Sometimes you don't know the size of all dimensions when you create a file, or you would like to arbitrarily extend the file along one of the dimensions.

For example, model output usually has a time dimension. Rather than specifying that there will be forty-two output times when creating the file, you might like to create it with one time, and then add data for additional times, until you wanted to stop.

For this purpose netCDF provides the unlimited dimension. By specifying a length of “unlimited” when defining a dimension, you indicate to netCDF that the dimension may be extended, and its length may increase.

In netCDF classic files, there can only be one unlimited dimension, and it must be declared first in the list of dimensions for a variable.

For programmers, the unlimited dimension will correspond with the slowest-varying dimension. In C this is the first dimension of an array, in Fortran, the last.

The third example in this tutorial, pres_temp_4D, demonstrates how to write and read data one time step at a time along an unlimited dimension in a classic netCDF file. See pres_temp_4D.

In netCDF-4/HDF5 files, any number of unlimited dimensions may be used, and there is no restriction as to where they appear in a variable's list of dimension IDs.