http://www.faa.gov/library/manuals/a...a/CH%2002a.pdf RUNWAY VISUAL RANGE
Runway visual range (RVR) is an instrumentally
derived value, based on standard calibrations,
that represents the horizontal distance a pilot will see down
the runway from the approach end. It is based on the
sighting of either high intensity runway lights or on the
visual contrast of other targets whichever yields the
greater visual range. RVR, in contrast to prevailing or
runway visibility, is based on what a pilot in a moving
aircraft should see looking down the runway. RVR is
reported in hundreds of feet, so the values must be converted
to statute miles if RVR is not being reported.
[Figure 2-11]
This visibility measurement is updated
every minute; therefore, the most accurate visibility
report will come from the local controller instead of a
routine weather report. Transmissometers near the runway
measure visibility for the RVR report. If multiple
transmissometers are installed, they provide reports for
multiple locations, including touchdown RVR, mid-
RVR, and roll-out RVR. RVR visibility may be reported
as RVR 5-5-5. This directly relates to the multiple locations
from which RVR is reported and indicates 500 feet
visibility at touchdown RVR, 500 at mid-RVR, and 500 at
the roll-out RVR stations.
RVR is the primary
visibility measurement used by Part
121 and 135 operators, with specific visibility reports
and controlling values outlined in their respective
OpsSpecs. Under their OpsSpecs agreements, the operator
must have specific, current RVR reports, if available,
to proceed with an instrument departure. OpsSpecs also
outline which visibility report is controlling in various
departure scenarios.