In a skydip, the atmospheric emission, as seen by the receiver (
), is
measured at equally spaced airmass (
).
is a combination of
the atmospheric emission
and of losses

each contribution computed from an equivalent atmospheric temperature and opacity

The zenith opacity can be written as a combination of a dry and wet components

(10) |

Assuming that and are independent of the elevation and that and are correctly modeled by an atmospheric model, we obtain that is a function of , , , and the airmass ( ). If and are assumed to be measured by other means, then and can be fitted through the measured couples ( , ).