http://www.ace.mmu.ac.uk/eae/Climate...ing_Winds.html
Prevailing Winds
The direction of wind is measured in terms of where the air is coming from. A northerly wind blows air from north to south. A southwesterly wind blows air from the southwest to the northeast.
The prevailing wind is the wind that blows most frequently across a particularly region. Different regions on Earth have different prevailing wind directions which are dependent upon the nature of the general circulation of the atmosphere and the latitudinal wind zones.