What is a "WS" Sigmet?

A "WS" Sigmet refers to a special report of significant meteorological conditions that pose a hazard to aircraft operations. Sigmets are issued to provide pilots and air traffic controllers with crucial information about severe weather phenomena that could impact flight safety, such as turbulence, severe thunderstorms, icing, or volcanic ash. These reports are significant because they help ensure that pilots are aware of potentially hazardous conditions along their flight path.

The other options do not accurately describe a "WS" Sigmet. While clear skies may be beneficial for flying, they do not represent significant weather events that warrant a Sigmet. Additionally, a warning of light turbulence is not encompassed within the scope of a Sigmet, which deals with severe or significant conditions. Lastly, river fog, while it can impact visibility, is not typically covered under the severe weather criteria that a Sigmet focuses on. Hence, the choice that defines a "WS" Sigmet as a special report of significant meteorological conditions is the correct understanding of this aviation term.