Vertical visibility (VV)

When the sky is obscured — by fog, heavy snow, dust or smoke — there is no identifiable cloud base to report, because the obscuration begins at or near the surface and has no distinct underside. Vertical visibility is reported instead: how far upward an observer can see into it.

The group is VV followed by three digits in hundreds of feet, so VV002 is a vertical visibility of 200 ft. When the value cannot be determined at all, the digits are replaced by solidi: VV/// means the sky is obscured but no figure is available, which is common from automatic stations.

Vertical visibility is not a ceiling and should not be read as one, although operationally it is often treated as the more limiting of the two. A cloud base is the underside of a discrete layer with clear air beneath it; vertical visibility describes a medium you are already inside. The distinction matters on approach: there is no layer to break out of, only obscuration that thins with height.