From AMS Glossary
In surface weather observations, a term used to denote one or more of the following: 1) the amount of sky covered but not necessarily concealed by clouds or by obscuring phenomena aloft; 2) the amount of sky concealed by obscuring phenomena that reach the ground; or 3) the amount of sky covered or concealed by a combination of 1) and 2).
Opaque sky cover is the amount of sky completely hidden by clouds or obscuring phenomena, while total sky cover includes this plus the amount of sky covered but not concealed (transparent). Sky cover, for any level aloft, is described as thin if the ratio of transparent to total sky cover at and below that level is one-half or more. Sky cover is reported in tenths, so that 0.0 indicates a clear sky and 1.0 (or 10/10) indicates a completely covered sky. Amount of sky cover for any given level is determined according to the summation principle. The following classifications of sky cover are used in aviation weather observations: clear, scattered, broken, overcast, partial obscuration, obscuration.