24 Jul 2013

How can it hail when it’s hot?

With violent thunderstorms over the UK in recent days, there have been some places that have experienced hail as well as torrential downpours of rain.

Quite a few people have asked me on Twitter, how is it be possible for hail, small balls of ice, to reach the ground with temperatures in excess of 20C?

So, I thought I’d write a blog to answer this question and give some insight into the processes that make this possible.

What is hail?

Hail is a solid form of precipitation that consists of small balls of ice. Each little ball of ice is known as a hailstone.

Hailstones are generally 5-10mm in diameter, but in extremely severe thunderstorms, hailstones can have a diameter as much as 15cm.

In order for them to form, a cumulonimbus cloud has to be present, as these are big and active enough to provide favourable conditions.

How does hail form?

Within a cumulonimbus cloud, there are many particles of ice and super-cooled water – water that remains in liquid form at temperatures below freezing due to a lack of condensation nuclei for them to freeze around.

Ice tends to be at near the top of the cloud, where the air temperature can be as low as minus 60C. Super-cooled water tends to be nearer the bottom half of the cloud, where temperatures are closer to freezing.

When a thunderstorm occurs, air moves violently up and down inside the cumulonimbus cloud – known as updraughts and downdraughts.

As a small particle of ice gets caught in these updraughts and downdraughts, layers of ice or water gather on its surface, causing it to grow in size and become a hailstone.

For as long as the updraughts and downdraughts are strong enough to carry the hailstone, it will continue to bounce around in the cloud, getting bigger.

However, when the hailstone is heavy enough, it will eventually no longer be able to be carried by the wind and will fall downwards – out of the cloud and towards the ground.

How can ice make it to the surface when it’s so warm?

Even in summer, you don’t have to go that high up for the temperature to drop to 0C. At the moment, the freezing level over the UK is just 3200 metres or so above the ground.

As hailstones drop from the cloud quickly, cooling the air around them as they fall, they are able to reach the ground intact.

But what you will notice is that they tend to gather in clumps. This is because they start melting on their way towards the ground as they fall through warm air – effectively sticking together.

If you have any other weather questions, please get in touch with me on Twitter – @liamdutton

Tweets by @liamdutton

One reader comment

  1. JACK says:

    I wish u would also give Imperial Measurements.

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