Partial obscuration

From Glossary of Meteorology

partial obscuration

A situation when a part of the celestial dome is hidden by a weather phenomenon.

In U.S. weather observing procedures, this is the designation for sky cover when part (0.1– 0.9) of the sky is completely hidden by surface-based obscuring phenomena. It is encoded "-X" in aviation weather observations; it never, by itself, constitutes a ceiling, for the overhead vertical visibility is not restricted.
Compare obscuration, thin.