Many of us have experienced those hot summer days which start off with sunshine, before towering cumulonimbus clouds bubble up through the day, eventually bringing thunder, lightning and torrential downpours. In some instances, those thunderous clouds can also deliver hail – small balls of ice that fall to the ground. But how does hail form?
When clouds reach the dizzy heights of cumulonimbus status, their peaks can ascend to 10-15km in the sky. By this point, they aren’t full of just ice crystals, but also supercooled water droplets; water still in liquid form, despite its temperature being below freezing.
Within cumulonimbus clouds, the air is very turbulent – moving up and down, causing updraughts and downdraughts, which are strong enough to bounce ice crystals around inside the cloud. As these ice crystals move around, they gather extra layers of ice, allowing a hailstone to form and grow.
Not to be tried at home, but if you sliced a hailstone down the middle, it would have concentric rings like a tree, denoting each layer of ice gathered on its journey.
For as long as the updraughts and downdraughts are strong enough to carry the hail, it will remain inside the cloud and continue to get bigger. However, when the hail becomes heavy enough, updraughts can’t hold it up anymore, and it falls to the ground.
So how does the ball of ice manage to make it to the surface without melting, when summer air temperatures can be in the range of 25 to 30°C? The answer is the speed at which it falls. Small hailstones typically fall to the ground at a speed of 15-40km/h. This speedy descent means that the ice doesn’t have enough time to melt before reaching the surface.
Read more:
- Why don’t hailstorms last as long as rainstorms?
- How can my cat know that a thunderstorm is on its way an hour before I do?
- How to read a weather map: Do you know your isobars from your weather fronts?
- Why does thunderstorm rain contain more nitrogen than ordinary rain?
Asked by: Lara Eaton, Manchester
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