Apparent wind in knots and degrees. Note that a number of additional factors come into play when converting the measurements from the masthead anemometer into the true wind if a high degree of accuracy is required, including the following: In the presence of a current, the true wind is considered to be that measured on the craft drifting with the water over the bottom, and wind with respect to the sea bed as the "ground" or "geographical wind". The apparent wind on board (a boat) is often quoted as a speed measured by a masthead transducer containing an anemometer and wind vane
Wind stress and sea states. The magnitude of this shear force per unit contact area ("τ", shear stress) is estimated through wind-shear or wind-drag formulas. These formulas parametrize the shear stress as a function of the wind speed at a certain height above the surface ("U") in the form where formula_2 is the density of the air; "C" is a dimensionless quantity wind-drag coefficient and is a repository function for all remaining dependencies. The height at which the wind speed is referred to in wind-drag formulas is usually 10 meters above the water surface. The expression of "C" contains, in first place,