Quote Originally Posted by Midgardsormr View Post
I had not known that. If you can explain it in a few words, what is the cause of this phenomenon?
First, I should note that this is only true in the northern hemisphere. It is the opposite in the southern. I have never had to forecast weather for the southern hemisphere so I forget about that.

Briefly, it is the combination of the pressure gradient force and the Coriolis force. The pressure gradient force causes air to flow from high pressure to low. The Coriolis force is a deflecting force caused by the rotation of the Earth. It causes moving air parcels to be deflected to the right in the northern hemisphere. Thus, air that is moving out from the center of an area of high pressure will be deflected to the right, or clockwise. Similarly, air moving into the center of an area of low pressure will also be deflected to the right, making it counterclockwise around the low in this case. In the southern hemisphere, the Coriolis force deflects parcels to the left, so clockwise and counterclockwise are reversed.