The rescuers trying to stop an avalanche

An avalanche is when a large amount of snow slips and falls down a mountain.

They can be very dangerous - and people like Mark Diggins from the Scottish Avalanche Information Service try and spot where they might happen.

He works in the Cairngorms - a mountain range in Scotland that gets lots of snow.

He shows BBC weatherman John Hammond what he does to try and spot where an avalanche could happen, so he can make it safe for people who want to use the mountain.