play / tactics / dirty air
Clear air is natural wind flowing onto the boat's sail undisturbed by any turbulence that may be due to the presence of another boat. Dirty air is disturbed air that first flows onto another boat before reaching your boat's sails thus reducing its speed. This means that in upwind the boat clear ahead gets clear air and can give dirty air to the trailing boat. In downwind the tables are turned and the boat clear ahead receives dirty wind from the trailing boat. Dirty air may also affect your point of sail but in tacking duels we only consider the effect of dirty air in slowing down another boat, and not the effect on its point of sail.