I am trying to calculate zonal stats as table with a python script for 45 .grib files from the ERA-40 Monthly Mean of Daily Mean database (http://data-portal.ecmwf.int/data/d/era40_moda/) for certain country shapefiles. I know that the data is in Kelvin rather than Celsius. But when I do it manually it's not working. I also have the same data in NetCDF format if that changes anything. But even in nc format, the zonal stats are not computing. Does anyone have any experience with using .grib and calculating zonal stats? Help please!