JAR OPS has a table giving a conversion factor for converting RVR to visibility. Generally speaking, the factors indicate that RVR will be equal to or greater than the associated visibility - I would have thought it was the other way round, but I'm hardly an expert.
Met Vis is the LOWEST vis observered in all directions. RVR is the vis observed only in the direction of the runway (at the average pilot's eye ht.....blah)
(1)I am on the threshold of R/W 27 looking west. Down the R/W I can see for 2000m, to the north I can see for 4000m , to the south I can see for 4000m and to the east I can see for 4000m. Now, here the met vis would be 2000m as would the RVR be.
(2) I am, again, stood on the threshold of R/W 27 looking west. Down the R/W I can see for 4000m, to the north I can see for 2000m, to the south I can see for 2000m and to the east I can see for 2000m. In this case the met vis would be 2000m; however, the RVR would be 4000m.
Hopefully you can see from these examples that if the direction of worst vis is in the same direction as the R/W RVR will equal met vis; however, the RVR can never be less than the met vis.
No doubt somebody will get all technical about the exact definitions of met vis and RVR, but hopefully that helps?
RVR - Runway Visual Range - the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. This value is normally determined by visibility sensors located alongside and higher than the center line of the runway. RVR is calculated from visibility, ambient light level, and runway light intensity
Visibility - The greatest distance under given weather conditions to which it is possible to see without instrumental assistance.
The important is: "RVR is calculated from....runway light intensity" So typically High Intensity Runway or Approach Lights are more visible than unlit objects and thats why RVR is greater than visibility.
Is this the bit in JAR-OPS about calculating an equivalent RVR from the met vis if RVR is not available?
If so, I would suggest that there is little, if any, real relationship between the two values. It is simply a way of controlling whether an approach can be made or continued in poor visibility. As such, the conversion is is just an approximation rather than an attempt to relate the two (different) measuring systems in a real-world situation.
BTW, met vis is now reported in terms of prevailing visibility, at least that is the ICAO standard. The exact rules escape me for the moment but it means that the vis in a met report may not be the lowest in any direction.
PS - Just my understanding as a simple controller.
Not so simple : Visibility. Visibility for aeronautical purposes is the greater of: a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background; b) the greatest distance at which lights in the vicinity of 1 000 candelas can be seen and identified against an unlit background. Note.— The two distances have different values in air of a given extinction coefficient, and the latter b) varies with the background illumination. The former a) is represented by the meteorological optical range (MOR). Runway visual range (RVR). The range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line. 188.8.131.52 Recommendation.— Runway visual range should be assessed at a height of approximately 2.5 m (7.5 ft) above the runway. 184.108.40.206 Recommendation.— Runway visual range should be assessed at a lateral distance from the runway centre line of not more than 120 m. The site for observations to be representative of the touchdown zone should be located about 300 m along the runway from the threshold. The sites for observations to be representative of the mid-point and stop-end of the runway should be located at a distance of 1 000 to 1 500 m along the runway from the threshold and at a distance of about 300 m from the other end of the runway. The exact position of these sites and, if necessary, additional sites should be decided after considering aeronautical, meteorological and climatological factors such as long runways, swamps and other fog-prone areas. 4.3.4 Averaging Where instrumented systems are used for the assessment of runway visual range, their output shall be updated at least every 60 seconds to permit the provision of current, representative values. The averaging period for runway visual range values shall be: a) 1 minute for local routine and special reports and for runway visual range displays in air traffic services units; and b) 10 minutes for METAR and SPECI, except that when the 10-minute period immediately preceding the observation includes a marked discontinuity in runway visual range values, only those values occurring after the discontinuity shall be used for obtaining mean values. 4.3.6 Reporting 220.127.116.11 In local routine and special reports and in METAR and SPECI, the runway visual range shall be reported in steps of 25 m when the runway visual range is less than 400 m; in steps of 50 m when it is between 400 m and 800 m; and in steps of 100 m when the runway visual range is more than 800 m. Any observed value which does not fit the reporting scale in use shall be rounded down to the nearest lower step in the scale. 18.104.22.168 Recommendation.— Fifty metres should be considered the lower limit and 2 000 metres the upper limit for runway visual range. Outside of these limits, local routine and special reports and METAR and SPECI should merely indicate that the runway visual range is less than 50 m or more than 2 000 m. -ICAO Annex 3 to the Convention on International Civil Aviation Meteorological Service for International Air Navigation
-Manual of Runway Visual Range Observing and Reporting Practices (Doc 9328)