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.