How is the visibility reported when it is below the minimum threshold?

When visibility falls below the minimum threshold for reporting, it is not reported. This means that the observation does not provide a specific visibility value because it is below the level at which accurate measurement can be reliably made. The rationale is that providing an inaccurate or unquantifiable measurement could lead to misunderstandings and possible safety risks in aviation operations.

In contrast, prevailing visibility and the reporting of zero statute miles (0sm) would indicate that there is some degree of visibility that can be quantified or that specific criteria have been met, while reporting the highest visibility observed is also not applicable when visibility is below the minimum threshold. In such cases, the most responsible action is to refrain from reporting visibility altogether to ensure that pilots and air traffic control have the most accurate and useful information for safe operations.