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{{Refimprove|date=May 2010}}
In [[physical oceanography]] and [[fluid dynamics]], the '''wind stress''' is the [[shear stress]] exerted by the [[wind]] on the [[free surface|surface]] of large bodies of water – such as [[ocean]]s, [[sea]]s, [[estuary|estuaries]] and [[lake]]s. It is the [[force]] component [[parallel (geometry)|parallel]] to the surface, per unit [[area]], as applied by the wind on the water surface. The wind stress is affected by the [[wind speed]], the shape of the [[wind wave]]s and the [[atmospheric stratification]]. It is one of the components of the [[air–sea interaction]], with others being the [[atmospheric pressure]] on the water surface, as well as the exchange of [[heat]] and [[mass]] between the water and the atmosphere.

==Dynamics==
The air blowing parallel to a water body imparts motion to the surface water by shear action.
This is a downward transfer of [[momentum (physics)|momentum]] from the air to the water that ultimately generates a drift current underneath.
The water surface also deforms under the action of wind and becomes wavy for increasing wind speeds, which modifies the grip that the wind has on the surface itself.
The mechanics of the interaction between wind and water becomes thus increasingly complex.
The [[Beaufort scale]], for example, shows the correspondence between wind speed and sea states.

The magnitude of this shear force per unit contact area (<math>\tau</math>, [[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 (<math>U_h</math>) in the form

:<math>\tau_\text{wind} = \rho_\text{air} C_D U_h^2, </math>

where <math>\rho_\text{air}</math> is the density of the air; <math>C_D</math> 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.<ref>Stuart D. Smith, Coefficients for sea surface wind stress, heat flux, and wind profiles as a function of wind speed and temperature, Journal of Geophysical Research, Vol. 93, No. C12, P15,467, doi:10.1029/JC093iC12p15467</ref>

The expression of <math>C_D</math> contains, in first place, the correction to dependency on the second power of <math>U_h</math> for different ranges of <math>U_h</math>.
The functional form of <math>C_D</math> is determined by an empirical formula that is determined from experiments in the laboratory and/or in the field.
Different formulas have been established by various authors for different wind-speed ranges and taking into account the mechanics of the wind to varying degrees of detail.

==References==
{{Reflist}}

{{DEFAULTSORT:Wind Stress}}
[[Category:Fluid dynamics]]
[[Category:Physical oceanography]]


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