Meteorological Aerodrome Reports (METARs) — Page 480, Lesson 449

I want to walk you through how cloud information is reported in a METAR. This is a core part of reading an aerodrome weather report, and we need to get the details right because they directly affect flight operations. Let's start with cloud base. In a METAR, cloud base is always given as a three-digit figure that shows the height in hundreds of feet. So if you see a number like 019, that means 1,900 feet. Crucially, that height is measured above aerodrome level, using the current aerodrome QFE — that's the atmospheric pressure setting that gives you height above the aerodrome, not above mean sea level. Now, the amount of cloud is reported in eighths of the sky, which we call oktas. The code uses standard abbreviations: FEW for 1-2 oktas, SCT for 3-4 oktas, BKN for 5-7 oktas, and OVC for 8 oktas — completely overcast. So if you have 6 oktas of cloud at 1,900 feet above aerodrome level, that appears in the METAR as BKN019. If you have 8 oktas at five hundred feet, that becomes OVC005. Here's an important point: the only cloud types that are specified in a METAR are the significant convective clouds. Those are cumulonimbus, abbreviated CB, and towering cumulus, abbreviated TCU. These are the clouds that produce hazardous weather like thunderstorms, heavy showers, and turbulence. No other cloud types are named in the METAR. Let me give you a concrete example. Look at the code OVC020CB. This tells us the sky is overcast — 8 oktas — covered by convective cumulonimbus cloud, and the cloud base is 2,000 feet above aerodrome level. Now, if you see a weather group like +SHRA before that cloud group, that indicates the cloud is producing a heavy shower of rain. The plus sign means heavy, SH means shower, and RA is rain. But what if there's no cloud of operational significance? The term "operational significance" here means no CB or TCU, and no cloud at or below the greater of 5,000 feet or the highest minimum sector altitude. If those conditions are met, and CAVOK doesn't apply — we'll get to CAVOK shortly — then the code NSC is used. NSC stands for "no significant cloud." Now, there's a special case for when the sky is obscured. If the sky at an aerodrome is hidden for reasons other than cloud cover — for example, fog, heavy rain, or blowing snow — and cloud coverage cannot easily be determined, then the code VV is used in place of the normal cloud information. VV stands for vertical visibility, and it's followed by a three-digit figure giving the vertical visibility in hundreds of feet. Let me decode a real METAR for you. Here's one: METAR EGTK 231020Z 26005KT 0300 FG VV002. Let's break it down piece by piece. METAR tells us this is a routine aerodrome report. EGTK is the ICAO code for Oxford/Kidlington. 231020Z means the observation was made on the 23rd of the month at 1020 UTC. 26005KT means the surface wind is from 260 degrees True at 5 knots. 0300 is the visibility — 300 metres. FG is fog. And VV002 means the sky is obscured and the vertical visibility is 200 feet. So the full decode is: METAR for Oxford/Kidlington, observed at 1020 UTC on the 23rd of the month; the surface wind is 260 degrees True at 5 knots; the visibility is 300 metres in fog; the sky is obscured with a vertical visibility of 200 feet. That figure shows you how cloud base is reported as a three-digit figure in hundreds of feet, measured above aerodrome level using QFE. Keep that relationship clear in your mind — the height is always relative to the aerodrome, not to sea level.