### Skydips and determination of the forward efficiency

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

(7)
And the atmospheric emission is the sum of the atmospheric emission in both receiver bands
(8)
each contribution computed from an equivalent atmospheric temperature and opacity
(9)
The zenith opacity can be written as a combination of a dry and wet components
(10)
where is the opacity due to the permanent components of the atmosphere (mainly oxygen) and is proportional to the varying amount of water vapor amount ( ) in the atmosphere:

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 ( , ).