Visibility Flashcards
What is CAVOK and when is it reported?
Cloud and Visibility OK - METAR code which replaces visibility, RVR, present weather and cloud groups when the following conditions exist:
- ) Prevailing visibility is 10km or more.
- ) No minimum visibility reported.
- ) No cloud below 5000ft or the minimum sector altitude - whichever is greater.
- ) No towering cumulus (TCU) or cumulonimbus (CB) clouds.
- ) No significant weather at or in the vicinity of the aerodrome.
What is Prevailing Visibility?
The highest visibility reached within at least half the horizon circle or within at least half the surface of the aerodrome.
What is Minimum Visibility?
Reported in addition to the prevailing visibility when the lowest visibility in any direction is less than half the prevailing visibility or less than 1500m.
N.B.
- Must be accompanied with a direction.
- If more than one direction, then most operationally significant is to be reported.
- No requirement to report minimum visibility if it is 10km or more.
How is visibility coded?
4 digits expressed in metres when the visibility is less than 10km and coded as 9999 if the visibility is 10km or more..
It is rounded down as follows:
- Nearest 50m when the visibility is less than 800m
- Nearest 100m when the visibility is 800m or more but less than 5000m
- Nearest 1000m when the visibility is 5000m or more but less than 10km
N.B.
- A visibility of less than 50m is encoded as 0000
- A visibility of 5000m or less must be explained by a present weather group.
At what height should visibility be assessed?
At a height of 1.5m above the ground. This should be taken into account if assessed from higher up.
What equipment is used to measure visibility?
Visiometer.
N.B.
- A manual estimation of visibility must always be taken to verify automated reading. This is because the visiometer only measures the visibility in a small area and assumes this to be representative of a general area.
