Question:

How do I tell ferret that NaN means "missing value" in a .cdf file?

Explanation:

If a variable in a NetCDF file contains NaN values, and if NaN is listed as the missing-value attribute for the variable, Ferret will recognize that and treat the variable accordingly.

If the netCDF file has NaN's among the values of a variable, but does not have NaN listed as the missing_value or _FillValue attribute, then to use the data with NaN as the missing-value flag, use the Ferret command to set the bad value for the variable to NaN.

yes? set var/bad=NaN varname