friction

(Or frictional force.) The mechanical resistive force offered by one medium or body to the relative motion of another medium or body in contact with the first.

Solid bodies in relative motion display sliding and rolling friction that depend upon the forces pressing the bodies together, but that are nearly independent of the shapes or relative speeds of the bodies. The resistance of fluids to the relative motion of a solid body is, however, dependent upon the relative speed and the shape of the body, as well as upon the character of the flow itself (
see drag). Turbulent resistance of the earth on the atmosphere (surface friction) has been represented as proportional to both the first and second power of the speed of the low-level winds. The first representation leads to the cross-isobar frictional wind and to an estimate of the low- level frictional convergence, while the second representation has been employed in studies of momentum and energy abstraction from the atmosphere at the earth's surface. The word "friction" is often inappropriately used in place of turbulent drag when describing the differences between the atmospheric boundary layer and the rest of the troposphere.