Definition
AIRMET Sierra is the AIRMET issued for instrument flight rules (IFR) conditions and mountain obscuration. It warns of ceilings less than 1,000 feet and/or visibility less than 3 statute miles affecting over 50 percent of the area at one time, and of extensive mountain obscuration caused by clouds, precipitation, smoke, haze, or other phenomena.
Plain English
A weather advisory that tells pilots about widespread low clouds, poor visibility, or mountains hidden by weather — conditions that make visual flying unsafe and can hide rising terrain.
Context Anchor
Seen in aviation weather products as “AIRMET Sierra.”
Derivation
The AIRMET system uses phonetic alphabet names (Sierra, Tango, Zulu) as memorable labels for each hazard type. Sierra is simply the radio-alphabet letter S, chosen as a clean, unambiguous tag — not because it stands for any specific word.
Why Pilots Care
An active AIRMET Sierra tells pilots that low ceilings or hidden terrain may require an instrument rating, alternate routing, or delayed departure for safety.
Intuition Check
Do not read Sierra here as just a mountain range or just a radio spelling word. In an AIRMET, Sierra is a weather-advisory category for low-visibility, low-cloud, and mountain-obscuration hazards.
Example Sentence 1
The briefer noted an active AIRMET Sierra over the route, with ceilings below 1,000 feet across the valley, so the pilot postponed the VFR departure.
Example Sentence 2
Because of the AIRMET Sierra, the pilot chose a lower route through the valley instead of crossing the ridges.