Indefinite ceiling

From Glossary of Meteorology

indefinite ceiling

(Formerly called ragged ceiling.) After U.S. weather observing practice, the ceiling classification applied when the reported ceiling value represents the vertical visibility upward into surface-based atmospheric phenomena (except precipitation).

Such phenomena include fog, blowing snow, and all of the lithometeors. All indefinite ceilings are estimations, but one of the following must be used as a guide: 1) the distance an observer can see vertically into the obstruction; 2) the height corresponding to the top of a ceiling-light beam; 3) the height at which a ceiling balloon completely disappears; 4) the height determined by the sensor algorithm at automated stations. The letters "VV" (vertical visibility) are used to designate an indefinite ceiling.