8.1 METAR Decoding

ACS PA.I.C.K2a asks you to read airport observations: the METAR (Aviation Routine Weather Report) and the SPECI (Aviation Selected Special Weather Report). This is not theory about why a cloud forms. It is a code. If you cannot turn BKN012 OVC025 08/07 A2985 into a ceiling, a temperature–dewpoint spread, and an altimeter setting, you will miss the item and you will mis-brief the airplane.

METAR versus SPECI

A METAR is the routine observation, typically issued hourly near the end of the hour (often at 53 or 55 minutes past, plus a 00Z/06Z/12Z/18Z synoptic METAR). A SPECI is an unscheduled observation issued when selected criteria change between those hours: a wind shift, visibility dropping through a breakpoint, a ceiling falling through a breakpoint, a thunderstorm beginning or ending, precipitation beginning or ending, or other published SPECI triggers in the Aviation Weather Handbook. The letters SPECI replace METAR at the front of the same coded string. Treat a SPECI as more recent than the last METAR, not as a different language.

AUTO after the time group means a fully automated report with no human logged on. COR means a corrected observation. No modifier means a human observer, or an automated station with a human logged on for oversight. When AUTO is in the body, the remarks tell you the sensor: AO2 (also written A02) has a precipitation discriminator that can tell liquid from frozen precipitation; AO1 does not.

The body, left to right

Wind. The first three digits are true direction in tens of degrees; the next two (or three) are speed in knots. 35012G22KT is wind from 350° at 12 knots, gusting 22. 00000KT is calm. VRB04KT is variable direction at 4 knots — typical when the speed is 6 knots or less. 35012KT 350V040 means the wind is 12 knots from 350° but the direction is varying between 350° and 040°. Gusts are peak speeds, not a second wind. Do not treat a gust group as a SPECI by itself unless the handbook’s SPECI wind criteria are met.

Visibility is statute miles in a U.S. METAR. 10SM is ten statute miles. 1/2SM is one-half statute mile. P6SM (more common in TAFs) is greater than 6 SM. International METAR visibility is often meters (9999 is 10 km or more). PAR will punish anyone who reads 3SM as three nautical miles or 3,000 meters. RVR is a runway-specific instrument value in feet: R17L/2000FT is 2,000 feet of visual range on runway 17 Left. M0600FT is less than 600 feet; P6000FT is more than 6,000 feet. RVR is not prevailing visibility.

Weather codes. Intensity is - light, no prefix moderate, + heavy. Then a descriptor, then a phenomenon. Common PAR codes: -RA light rain, +TSRA thunderstorm with heavy rain, BR mist (visibility 5/8 SM to 6 SM), FG fog (visibility less than 5/8 SM), HZ haze, SN snow, FZRA freezing rain. TS is a thunderstorm whether or not rain is coded. SH is a shower. VC is vicinity (not at the station). UP is unknown precipitation on an automated station. Do not invent a private intensity for BR or FG — those words already carry a visibility idea.

Sky. Coverage is in eighths (oktas): FEW greater than 0 to 2/8, SCT 3/8 to 4/8, BKN 5/8 to 7/8, OVC 8/8. Heights are hundreds of feet AGL: BKN012 is broken at 1,200 feet AGL. SKC is a human observer reporting a clear sky. CLR is an automated station that detected no clouds below 12,000 feet — clouds may sit above that. VV is vertical visibility into an obscuration (VV003 is 300 feet). CB or TCU may be appended when those clouds are identified.

A ceiling is the lowest BKN or OVC layer, or VV. FEW080 SCT140 has no ceiling. SCT012 BKN080 has an 8,000-foot ceiling, not 1,200. BKN008 OVC015 has an 800-foot ceiling. That is the same definition you used in weather theory; the METAR is where you apply it.

Temperature, dew point, and altimeter. 22/M02 is 22 °C and dew point −2 °C. A large spread is drier air; 18/17 is nearly saturated. A3018 is 30.18 inHg. That is the Kollsman-window number, not a millibar field.

Remarks that PAR actually samples

After RMK: AO2 is the discriminator. SLP189 is sea-level pressure 1018.9 mb (the leading 10 or 9 is implied). T02221022 is temperature +22.2 °C and dew point −2.2 °C in tenths — the first digit of each four is the sign (0 positive, 1 negative). TSB38 means a thunderstorm began at 38 minutes past the hour; TSE is ended. Lightning remarks name type and location (LTG DSNT NE, cloud-to-ground CG, in-cloud IC). Do not skip remarks on a convective or low-visibility METAR. The body tells you what is happening at the station; the remarks often tell you when it started and how the sensor knows.

Worked example 1 — routine VFR

METAR KCOS 141753Z 35012G22KT 10SM FEW080 SCT140 22/M02 A3018 RMK AO2 SLP189 T02221022

• Routine METAR, Colorado Springs, 14th day at 1753Z.
• Wind 350° at 12 knots, gusting 22.
• Visibility 10 statute miles. No RVR, no weather group.
• Few at 8,000, scattered at 14,000. No ceiling.
• Temperature 22 °C, dew point −2 °C (spread 24 °C — dry).
• Altimeter 30.18 inHg.
• Automated with discriminator; SLP 1018.9 mb; precise temp/dew 22.2 / −2.2 °C.

Jordan can legally depart VFR on this observation if the rest of 91.155 and his personal minimums are met. The gusts are a crosswind-component problem, not a ceiling problem.

Worked example 2 — convective SPECI

SPECI KOKC 141642Z AUTO 08018G32KT 080V140 1/2SM R17L/2000FT +TSRA BR BKN008 OVC015CB 18/17 A2978 RMK AO2 TSB38 LTG DSNT NE SLP088 T01830172

• SPECI, not METAR — something crossed a threshold at 1642Z.
• AUTO: no human is logged on.
• Wind 080° at 18 gusting 32, direction varying 080° to 140°.
• Prevailing visibility 1/2 statute mile. RVR runway 17L is 2,000 feet — that is not 2 SM.
• Weather: thunderstorm, heavy rain, and mist.
• Sky: broken 800, overcast 1,500 with cumulonimbus. Ceiling = 800 feet AGL (lowest BKN/OVC).
• Temperature 18 °C, dew point 17 °C (almost saturated).
• Altimeter 29.78 inHg.
• Thunderstorm began at 38 minutes past the previous hour; lightning distant northeast; SLP 1008.8 mb; precise 18.3 / 17.2 °C.

Maya does not “see if she can stay under it.” This SPECI is IFR, convective, and a no-go for a typical VFR training airplane. The next product family — TAF, GFA, and convective advisories — tells her whether the storm is a 40-minute TEMPO or a line that will still be there at her ETA.