Definition
AIRMET SIERRA is the specific AIRMET issued for instrument flight rules (IFR) conditions and mountain obscuration. It warns of widespread ceilings less than 1,000 feet and/or visibility less than 3 statute miles affecting more than 50 percent of an area at one time, and of mountain passes or ridges obscured by clouds, precipitation, or haze.
Plain English
It's the weather advisory that tells pilots where the cloud bases are low, where visibility is poor, and where mountains are hidden in clouds or haze.
Context Anchor
You will see AIRMET SIERRA during a preflight weather briefing, especially when planning an IFR flight route through areas of low clouds, poor visibility, or mountainous terrain.
Derivation
AIRMET stands for Airmen's Meteorological Information. The three AIRMET types are given phonetic alphabet names so they're easy to call out by radio: SIERRA covers IFR and mountain obscuration, TANGO covers turbulence, and ZULU covers icing. SIERRA was simply the next available phonetic letter when the system was set up — it has no meaning beyond identifying this category.
Why Pilots Care
It signals the need to file IFR or adjust the route to avoid reduced visibility and terrain risks.
Grounding Statement
Picture planning a route through hills while the forecast shows low clouds covering a large area; AIRMET SIERRA is the weather product warning you that terrain or forward visibility may be hard to see.
Intuition Check
Sierra does not mean the Sierra Nevada mountains here. It is the aviation category label for low-visibility conditions and mountain obscuration.
Example Sentence 1
During his preflight briefing, he noted an AIRMET SIERRA covering the entire route, with ceilings forecast below 1,000 feet across the valley.
Example Sentence 2
We delayed departure after noting an AIRMET SIERRA that would obscure the mountain passes.