In geophysics, the term admittance (or barometric admittance) refers to the apparent effect of atmospheric pressure on the measured force of gravity. Accommodation of this effect allows more reasonable use of the high precision of current state-of-the-art gravimeters. The classic rule of thumb is that the measurement varies by -0.3 μGal per mbar, which is basically a correlation averaged over many circumstances. It is common to use a ratio derived through a straight-forward analysis of local measurements over some time and various pressures. More complex models may incorporate more factors, and have been developed using harmonic analysis and associated power spectra.
The effect is presumed to be due to the mass of the nearby atmosphere indicated by the pressure, and to atmospheric-pressure effects on the terrain.
Depending upon context, admittance may mean something more general than barometric admittance, more or less a synonym for gravity anomaly, generally used when some formula has been devised to calculate it from measurable data (such as the above-described example of a calculated barometric admittance based upon barometric pressure).