Under what visibility conditions can fog (FG) be reported in a METAR?

The visibility conditions under which fog can be reported in a METAR are defined as when the visibility is less than 1 statute mile. Fog occurs when the water vapor in the air condenses into tiny water droplets, leading to reduced visibility. Typically, a visibility of less than 1 mile is classified as fog in aviation meteorology.

In contrast, the other options do not correctly reflect the criteria for reporting fog. For instance, while visibility less than 5/8 miles may indicate dense fog, the more accurate threshold for reporting the presence of fog is less than 1 mile. Furthermore, fog can occur at any time, and is not solely dependent on precipitation conditions, which is why the choice of being only during rain is inaccurate. Thus, the selection that defines fog conditions by visibility being less than 1 statute mile is the correct one.