From my book, How to Read a Tree,
The whole tree responds to wind and this includes the trunk. Most are ‘skinny into wind’. Walk a few times around a large mature tree in an open area and you will soon see how the trunk appears to grow fatter, then thinner, then fatter again. (This is one of the reasons that foresters record tree size by circumference and not diameter.) The trunk is skinniest when you look in line with the prevailing wind direction, from south-west to north-east in the UK, and it appears fattest when you are looking across the direction of the prevailing wind, from south-east to north-west.
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How to Read a Tree – The Book