is ~15 cm.

>Reading the sky

Large hail often arrives just before or beside the heaviest rain, from the part of the storm that looks strangely green-tinted — sunlight filtered through a deep core of suspended ice. On the radar, the tallest, most intense core of a supercell is the hail factory. And since giant hail needs the same rotating updraft that tornadoes do, a report of golf-ball hail is itself a warning: the storm has the engine for worse.