Figure 1-12 -- Prevailing visibility.
When the prevailing visibility or tower visibility is less than 4 miles and the tower visibility
differs from the prevailing visibility, place the tower visibility in the remarks section of a
METAR or SPECI. In this case, the tower visibility is used to determine aircraft
approach and departure weather minimums.
The tower visibility is a prevailing visibility. The difference is
the prevailing visibility in the body of a METAR is the
visibility value observed at the surface observation point,
whereas the tower visibility is a prevailing visibility
observation taken at the tower cab level. Both are
Runway Visual Range (RVR) Group
The RVR is an instrumentally derived value that represents the horizontal distance a
pilot will see down the runway from the approach end.