How does hail form?

How it works

Hail usually comes from cumulonimbus clouds, the towering thunderstorm clouds with strong updrafts: rising air currents that act like a giant hair dryer blowing upwards. High in the cloud it is well below 0 °C, and there are supercooled droplets, which are water colder than freezing that has not turned to ice yet.

A small ice pellet tossed around by the updraft bumps into these droplets, which freeze onto it instantly, layer after layer. Cut a big hailstone in half and you can often see rings of clear and cloudy ice, like an onion, recording its journey through the cloud.

When a hailstone grows too heavy for the updraft to hold, it falls. Stronger updrafts can hold bigger hailstones. Large hail mostly comes with strong thunderstorms in warmer months, not in the depths of winter. Most hailstones are about the size of a pea or smaller, but severe storms can produce much bigger ones.

Where to see it in New Zealand

New Zealand weather forecasts often mention showers with hail, especially when cold air sweeps up from the south; this hail is usually small. Bigger hail tends to come with strong thunderstorms and can damage crops and orchards, so some growers put hail netting over their fruit trees.

Try it together

Once the hail has stopped and there has been no thunder for a while, ask an adult to help you collect a few hailstones in a cup. Compare their sizes, hold one up to the light to look for rings, and time how long they take to melt. Never go outside while hail is falling.