For example, meteorological upper air analyses do not show isolines of pressure but of geopotential height. A specific barometric pressure is chosen for the plot (say, 500 hPa) and at every gridpoint the altitude of that pressure level above the reference ellipsoid (i.e. the mean sea level) is calculated. 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.
In atmospheric equations of motion, such as the primitive equations, geopotential height is used to eliminate centrifugal force and air density.