The prevailing visibility roughly represents the average visibility.
It is the greatest distance that can be seen throughout at least half the horizon circle.
The areas could comprise contiguous or non-contiguous sectors.
The lowest visibility observed will also be reported if the visibility in any direction is either:
a) Less than 1500 metres
b) Less than 50% of the prevailing visibility.
If the lowest visibility is observed in more than one direction then the most operationally significant direction will be reported.
When visibility is fluctuating rapidly and the prevailing visibility cannot be determined then only the lowest visibility will be reported without direction.
e.g. If visibility near the airport is 900 meters in the North East quadrant, 5 km in South East, 3 km in the South West and 4 km in the North West quadrant then what would the prevailing visibility be reported as?
The maximum is 5 and the second highest is 4, so prevailing visibility reported will be the more restrictive of the two i.e. 4 km.
The visibility (900m) in one particular direction i.e. NE is less than 1500 and less than half the prevailing visibility so it will be reported together with its direction.
So for the above example, the reported visibility format will be 4000 0900NE.
RVR – Runway Visual Range
The maximum distance in the direction of takeoff or landing at which the runway, or specified lights delineating the runway, can be seen from a position on the centreline corresponding to the average eye level of a pilot at touchdown.
– RVR is not normally reported if it is 1500m or more.
– Between 1500 and 800m it is reported in steps of 100m.
– Between 800 and 200m it is reported in steps of 50m.
– Between 0 and 200m it is reported in steps of 25m.
– e.g. R36L/P1500: Runway 36 Left touch-down RVR is more than 1500m.
– If RVR is more than the maximum that the equipment is calibrated, then that maximum is given preceded by P (plus).
– If it is less than the minimum, the minimum is given preceded by M (minus).
– If the RVR has been steady the group can be followed by N (No change).
– If it has been changing rapidly then the group is followed by “U” for up or “D” for down.
– If it has been very variable over the 10 minute observation period, the maximum and minimum is given separated by a “V”.
– RVR is not normally recorded or reported if it is more than 1500m.
– METAR reports only touchdown RVR.
– ATIS and ATC voice warnings reports mid-point and stop-end RVR.
Exam Question Tips:
Arrow In general, the meteorological visibility during rainfall is “Greater” as compared to during drizzle.
Arrow Flight visibility from the cockpit during approach in a tropical downpour can decrease to minimal tens of metres
Arrow In unstable air, surface visibility is most likely to be restricted by showers of rain or snow
Arrow Reduction in visibility due to precipitation depends on precipitation intensity and type
Arrow In the weather pattern behind a cold front, the visibility outside precipitation is good and the precipitation is showers
Arrow Conditions necessary for vertical visibility to be reported is when sky is obscured by fog or heavy precipitation and the height of the cloud base cannot be measured.
Arrow The meteorological visibility is generally less than the RVR in homogeneous fog.
Arrow The visibility transmitted in a METAR is the lowest observed in a 360 deg scan from the meteorological station.
Arrow Units kt / 100 feet are used to report vertical wind shear.