ncra averages record variables across an arbitrary number of input files. The record dimension is retained as a degenerate (size 1) dimension in the output variables.

Input files may vary in size, but each must have a record dimension. The record coordinate, if any, should be monotonic for (or else non-fatal warnings may be generated). Hyperslabs of the record dimension which include more than one file are handled correctly. ncra supports the stride argument to the -d hyperslab option for the record dimension only, stride is not supported for non-record dimensions.

ncra weights each record (e.g., time slice) in the input-files equally. ncra does not attempt to see if, say, the time coordinate is irregularly spaced and thus would require a weighted average in order to be a true time average.

Average files 85.nc, 86.nc, 89.nc along the record dimension, and store the results in 8589.nc:

ncra 85.nc 86.nc 87.nc 88.nc 89.nc 8589.nc ncra 8[56789].nc 8589.nc ncra -n 5,2,1 85.nc 8589.nc

Assume the files 85.nc, 86.nc, 89.nc each contain a record coordinate time of length 12 defined such that the third record in 86.nc contains data from March 1986, etc. NCO knows how to hyperslab the record dimension across files. Thus, to average data from December, 1985 through February, 1986:

ncra -d time,11,13 85.nc 86.nc 87.nc 8512_8602.nc ncra -F -d time,12,14 85.nc 86.nc 87.nc 8512_8602.nc

ncra -F -d time,3,,12 -v temperature 85.nc 86.nc 87.nc 858687_03.nc

Assume the time coordinate is incrementally numbered such that January, 1985 = 1 and December, 1989 = 60. Assuming ?? only expands to the five desired files, the following averages June, 1985--June, 1989:

ncra -d time,6.,54. ??.nc 8506_8906.nc

If the info and NCO programs are properly installed at your site, the command

info nco

should give you access to the complete manual, except for the TeX-intensive portions.