geopotential thickness
From Glossary of Meteorology
The difference in the geopotential height of two constant-pressure surfaces in the atmosphere, proportional to the appropriately defined mean air temperature between the two surfaces:
where Δφ is the geopotential thickness; R the gas constant for air; p1 and p2 the pressure at the lower and upper isobaric surfaces, respectively; T the Kelvin temperature; and Tm the mean temperature.
