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.