Hailstorm blocks roads in northern Spain

On Twitter on Wednesday evening, I saw some amazing footage of a hailstorm that hit town of Almazan, just outside the city of Soria in northern Spain.

The hailstorm lasted around 45 minutes and caused flooding of homes and businesses, as well as blocking roads, leading to traffic chaos.

There were reports of hail one metre deep next to the Duero river. Local police, the civil guard and firefighters used snow ploughs to help clear the streets. But you’re probably thinking, how can hail in summer when it’s hot? 

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

But what you sometimes see 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

Video: YouTube