Wind stress

(meteorology) The drag or tangential force per unit area exerted on the surface of the earth by the adjacent layer of moving air.

The drag or tangential force per unit area exerted on the surface of the Earth by the adjacent layer of moving air. Erosion of ground surfaces and the production of waves on water surfaces are manifestations of wind stress. Surface wind stress determines the exchange of momentum between the Earth and the atmosphere and exerts a strong influence on the typical variation of wind through the lowest kilometer of the atmosphere. Estimated values of the surface wind stress range up to several dynes per square centimeter (0.1 pascal), depending on the nature of the surface and the character of the adjacent airflow. See also Meteorology.

Significant stresses arise within the lower atmosphere because of the strong shear of the wind between the slowly moving air near the ground and the more rapidly moving air a kilometer above and because of the turbulent nature of the airflow in this region. The turbulent eddies referred to here have characteristic dimensions ranging up to a few hundreds of meters.

Wind pressure is the force exerted by the wind per unit area of solid surface exposed normal to the wind direction and is also known as dynamic pressure. In contrast to shearing stresses, the wind pressure arises from the difference in pressure between the windward and lee sides of the exposed surface. Wind pressure thus represents a substantial force when the wind speed is high. See also Wind.

The drag or tangential force of the wind on the sea is expressed in units of dynes per square centimeter or micronewtons per square meter but is normally taken to represent the mean drag over an undefined area, perhaps several kilometers square, containing many waves. It is usually related to an appropriate time and space average of the wind near the sea surface (at 10 m above the mean level, for example).

The drag coefficient over the sea is an important quantity in both meteorology and oceanography since it relates the wind speed to the drag, which generates ocean waves, drives the ocean currents, and sets the scale of the atmospheric turbulence that transfers water vapor and heat from the ocean to the atmosphere to provide the energy for clouds and weather systems. The drag coefficient of the sea surface depends on the wave field and on the turbulent structure of the flow in the air and the water. Present knowledge of the complicated fluid mechanics involved is not sufficient to allow theoretical calculation of it. See also Atmospheric general circulation; Maritime meteorology; Ocean circulation; Ocean waves.

There is substantial agreement that the drag of the wind on the sea is small relative to that of a fixed soil surface with the same geometry. It is largely independent of the fetch and so seems to depends less on the larger waves than on the short waves and ripples. Surface-active agents, which affect the shortest waves, may therefore be important.