What does AIRMET Sierra describe?

AIRMET Sierra specifically addresses weather conditions that can significantly affect aviation safety, particularly in terms of visibility and flight conditions. It highlights IFR (Instrument Flight Rules) conditions, which refer to weather conditions that require pilots to fly using instruments rather than visual references due to low visibility. Additionally, AIRMET Sierra covers extensive mountain obscurations, indicating situations where mountains may be hidden from view due to weather, such as fog, clouds, or precipitation. This is particularly important for pilots flying in mountainous regions, as it can lead to navigational difficulties and increased hazards.

The other options describe different aviation weather concerns that do not fall under the specific description of AIRMET Sierra. Severe icing and moderate turbulence, for example, are covered under different AIRMET designations. Therefore, understanding AIRMET Sierra's focus on IFR conditions and mountain obscurations is crucial for pilots to prepare adequately for flying in adverse weather situations.