Turbulence can be caused by many different things - thunderclouds, low level high winds, jet streams and thermal activity close to the ground to name a few. Of these only thermal activity close to the ground can really be forecast with any certainty in advance. If the sun is not up, there will be little or no thermal activity close to the ground - but this only relates to the very short period of time the aeroplane spends close to the ground. It can be argued that there will be less thermally triggered clouds at night but the really nasty thunder clouds are there 24 hours a day. Jet streams tend to be stronger in the winter months in the northern hemisphere due to a larger thermal gradient from the equator to the poles. Similarly in the northern hemisphere winter you tend to get more low pressure systems across the uk which lead tend lead to high wind speeds close to the ground.

So all in all it's almost impossible to say whether it is best to fly at day or night or winter or summer to avoid turbulence. Rest assured though BA and every airline do their very best to make the flight as smooth as possible. One final word, it is always important to wear your seat belt at ALL times. Clear Air Turbulence is hard to predict and can happen with no warning. The chances of being injured by turbulence are dramatically increased by not wearing the belt - that is why we always remind you to wear the belt in the captain's welcome aboard PA.