So estimate the "water content of the atmosphere" and post your answer.

Sorry, it's only now that I checked here again for an answer to my
initial question. (Since my question didn't get a reply for a while, I
actually lost hope to receive some help through this forum.)

@Szczepan: That's interesting. Unfortunately such measurements are not
(yet?) done or at least the results not yet published from the
different meteorological stations.

@Bill: Yes, I was asking for an *estimation*. Thank you for providing
the link to this paper. Will check now how well the results of this
fitted function compare with some extreme cases.