From AMS Glossary
- After U.S. weather observing practice, the height ascribed to the lowest layer of clouds or obscuring phenomena when it is reported as broken, overcast, or obscuration and not classified "thin" or "partial." The ceiling is termed unlimited when the foregoing conditions are not satisfied. Whenever the height of a cirriform cloud layer is unknown, a slant / is reported in lieu of a height value. At all other times, the ceiling is expressed in feet above the surface, which is a horizontal plane with an elevation above sea level equal to the airport elevation. At stations where this does not apply, "surface" refers to the ground elevation at the point of observation.
For obscurations, the ceiling height represents vertical visibility into the obscuring phenomena rather than the height of the base, as in the case of clouds or obscuring phenomena aloft. In an aviation weather observation, the ceiling height is always preceded by a letter that designates the ceiling classification.