Runway Visual Range (RVR) and Sky Condition Reporting

When visibility drops and ceilings lower, the difference between a safe approach and a dangerous one can come down to a few hundred feet of visibility or a single cloud layer. Runway Visual Range (RVR) and sky condition reporting are the two METAR elements that most directly define whether an airport is legal and safe for an instrument approach. Understanding how these values are measured, encoded, and used in real operations is essential knowledge for any instrument-rated pilot — and a frequent focus of FAA knowledge test questions.

Both RVR and sky condition data appear in the body of the Aviation Routine Weather Report (METAR) and the Aviation Selected Special Weather Report (SPECI), the standard code forms used for aviation surface observations worldwide under World Meteorological Organization (WMO) and ICAO requirements. While many modern preflight services decode these reports into plain language, every pilot must be able to read the raw coded format independently, as the FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 24, makes clear.

How RVR Works

Runway Visual Range is an instrumentally-derived value representing the horizontal distance a pilot can see down the runway from the approach end, measured by a transmissometer or forward-scatter sensor mounted alongside the runway. Unlike prevailing visibility, which describes general horizontal visibility across an area, RVR is runway-specific and is expressed in feet in the United States. This makes it far more operationally relevant during low-visibility approaches, because it tells you exactly what you can expect to see once you are on final and crossing the runway threshold.

In a METAR, RVR appears immediately after the visibility group and is coded with the letter R, followed by the runway designator, a forward slash, and the RVR value in feet followed by FT. For example, in the classic training METAR METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132, the group R17L/2600FT means Runway 17 Left has an RVR of 2,600 feet. When RVR exceeds the highest reportable value, a prefix of P is added (e.g., P6000FT); when it is below the lowest measurable value, an M prefix appears (e.g., M600FT). A variable RVR is shown with the low and high values separated by a V (e.g., 1200V2400FT).

RVR is only reported when the prevailing visibility is less than one mile or the RVR is less than 6,000 feet. Up to four RVR values can appear in a single METAR, corresponding to different runway touchdown, midpoint, and rollout sensors, though in practice one or two values are most common.

How Sky Condition Is Reported

Sky condition describes the amount of sky covered by clouds and the height of each layer above ground level (AGL). In METAR coding, sky coverage uses standard contractions based on eighths of sky cover (oktas):

• SKC — Sky Clear (used at manual stations; zero cloud cover).
• CLR — Clear below 12,000 feet AGL (used at automated stations; no clouds detected below 12,000 ft).
• FEW — Few clouds: 1–2 oktas (1/8 to 2/8 coverage).
• SCT — Scattered: 3–4 oktas (3/8 to 4/8 coverage).
• BKN — Broken: 5–7 oktas (5/8 to 7/8 coverage). This constitutes a ceiling.
• OVC — Overcast: 8 oktas (total coverage). This also constitutes a ceiling.
• VV — Vertical Visibility: used when the sky is obscured and a ceiling height cannot be determined in the normal sense; the number following VV is the vertical visibility in hundreds of feet into the obscuration.

Cloud heights are expressed in hundreds of feet AGL, written as a three-digit number directly after the coverage code. For example, OVC010 means an overcast ceiling at 1,000 feet AGL. BKN035 means a broken layer at 3,500 feet AGL. In the example METAR above, OVC010CB indicates an overcast layer at 1,000 feet AGL composed of cumulonimbus clouds — a significant convective hazard that is noted with the CB suffix. Similarly, TCU (towering cumulus) may appear as a cloud type suffix.

The ceiling is officially defined as the lowest broken (BKN) or overcast (OVC) layer, or a vertical visibility (VV) value when the sky is obscured. FEW and SCT layers are not ceilings. In the METAR body, layers are reported in ascending order, lowest first, so the first BKN or OVC group encountered is the ceiling.

SPECI Triggers Related to RVR and Sky Condition

The SPECI is an unscheduled special observation issued whenever certain rapidly changing conditions are observed between hourly METARs. Chapter 24 of FAA-H-8083-28B lists specific RVR and ceiling thresholds that trigger a SPECI, reinforcing why pilots must monitor for these reports during active weather:

• RVR trigger: A SPECI is issued when the highest RVR value from the designated runway decreases to less than 2,400 feet during the preceding 10 minutes, or — if already below 2,400 feet — increases to equal or exceed 2,400 feet during the preceding 10 minutes.
• Visibility triggers: Visibility crosses through 3, 2, 1, 1/2, or 1/4 mile thresholds (decreasing below or increasing to meet these values).
• Ceiling triggers: The ceiling crosses through 3,000, 1,500, 1,000, or 500 feet, or the lowest standard instrument approach procedure (IAP) minimum for that airport.
• New low layer: A cloud or obscuration layer appears below 1,000 feet that was not present in the preceding METAR or SPECI.

These thresholds are not arbitrary — they align directly with published instrument approach minimums, ensuring that pilots receive updated surface observations precisely when conditions are crossing decision-relevant boundaries.

Why RVR and Sky Condition Reporting Matter Operationally

Instrument approach minimums are expressed in terms of ceiling and visibility — and often RVR for lower-visibility approaches. A Category I ILS approach, for example, typically requires a 200-foot decision altitude and either 1/2-statute-mile visibility or 2,400 feet RVR. A pilot cannot legally commence or continue that approach unless the reported weather meets the published minimums at the time of the approach. The specific RVR value in the METAR or a real-time ATIS/ASOS broadcast is what makes that determination possible.

Sky condition reporting also directly governs VFR flight. The legal VFR minimums at towered airports require a ceiling of at least 1,000 feet and 3 statute miles visibility. The BKN or OVC designation in the METAR body is the controlling value — a scattered layer (SCT) at 800 feet does not constitute a ceiling for regulatory purposes, even though it may severely restrict visibility beneath it in practical terms.

Automated surface observation systems (ASOS and AWOS) produce most METAR sky condition data using laser ceilometers that sense backscatter from cloud bases. AWOS-3 and higher include cloud height sensors; lower-tier systems like AWOS-1 and AWOS-2 do not report cloud data. This means that at AWOS-equipped airports below the AWOS-3 tier, sky condition will be absent from the report — a critical limitation pilots must know, especially at remote or rural airfields.

Key Numbers and Rules

• RVR is reported in feet in the U.S.; it appears in the METAR only when prevailing visibility is less than 1 SM or RVR is less than 6,000 ft.
• RVR prefix M = below minimum measurable value; prefix P = above maximum measurable value.
• BKN (5–7 oktas) and OVC (8 oktas) define a ceiling; FEW (1–2) and SCT (3–4) do not.
• VV followed by three digits = vertical visibility in hundreds of feet when sky is obscured.
• Cloud heights in METARs are always AGL, in hundreds of feet (e.g., BKN025 = 2,500 ft AGL).
• SPECI RVR trigger: 2,400 feet — the RVR crosses this threshold in either direction over the preceding 10 minutes.
• SPECI ceiling triggers: 3,000 / 1,500 / 1,000 / 500 ft and the lowest published IAP minimum.
• CB or TCU suffixes on a sky condition group indicate cumulonimbus or towering cumulus — always operationally significant.
• CLR (automated) means no clouds detected below 12,000 ft AGL; SKC (manual) means sky clear.

Common Test Traps

• SCT is not a ceiling. A common error is reading a scattered layer as a ceiling. Only BKN and OVC (and VV) constitute official ceilings. FEW and SCT layers still appear in the METAR and affect VFR flight practically, but they are not ceilings by definition.
• CLR vs. SKC. CLR is used by automated stations and only means no clouds below 12,000 ft — it does not guarantee clear skies above that altitude. SKC is used at manual stations and means sky is completely clear.
• RVR vs. visibility. RVR and prevailing visibility are not interchangeable. They are measured differently and used for different purposes. On CAT I/II/III approach minimums, RVR is the controlling value when listed, not statute-mile visibility.
• M and P prefixes. Students often confuse M (below sensor minimum) with a good value. M600FT RVR means the sensor cannot measure how low it actually is — conditions may be worse than 600 feet, not better.
• AWOS tier capability. An AWOS-2 reports visibility but not cloud height; only AWOS-3 and above include cloud height. Do not assume a METAR from an AWOS-equipped field contains a complete sky condition report without knowing the AWOS tier.