- 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 ofgravity withlatitude 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.At an elevation of h, the geopotential is defined as
Phi = int_0^h g(phi,z),dz, ,
where g(phi,z) is the acceleration due to gravity, phi 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 g_0 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 andair density (which is very difficult to measure) in the equations.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.Further reading
Hofmann-Wellenhof, B. and Moritz, H. Physical Geodesy, 2005. ISBN 3211235841
Eskinazi, S. Fluid Mechanics and Thermodynamics of our Environment, 1975. ISBN 0122425405
External links
* [http://www.joseph-bartlo.net/articles/011099.htm Height & Pressure coordinates]
* [http://www.joseph-bartlo.net/articles/012199.htm Upper Air Charts]
* [http://www.joseph-bartlo.net/articles/021799.htm Upper Air Chart Analysis]
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