How is fog defined in terms of visibility?

Fog is defined as a weather phenomenon that significantly reduces visibility, specifically to less than 5/8 statute miles (SM). This classification is important in aviation as fog can greatly impact flight operations, making it essential for pilots to understand visibility readings when encountering fog.

When visibility drops to this level or lower, it can create hazardous conditions for pilots and require special procedures or considerations for takeoff, landing, and navigation. Notably, the definition of fog is rooted in its ability to obstruct the view of the surroundings, which is quantified by the visibility measurement. Therefore, understanding that visibility under 5/8 SM constitutes fog helps pilots prepare for and mitigate risks associated with these conditions in flight operations.