In a METAR report, what does "M" indicate before a visibility value?

In a METAR report, the letter "M" before a visibility value indicates that the visibility is measured in meters and that the reported value is less than the standard threshold for reporting visibility in statute miles. This notation is often found when visibility is low, providing a clear indication to pilots and meteorologists that the visibility condition is not favorable.

For example, if a METAR report states visibility as "M1", it means that the visibility is less than 1 meter. In contrast, visibility values without the "M" are typically reported in statute miles, which are more common in the United States. Understanding this distinction is crucial for interpreting METAR reports effectively, especially in aviation operations where visibility plays a critical role in flight safety.