Geopotential height

Geopotential height or geopotential altitude is a vertical coordinate referenced to Earth's mean sea level, an adjustment to geometric height (altitude above mean sea level) that accounts for the variation of gravity with latitude and altitude. Thus, it can be considered a "gravity-adjusted height". It is the altitude all aircraft's barometric altimeters are calibrated to.

Definition

At an elevation of $h$, the geopotential is defined as:

$\Phi (h)=\int _{0}^{h}g(\phi ,z)\,dz\,,$

where $g(\phi ,z)$ is the acceleration due to gravity, $\phi$ is latitude, and $z$ is the geometric elevation. Thus geopotential is the gravitational potential energy per unit mass at that elevation.

The geopotential height is:

${Z_{g}}(h)={\frac {\Phi (h)}{g_{0}}}\,,$

which normalizes the geopotential to $g_{0}$ = 9.80665 m/s², the standard gravity at mean sea level.

Usage

Geopotential height analysis on the North American Mesoscale Model (NAM) at 500 hPa.

Geophysical sciences such as meteorology often prefer to express the horizontal pressure gradient force as the gradient of geopotential along a constant-pressure surface, because then it has the properties of a conservative force. For example, the primitive equations which weather forecast models solve use hydrostatic pressure as a vertical coordinate, and express the slopes of those pressure surfaces in terms of geopotential height.

A plot of geopotential height for a single pressure level in the atmosphere 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 National Weather Service defines geopotential height as:

"...roughly the height above sea level of a pressure level. For example, if a station reports that the 500 mb [i.e. millibar] height at its location is 5600 m, it means that the level of the atmosphere over that station at which the atmospheric pressure is 500 mb is 5600 meters above sea level. This is an estimated height based on temperature and pressure data."

See also

• Atmospheric model
• Above mean sea level
• Dynamic height

Further reading

• Hofmann-Wellenhof, B. and Moritz, H. "Physical Geodesy", 2005. ISBN 3-211-23584-1
• Eskinazi, S. "Fluid Mechanics and Thermodynamics of our Environment", 1975. ISBN 0-12-242540-5

External links

• Media related to Geopotential height at Wikimedia Commons