Feulner also asserts that there may be two simultaneous effects: (i) that
the precipitable water content, pyranometry and the pyrheliometry measurements exhibit pronounced regular seasonal variations, and (ii) the seasonal distribution of sunspot numbers can give the impression of a change with solar activity.
By accounting for these aspects, he subtracted the median of the measurements (
precipitable water content, etc) for the corresponding calendar month for all the data, and excluded the years with volcanic activity.