METAR & TAF Glossary

What each code in an aviation weather report actually means, one page at a time. 15 of the codes that come up most often, explained in plain language.

• CAVOKCeiling And Visibility OK — a single code that replaces the visibility, weather and cloud groups when all three are unremarkable.
• QNHThe pressure setting that makes an altimeter read altitude above mean sea level.
• BECMGA gradual, lasting change: once it happens, the new conditions hold.
• TEMPOTemporary fluctuations that come and go without changing the underlying forecast.
• PROB30 and PROB40A stated 30 % or 40 % probability that the conditions which follow will occur.
• RVRRunway Visual Range — the distance a pilot on the centreline can see down a specific runway.
• Cloud cover: FEW, SCT, BKN, OVCHow much of the sky is covered, measured in eighths, and how high the base sits.
• Vertical visibility (VV)How far up you can see when the sky is obscured and no cloud base can be identified.
• NOSIGNo significant change expected in the two hours after the observation.
• AUTOA fully automated observation, with no human check of what the instruments reported.
• SPECIAn unscheduled observation, issued because conditions changed enough to matter.
• Wind shear (WS)A sharp change in wind speed or direction over a short distance, reported where it affects a runway.
• Thunderstorm (TS)Thunder, lightning, and the convective hazards that come with them.
• Freezing precipitation (FZ)Supercooled liquid that freezes on contact — the most dangerous icing in the code.
• Visibility: 9999, 0000 and SMHow the visibility group encodes distance, and what its two limiting values mean.