An AIRMET Sierra is issued to warn pilots of:

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.