In Aerodynamics, skin friction is the component of parasitic drag http://en.wikipedia.org/wiki/Parasitic_drag arising from the friction of the fluid against the "skin" of the object that is moving through it. Skin friction is a function of the interaction between the fluid and the skin of the body, as well as the wetted surface, or the area of the surface of the body that would become wet if sprayed with water flowing in the wind. As with other components of parasitic drag, skin friction follows the drag equation http://en.wikipedia.org/wiki/Drag_equation and rises with the square of the velocity.