Definition
A widespread condition in which mountains and mountain passes are hidden by clouds, precipitation, smoke, haze, or other weather phenomena, making visual flight through or over mountainous terrain unsafe. It is one of the conditions that triggers an AIRMET Sierra advisory.
Plain English
A large area where mountains are blocked from view by clouds or weather, so a pilot flying visually cannot see the high ground or safely fly through the passes.
Context Anchor
Seen in inflight weather advisories and preflight weather briefings, especially in or near mountainous areas.
Derivation
Obscurement comes from the Latin obscurare, meaning to darken or hide. In aviation weather, an obscurement is anything that hides terrain or the sky from view. Extensive simply means widespread, covering a large area rather than one isolated peak.
Why Pilots Care
VFR pilots may suddenly lose visual reference to terrain, raising the risk of controlled flight into terrain or spatial disorientation.
Grounding Statement
Picture a mountain ridge that should be visible ahead, but clouds or haze have covered it so completely that the terrain blends into the sky.
Intuition Check
Extensive does not mean every mountain is hidden; it means the problem covers a broad enough area to matter operationally. Obscurement does not just mean “a little less clear”; it means the terrain may be hidden from view.
Example Sentence 1
The preflight briefing included an AIRMET Sierra for extensive mountain obscurement across the Sierra Nevada, so the pilot delayed the flight until the next morning.
Example Sentence 2
With extensive mountain obscurement forecast, the pilot filed IFR to maintain safe terrain clearance instead of attempting visual navigation.