The geostrophic wind is defined as the wind resulting from the balance between the Coriolis force and the pressure gradient force. Frictional effects are neglected, which is usually a very good approximation for the synoptic scale instantaneous flow in the midlatitude mid-troposphere. However, although ageostrophic terms are relatively small, they are important for the time evolution of the flow. In physics, the Coriolis effect is an inertial force first described by Gaspard-Gustave Coriolis, a French scientist, in 1835. ... In physics, friction is the non-conservative resistive force that occurs when two surfaces travel along each other when forced together. ... synoptic literally means an overall view - hence Synoptic Gospels. ... The troposphere is the lowermost portion of Earths atmosphere and the one in which most weather phenomena occur. ...

The geostrophic wind (u_g,v_g) can be derived from the primitive equations, using the geostrophic approximation: The primitive equations are a version of the Navier-Stokes equations which describe hydrodynamical flow on the sphere under the assumptions that vertical motion is much smaller than horizontal motion (hydrostasis) and that the fluid layer depth is small compared to the radius of the sphere. ...

where g is the force of gravity (9.81 m/s^2), f is the Coriolis parameter (approximately 1e-4; varies with latitude) and Z is the geopotential height field. The validity of this approximation is dependent on the local Rossby number: It is invalid at the equator because f is zero there, and therefore generally not used in the tropics. It has been suggested that Law of universal gravitation be merged into this article or section. ... Hurricane Isabel east of the Bahamas on September 15, 2003. ... In the geosciences, geopotential height is a measure of geometric height that accounts for the dependence of gravity on latitude and height. ... The Rossby number, named for Carl-Gustav Arvid Rossby, is a dimensionless number used in describing geophysical phenomena in the oceans and atmosphere. ... A tropic is either of two circles of latitude: Tropic of Cancer, at 23½°N Tropic of Capricorn, at 23½°S Tropic is also the name of a town in Utah, United States. ...

Other variants of the equation are possible, e.g. using the pressure field instead of Z, but are slightly more complex.

See also

• thermal wind

The thermal wind is not actually a wind, but a wind difference between two pressure levels and , with . ...

External links

• Geostrophic approximation
• Definition of geostrophic wind