An AIRMET Sierra is issued to warn pilots of:
AModerate turbulence and strong surface winds.
BIFR conditions (ceiling below 1,000 feet and/or visibility below 3 miles) and mountain obscuration.✓
CModerate icing not associated with thunderstorms.
Why →AIRMETs come in three types: Sierra (IFR conditions and mountain obscuration), Tango (turbulence, strong surface winds, low-level wind shear), and Zulu (icing and freezing levels). AIRMET Sierra indicates ceilings expected below 1,000 feet and/or visibility below 3 miles over 50% or more of an area.
The trap →Three AIRMET types. Three different hazards. S = sky obscured/IFR. T = Turbulence. Z = Zero degrees (freezing/icing). Mixing them up is the classic test trap.
Field note →An active AIRMET Sierra over your planned flight area means IFR conditions. Ceilings and visibility below VFR minimums. Consider it a strong go/no-go input even if your immediate location currently looks clear.
SOURCE → FAA Aviation Weather Services / PHAK Chapter 13CHECKED JUL 16ACS III.A.K1MED