Flight turbulence: What is it, when does it happen and why?

Frequent flyers will be familiar with the sudden jolting that can happen when an aircraft gets into turbulence. It can move the plane and cause sudden changes in height.

Most turbulence occurs in cloud where there are up- and downdrafts of wind, according to BBC Weather’s Simon King, a former RAF officer.

Much of that will be fairly mild, but in bigger clouds – such as the cumulonimbus thunderstorm cloud – the chaotic movements of air can cause moderate or even severe turbulence.

There’s another type of turbulence called “clear air” turbulence – which as the name suggests is cloudless and can’t be seen. This is much more problematic as it’s very difficult to detect.

This type of turbulence happens around the jet stream, a fast flowing “river” of air that’s typically found at 40,000-60,000ft, aviation academic and commercial pilot Guy Gratton says.

You can easily have 100mph speed difference between the air in the jet stream and the surrounding air, he says. The friction around the jet stream between the slower and faster air causes turbulence. This is always there and it moves, making it difficult to avoid.

If you are flying from Europe to North America, for example, it’s hard to completely avoid it, Mr Gratton says, and this can result in periods of severe turbulence.