Turbulence

[lafrancaise]

I've finally got around to asking this question. Why is it, that when a plane goes through cloud there's turbulence? after all a cloud is only moisture....

[heathfield]

When a plane travels through the air, the air parts, flows round the plane, and comes together again. It doesn't come together smoothly, but in gusts and whirls. This is called turbulence.
It's exactly the same as you can see behind a car travelling fast on a wet road.

[JudgeJ]

Clouds are only a symptom of disturbed air. It is not the clouds themselves which provide the turbulence, but violent air movements.

These movements take place for a variety of reasons but most notably when air masses of different temperatures meet each other. Clouds are formed as a result of these meetings. So although, when you move through clouds in an aircraft, turbulence is very often present, it is the turbulence which forms the clouds, and not the other way around.

[Clanad]

To add a small addendum to Judge 1's information, not all clouds have noticeable turbulence associated. The cumulus type clouds (irregular shaped and tending to the vertical development) almost always have turbulence caused by the previous description. However, the stratus type clouds associated with longer lasting rain storms (the leaden colored, featureless clouds) are usually very smooth to fly through... at least in my nearly 30,000 hours of flight experience...