In the ECMWF model (IFS), snow is represented by an additional layer on top of the uppermost soil level. The whole grid box may not be covered in snow. The snow cover gives the fraction of the grid box that is covered in snow.
The method for calculating snow cover depends on the particular version of the IFS that was used to produce the data. The two methods used for ERA-Interim and ERA5, respectively, are given below.
For ERA-Interim and any IFS cycle before CY35R3, the snow cover (SC) was computed directly using snow water equivalent (ie parameter SD (141.128)) as:
For ERA5, the snow cover (SC) is computed as follows:
For more information on snow cover in the IFS and ECMWF datasets, see Part IV: Physical processes, of the IFS documentation: