How should the presence of fog be reported in a METAR?

In a METAR report, fog is specifically characterized by visibility of less than 1 statute mile (SM). This definition is crucial because it provides standardized criteria for recognizing fog conditions that could impact visibility for pilots and other aviation personnel.

When visibility drops below this threshold, it is essential to report it as fog, as it can significantly affect flight operations and safety. The use of the term "fog" in this context ensures clarity and consistency throughout METAR reports. This clarity is vital for pilots making decisions regarding flight safety and navigation.

The other terms like "mist" or "scattered fog" do not accurately convey the critical conditions associated with reduced visibility. "Mist" generally refers to reduced visibility that is greater than that which qualifies as fog but still affects visibility. "Scattered fog" is not a standard term used in reports, and "density changes" does not refer to visibility conditions directly related to fog.

In summary, reporting fog with visibility less than 1 SM in a METAR follows established aviation standards that prioritize accurate and actionable weather reporting for safety.