What are NetCDF files and how can I decode them?

Before downloading data check if your processing software supports the GRIB and/or NetCDF formats. If your software supports both, use GRIB as first choice. If your software supports only NetCDF or if you run into problems with GRIB, use the NetCDF format.

NetCDF (Network Common Data Form) is a set of software libraries and self-describing, machine-independent data formats that support the creation, access, and sharing of array-oriented scientific data. NetCDF is commonly used to store and distribute scientific data. The NetCDF software was developed at the Unidata Program Center in Boulder, Colorado, USA (Unidata NetCDF Factsheet; Also see Wikipedia article). NetCDF files usually have the extension .nc.

An agreement about how to name and describe the data in a NetCDF file is a NetCDF Convention. The most widely used is the CF (Climate and Forecast Metadata) Convention (see CF full definition).

NetCDF files which are CF compliant can be interpreted by the widest range of software tools to read, process and visualise the data (e.g. Metview, NCView, Xconv).

To decode NetCDF files, there is an official NetCDF Application Programming Interface (API) with interfaces in Fortran, C and C++ available from Unidata. Unidata NetCDF API also comes with some useful command-line tools (e.g. ncdump -h file.nc gives a nice summary of file contents - see ncdump guide).

There are ways also to convert a NetCDF file to ASCII or text (e.g. netcdf4excel).

For writing NetCDF files, please check through Unidata 6 Best Practices (6.8 Packed Data Values and 6.9 Missing Data Values are of particular interest).

NetCDF version 3 format is widely used. In 2008, an enhanced version, NetCDF4, was introduced.

NOTE: The NetCDF format is not formally supported by ECMWF.

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