Cloud Layers, Ceiling and Vertical Visibility
Updated: 2026-09-27 · First published: 2026-09-27 · About this site
The cloud group is where a METAR decides your flight category, your approach minima and, at a VFR field, whether you can go at all. It is also where the units are least intuitive: coverage in eighths of the sky, heights above the aerodrome rather than the sea, and a special code for the case where there is no cloud base to measure because you cannot see the sky. This guide sets out the rules and the edge cases.
Coverage in oktas
| Code | Oktas (eighths) | Ceiling? |
|---|---|---|
| SKC / CLR | 0 (SKC: human observer; CLR: automated, nothing below 12,000 ft) | — |
| FEW | 1–2 | No |
| SCT | 3–4 | No |
| BKN | 5–7 | Yes |
| OVC | 8 | Yes |
| VV | sky obscured | Yes (the VV value) |
Coverage is cumulative from the ground up: a second layer's oktas include the layers below it. FEW010 SCT025 BKN040 means the sky is 5–7 oktas covered when you count everything up to 4,000 ft, not that the 4,000 ft layer alone is broken. This is why a low FEW layer under a broken deck is often solid cloud from the ground's point of view along the approach.
Heights
The three digits after the coverage are the base in hundreds of feet above aerodrome level. BKN008 is 800 ft above the runway. To compare with a chart, an obstacle or a minimum descent altitude, add the field elevation: at LTAC Ankara Esenboğa (about 3,125 ft AMSL) BKN008 is a cloud base near 3,900 ft AMSL. Heights are rounded to the nearest 100 ft below 5,000 ft and to 1,000 ft above 10,000 ft, and a ceilometer measures only the point directly above it — a ridge two miles away can be in cloud while the station reports SCT.
Which layer is the ceiling
The ceiling is the lowest layer that is BKN or OVC, or the vertical visibility. FEW003 SCT012 BKN025 OVC080 has a ceiling of 2,500 ft; the few at 300 ft is not a ceiling, although on a dark approach it will look like one. The flight category is derived from this ceiling and the visibility, and the flight categories guide gives the thresholds. On the map and station pages the category colour reflects exactly this rule; the reasoning is shown beside the raw report.
CB and TCU — the only types named
A METAR names two cloud types, appended to the layer they belong to: CB (cumulonimbus) and TCU (towering cumulus). SCT030CB is scattered cumulonimbus with a base at 3,000 ft. CB means a thunderstorm, or the capability of one, within sight of the aerodrome — expect severe turbulence, hail, icing and downbursts near it even if no TS is in the weather group yet. TCU is cumulus with strong vertical development that has not yet produced an anvil; it is usually a CB thirty minutes from now. Neither is reported by a fully automated station unless it has a lightning detector, which is why AUTO reports on a convective day deserve a look at the satellite picture. The thunderstorm guide covers what to do with them.
Vertical visibility
When fog, heavy precipitation, blowing snow or smoke hides the sky completely, there is no cloud base to report. Instead the station gives VV followed by the vertical visibility in hundreds of feet: VV002 is 200 ft. It counts as a ceiling for every purpose — approach minima, alternate rules, flight category. VV/// means the sky is obscured and the vertical visibility could not be measured. A report of VV001 0200 FG is about as low as aviation weather gets and is a LIFR that no Category I approach will get into.
Special values
- CAVOK — replaces visibility, weather and cloud: visibility 10 km or more, no cloud below 5,000 ft (or the highest MSA, whichever is greater), no CB or TCU, no significant weather.
- NSC — no significant cloud: the cloud criteria of CAVOK are met but the visibility or weather criteria are not.
- NCD — automated station, no cloud detected. The ceilometer's range is finite (often 25,000 ft, sometimes 10,000); NCD can mean clear or can mean everything is above the sensor.
- /// in a cloud group — coverage or height could not be determined.
///015///is a layer at 1,500 ft of unknown coverage;BKN///is a broken layer of unknown height. - SKC vs CLR — SKC is a human observer reporting a clear sky; CLR is an automated station reporting nothing below 12,000 ft (US) — cirrus at 30,000 ft can be present.
In a TAF
Cloud in a TAF follows the same codes and the same AAL reference, but forecast bases are rounded more coarsely and forecasters lean toward the pessimistic edge when the base is uncertain. TEMPO 2806/2810 BKN004 in a TAF is the forecaster telling you that an approach with 400 ft minima may not work for parts of that window; it is a stronger signal than the same layer would be in a METAR because it is deliberate. Every TAF period on this site is decoded and categorised separately for that reason, and the 3D profile on the airport page draws the forecast cloud layers hour by hour so the gap between them and your minima is visible rather than arithmetic.
Frequently asked questions
Are METAR cloud heights above sea level?
No. Cloud base heights in a METAR and TAF are above aerodrome level (AAL), in hundreds of feet. BKN020 at a field 2,000 ft above sea level is a cloud base at 4,000 ft AMSL. Charts and minimum altitudes are AMSL, so add the field elevation before comparing.
What counts as a ceiling?
The lowest layer reported as BKN (5–7 oktas) or OVC (8 oktas), or the vertical visibility (VV) when the sky is obscured. FEW and SCT layers are not ceilings no matter how low they are.
What does VV002 mean?
Vertical visibility 200 ft. The sky is totally obscured — by fog, heavy snow, smoke or dust — so no cloud base can be measured; instead the report gives how far up the observer or ceilometer can see. It is treated as a ceiling of 200 ft and is almost always LIFR.
Why are only CB and TCU reported as cloud types?
Because they are the two that matter operationally: cumulonimbus brings thunderstorms, severe turbulence, hail and icing; towering cumulus is a cumulonimbus in the making. Other types (stratus, altocumulus) are not identified in a METAR; you infer them from height and coverage.
What does NSC or NCD mean?
NSC (no significant cloud): no cloud below 5,000 ft or the highest minimum sector altitude, no CB or TCU, but conditions do not meet CAVOK. NCD (no cloud detected) is an automated station's statement that its ceilometer saw nothing — which may mean clear sky or may mean the cloud is above the sensor's range.
Read next
- How to Read METAR Reports: A Pilot's Guide — Learn to decode a METAR line by line: station, time, wind, visibility, weather, clouds, temperature, QNH and trend. Worked example plus the mistakes that catch pilots out.
- VFR vs IFR Flight Categories Explained — What the four flight categories mean, the exact ceiling and visibility thresholds, how to derive them from a METAR or TAF, and how they differ from flight rules and European colour states.
- How to Read a TAF Forecast — Decode a TAF: validity period, FM, BECMG, TEMPO and PROB groups, what each change group promises, and how to pick the worst period for alternate planning. Worked example included.
- Crosswind Component and Runway Selection — The sine rule, the clock-face shortcut, gust handling and a worked example. How crosswind, headwind and tailwind components decide which runway to use and when to divert.
- QNH, QFE and QNE: Altimeter Settings Explained — What each altimeter setting measures, when to use which, how to convert hPa and inHg, what a wrong setting does to your altitude, and how transition altitude and level work.
- All pilot guides — the full library, 15 articles.
- Airports by country — live METAR and TAF for every reporting station
- World weather rankings from the observation archive