Sometimes there are missing values in the data, and some value is needed to represent them.

For example, what value do you put in a sea-surface temperature variable for points over land?

In netCDF, you can create an attribute for the variable (and of the same type as the variable) called “_FillValue” that contains a value that you have used for missing data. Applications that read the data file can use this to know how to represent these values.

Using attributes it is possible to capture metadata that would otherwise be separated from the data. Various conventions have been established. By using a set of conventions, a data producer is more likely to produce files that can be easily shared within the research community, and that contain enough details to be useful as a long-term archive. Conventions also make it easier to develop software that interprets information represented in data, because a convention selects one conventional way to represent information when multiple equivalent representations are possible.

For more information on _FillValue and other attribute conventions, see Appendix A: Attribute Conventions.

Climate and meteorological users are urged to follow the Climate and Forecast (CF) metadata conventions when producing data files. For more information about the CF conventions, see http://cf-pcmdi.llnl.gov.