What does 'M' signify when reporting negative altitudes for clouds?

When reporting negative altitudes for clouds, the letter 'M' signifies a measurement below ground level. This notation is used in aviation meteorology to indicate that the base of the cloud layer is lower than the reference point, which is the surface of the earth. For instance, if a cloud is reported as being at "M500," it means the base of that cloud is 500 feet below the ground. This can occur in situations like when there are low valleys or during certain meteorological conditions where clouds are present in depressions or basements.

This choice effectively communicates the significance of the 'M' designation in the context of observations, helping pilots and meteorologists understand that they are dealing with cloud bases not just at an altitude reference above sea level, but rather below the level of the ground itself.