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Encyclopedia > Geopotential height

Geopotential height is a vertical coordinate referenced to Earth's mean sea level — an adjustment to geometric height (elevation above mean sea level) using the variation of gravity with latitude and elevation. Thus it can be considered a "gravity-adjusted height." One usually speaks of the geopotential height of a certain pressure level, which would correspond to the geopotential height necessary to reach the given pressure. For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ... Basic Definition In geography, the elevation of a geographic location is its height above mean sea level (or some other fixed point). ... Gravity is a force of attraction that acts between bodies that have mass. ... Latitude, usually denoted symbolically by the Greek letter phi, , gives the location of a place on Earth north or south of the equator. ...


At an elevation of h, the geopotential is defined as


Phi = int_0^h g(phi,z),dz, ,


where g(φ,z) is the acceleration due to gravity, φ is latitude, and z is the geometric elevation.


Thus, it is the gravitational potential energy per unit mass at that level. The geopotential height is


{Z_g} = frac{Phi}{g_{0}},,


where g0 is the standard gravity at mean sea level.


Geophysical scientists often use geopotential height rather than geometric height, because doing so in many cases makes analytical calculations more convenient. For example, the primitive equations which weather forecast models solve are more easily expressed in terms of geopotential than geometric height. Using the former eliminates centrifugal force and air density (which is very difficult to measure) in the equations. The primitive equations are a version of the Navier-Stokes equations that 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. ... The density of air, ρ (Greek: rho) (air density), is the mass per volume of Earths atmosphere, and is a useful value in aeronautics. ...


A plot of geopotential height for a single pressure level shows the troughs and ridges, Highs and Lows, which are typically seen on upper air charts. The geopotential thickness between pressure levels — difference of the 850 hPa and 1000 hPa geopotential heights for example — is proportional to mean virtual temperature in that layer. Geopotential height contours can be used to calculate the geostrophic wind, which is faster where the contours are more closely spaced and tangential to the geopotential height contours. The geostrophic wind is defined as the wind resulting from the balance between the Coriolis force and the pressure gradient force. ...


External links

  • Height & Pressure coordinates
  • Upper Air Charts
  • Upper Air Chart Analysis

  Results from FactBites:
 
Geopotential height - definition of Geopotential height in Encyclopedia (228 words)
This height depends both on the mass of air at that point (as is measured by surface pressure) and the vertical temperature distribution in the air column.
The result is that such a plot of geopotential height for a single level shows the midlatitude troughs and ridges, that is the Rossby wave pattern of cyclones and anticyclones, while the layer depth, between say, the 700 hPa and 500 hPa geopotential heights is proportional to the temperature in that layer.
Geopotential height contours can be used to calculate the geostrophic wind, which blows stronger where the contours are more closely spaced and tangential to the isolines.
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