What must accompany visibility being less than 6 statute miles?

When visibility is reported as being less than 6 statute miles, it must be accompanied by a reason for the reduced visibility. This is critical for flight operations and safety, as knowing the cause of limited visibility can help pilots make informed decisions when preparing for takeoff, landing, or navigating in the airspace. Common reasons might include fog, rain, smoke, or other atmospheric phenomena that significantly affect visibility.

In aviation reporting, particularly in METAR reports, providing the reason for reduced visibility complements the visibility data and enhances situational awareness. This allows pilots and air traffic control to understand potential hazards associated with the weather conditions at that time. Visibility reports without reasons do not convey the necessary context for safe operations.

Including a reason helps to provide a clearer picture of the environmental conditions, which is essential for effective flight planning and for ensuring the safety of aircraft operations.