The files *.dec contain the raw data used form the main Umkehr program.
The columns are explained in the original files provided by WMO-WODC
The files nper???.dat (where ??? is the WMO number of the station) contain
The flagged data
The first column is the dcimal data
The 2nd 3rd and 4th is the day, month and year correspondingly.
Then it is the deseasonalized N-values in 
60,65,70,74,77,80,83,85,86.5,88,89,90 degrees SZA.
Then there are as many columns as the identified number of shifts.
(which are described in file ???.doc along with the starting date of data).
So if in the file ???.doc there are five lines with dates (month and year)
then this means that there are 4 shifts. (the first date is the beggining
of the period). The dates following are the dates of the 4 shifts.
Now back in the *.dat file if there is 1 in the colum then this means that 
the data for this date BELONG to a period within the corresponding shift.
If there is a 0 then this means that the data do NOT belong to the corresponding
shift. The number of the column of the 1's and 0's determine the number of 
the shift.