In a METAR, the prevailing visibility is the key value — Page 473, Lesson 443

Let’s start with the visibility reporting itself. In a METAR, the prevailing visibility is the key value. In the example you see here, it’s reported as 0800 — that means 800 metres. Now, what exactly is prevailing visibility? It’s the visibility value that is either reached or exceeded around at least half the horizon circle, or within at least half of the surface of the aerodrome. So if you look all around you, the visibility that covers at least half of that circle — that’s the prevailing visibility. But there’s an important extra rule. If the visibility in one particular direction — which is not the prevailing visibility — is less than 1500 metres, or less than 50% of the prevailing visibility, then you must also report the lowest visibility observed and its general direction. So you’re not just giving one number; you’re flagging a dangerous patch of poor visibility that could affect an approach or departure. Now, how do we code these numbers? Up to 10 kilometres, visibility is reported in metres. So 6000 means 6000 metres. Once visibility reaches 10 kilometres or more, the code figure used is 9999. And if visibility drops below 50 metres, the code is 0000. In our example, the prevailing visibility is 800 metres — that’s 0800 in the METAR. Let’s look at the full METAR line: METAR EGTK 231020Z 26012G25KT 220V300 0800 That’s Oxford Kidlington, on the 23rd at 1020 Zulu, wind from 260 degrees at 12 knots gusting 25, varying between 220 and 300, and the prevailing visibility is 800 metres. Now, in some cases, you’ll see runway visibility information in a METAR. That’s called Runway Visual Range — RVR. RVR is only given when either the horizontal visibility or the RVR itself is less than 1500 metres. The RVR group starts with the letter R, then the runway in use, then the threshold visibility in metres. Here’s the example for Oxford Kidlington: METAR EGTK 211020Z 26012G25KT 0800 R30/1100 So the prevailing visibility is still 800 metres, but the RVR at the threshold of Runway 30 is 1100 metres. That tells you that even though the general visibility is 800 metres, the visibility right at the runway threshold — where you need it for takeoff and landing — is 1100 metres. That’s a useful distinction for a pilot. So to summarise: prevailing visibility is the value covering at least half the horizon or half the aerodrome surface. If a different direction has visibility below 1500 metres or below 50% of the prevailing value, you report that lowest visibility and its direction. Visibility is in metres up to 10 km, then 9999, and below 50 metres is 0000. RVR is reported when visibility or RVR is under 1500 metres, and it’s coded with R, the runway, and the threshold visibility in metres.