David Shirley of Wellington asks :-

Why is it bumpy flying through clouds?

Gavin Fisher, an meterological physicist with the National Institute for Water and Air, Auckland, responded.

We need to think about two things - firstly what is happening inside clouds, and secondly what is happening when we hurtle through them in an airplane.

Not all clouds are bumpy. There are two basic kinds - 'cumulus' type clouds (the bumpy ones, and they look bumpy too!), and 'stratus' type that are usually not bumpy. Clouds mostly form when the air somehow cools enough for water vapour to condense. In stratus clouds this can occur by the air moving over a cool surface, or by loosing heat to space. But in cumulus it occurs because the air is rising and starts to cool, often because sunshine heats the ground, causing the air to rise - which is why 'fluffy' cumulus are often seen on fine days. Once the air starts to rise, some things work to keep this going - such as the very act of forming a cloud. In some conditions this can cause thunderstorms with hail and lightening.

So inside a cumulus cloud we have all these parcels of air rising and mixing - sometimes they are just a few metres across, sometimes much larger. They can be mild updrafts - just a few metres per second - or huge gales - rising at 20-30 metres per second (even enough to stop a large cricket ball sized hailstone from falling out).

Now what happens it we fly an airplane through this? The wings 'feel' all these updrafts, which make the airplane 'bump'. It's much worse for small airplanes because they go from one updraft to the next, and there might be a big difference. It's not so bad for big airplanes, because most clouds have updraft sizes that are smaller than the wingspan.

It is extremely rare for airplanes to be in danger - they are designed to survive 'bumpiness' which is far worse than most of us will ever experience!