From AMS Glossary
It is the magnitude of the wind vector component parallel to the heading(s) of an aircraft, averaged over the entire flight; positive if a tailwind, negative if a headwind:The wind factor accounts for the retarding effect of a crosswind.
Its numerical value is of the order of a few hundredths and is inversely dependent upon the depth of frictional influence (
see Ekman spiral) at that point in the ocean. In sufficiently shallow water, the actual depth is the controlling factor.