I want to aggregate monthly averages of temperature and precipitation over a 10-year period into weighted seasonal averages (DJF) for use in interpolation. I have downloaded Shapefiles for each month (i.e. DJF) and each year from NCAR (http://gisclimatechange.ucar.edu/). One way to do this would be to copy-paste the attribute table values and average that way but I'm sure there is a more efficient method. I'm wondering if QGIS has any sort of plugin or preexisting tool which could make the process easier.