Some parts of Australia have seen a lot of hail falling this spring, and some of the pieces have been big enough to cause some damage. But how do these big hailstones form?

It all starts with a thunderstorm.

If you’ve seen the steam rising from a pot of boiling water, you might be able to imagine this happening on a large scale in the atmosphere. During a thunderstorm warm, moist air is carried high up into the atmosphere in an updraft. The upper parts of the atmosphere are very cold, and if the updraft carries moisture and rain droplets high enough then they can freeze. This is where hail begins to form.

The size of the hailstones depends on the strength of the updraft. Once the hail begins to freeze it is kept floating in the air by the updraft of warmer air. The updraft brings more moisture which freezes onto the hailstone and it grows bigger. When the hailstone is too big for the updraft to hold up, it falls back down to Earth. If the updraft is weak, it will only be able to hold up small hailstones. However, if the updraft is very strong then is will be able to hold up much larger hailstones.

The size of the hailstones, and the amount of damage they could cause, all comes back to that thunderstorm updraft. Thunderstorms also have a downdraft, where cooler air sinks down towards the earth’s surface. If a storm has a stronger updraft than a downdraft, the storm will produce a lot of rain or hail. If the downdraft is stronger, the storm tends to have less rain and stronger and potentially damaging winds.

The largest hailstone ever recorded fell in South Dakota, USA, during 2010 and measured 20cm across and weighed almost 900g.