admittance

In geophysics, the term admittance is often short for barometric admittance, 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, a rate derived by averaging many circumstances. It is also common to use a ratio derived through a straightforward analysis of local measurements taken at various times with various pressures. More complex models may incorporate additional 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 (the exact mass and its distribution being related to the atmosphere's pressure) and to atmospheric-pressure effects on the terrain.

Depending upon context, admittance may mean something more general than barometric admittance: the word is basically a synonym for the term correction (as used regarding gravity anomalies); the term admittance is generally used in cases where a formula has been devised to calculate such a correction from measurable data (such as the above-described example of a calculated barometric admittance based upon barometric pressure).