Definition
A ceiling classification assigned by a weather observer when the sky is obscured by surface-based phenomena such as fog, smoke, heavy precipitation, or blowing snow, and the actual height of any cloud layer above cannot be determined. The reported value is the vertical visibility upward into the obscuring layer, not the height of a cloud base.
Plain English
When something on the ground — like fog or heavy snow — blocks the view straight up, the observer can't see how high the clouds really are. So instead, they report how far up you can see into the murk. That number is called an indefinite ceiling.
Context Anchor
Seen in airport weather reports and preflight weather briefings when low visibility weather is sitting on the airport.
Derivation
Indefinite' comes from the Latin indefinitus, meaning 'not fixed' or 'not clearly defined.' The term fits because the observer cannot fix a definite cloud base — only how far up the eye can penetrate the obscuration.
Why Pilots Care
Determines whether conditions meet VFR or IFR requirements and affects minimum safe altitudes and go/no-go decisions.
Analogy
It is like standing in thick fog and looking up. You cannot tell where the clouds begin; you can only tell how far upward your view goes before it disappears.
Grounding Statement
When fog or blowing snow starts at the ground and hides the sky, the reported ceiling may be the upward seeing distance, not a cloud height.
Intuition Check
Indefinite does not mean the ceiling is unimportant or simply unknown. It means the normal cloud ceiling cannot be clearly identified, so the report uses how far upward a person or sensor can see.
Example Sentence 1
The METAR reported an indefinite ceiling of 200 feet in fog, so the flight was held until conditions improved.
Example Sentence 2
With an indefinite ceiling reported, the approach required the pilot to use the published minimums based on vertical visibility.