Science Questions

What causes hail?

Tue, 7th Jan 2014

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Amos asked:

What causes hail? does it stay in the atmosphere? can it cause damage to the aeroplane?


Chris - Well, hail is effectively frozen rain and when water rises in the atmosphere on Hailstonewhat are called updrafts, so, when air is warmer and less dense, it tends to rise upwards into the sky. It will coalesce as droplets in clouds and if those droplets fall because the draft of air rising is not strong enough to hold them up against the pull downwards of gravity, then they'll fall down a bit. But then if another updraft comes along and pushes them back up again and they go through more cold air then because they're already a droplet, they can get frozen in the cloud. They come down again and go back up again. As they're doing that, they can continuously meet more damp air which will freeze on to the surface which is your hail stone. Normally, what happens is that that falls out of the cloud, gets down towards the Earth’s surface and falls through air that's warm. And so, it melts and it falls as rain. But if the air is already very cold on the way down, the ice doesn’t melt on the way down, so it just falls as a solid frozen – what we call hydrometeor, an ice crystal on the way down.


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