Thin
From Glossary of Meteorology
thin
As used in aviation weather observations, descriptive of a sky cover that is predominantly transparent.
According to the summation principle, at any level, if the ratio of the transparent sky cover to the total sky cover (opaque plus transparent) is one-half or more, then the cloud layer at that level must be classified as "thin." It is denoted by the symbol "-" preceding the appropriate sky cover symbol.
