In order for fog to be observed or forecast, what must the visibility be?

Fog is defined as a cloud that reduces visibility at the Earth's surface. For fog to be officially recognized or forecasted, visibility must be less than 5/8ths of a mile (approximately 1,000 meters). This standard arises because at visibility levels greater than 5/8ths of a mile, the atmospheric conditions are generally considered to be clear enough that fog would not significantly impact visibility.

Therefore, the answer which states that fog is observed or forecast when visibility is less than 5/8ths of a mile is correct. Options suggesting a visibility limit of less than 5 miles, less than 1 mile, or less than 2 miles are not precise because they do not conform to the specific threshold that defines fog. It's the 5/8ths of a mile criterion that separates fog conditions from other less impactful reductions in visibility. Understanding this measurement is crucial for flight operations, meteorology, and safety in navigation.