The excess in geopotential difference over the standard geopotential difference [at a standard specific volume at 35 parts per thousand (‰) and 0 degrees C] between isobaric surfaces. See: Geopotential And Geopotential Topography. 2 )D = I *dp1 Pwhere p is the pressure and *, the specific volume anomaly. P1 2 and P are the pressures at the two surfaces