How are low, medium and high cloud cover defined?

Let sigma = pressure / surface pressure.

• Low cloud cover is for 1.0 > sigma > 0.8
• Medium cloud cover is for 0.8 >= sigma > 0.45
• High cloud cover is for 0.45 >= sigma

The low cloud cover (LCC) in ERA-Interim is underestimated if there is both cloud in the profile either side of the the medium cloud cover (MCC)/LCC boundary and the cloud fraction is less in the lower layer than in the higher layer. This problem also applies to MCC for the high cloud cover (HCC)/MCC interface.