METAR, TAF & SIGMET — the language of aviation weather

Decoded field by field

Working through our example in order. The groups always appear in this sequence; groups that do not apply are simply omitted.

METAR LEMG 191300Z 12008KT 9999 FEW035 27/18 Q1018 NOSIG

Groups missing from the example

Reading cloud cover: FEW, SCT, BKN, OVC

Cloud cover is given in eighths of the sky — the technical term is okta. Each cloud group states the coverage first, then the height of its base in hundreds of feet above the aerodrome: SCT035 means “three to four eighths at 3,500 feet”. Up to four layers are reported, lowest first.

Cloud heights: what 035 means

The height always follows the coverage as a three-digit figure in hundreds of feet. So 035 is 3,500 feet, 003 only 300 feet, 250 a full 25,000 feet. A foot is just under 30 centimetres — to convert roughly, treble the figure and multiply by ten: 035 ≈ 1,050 metres.

The reference point is what matters: heights are measured above aerodrome level, not above sea level. At a high airport such as Denver, which itself sits at around 5,400 feet, BKN035 means 3,500 feet above the runway — and almost 8,900 feet above the sea. The first figure counts for the landing, the second for route planning.

Only what matters for flight operations is reported: the lowest layers, at most four, in ascending order. High cirrus above 25,000 feet often does not appear at all — a missing entry does not necessarily mean “nothing up there”. The TAF forecast uses exactly the same notation.

Why the distinction matters: only the lowest BKN or OVC layer counts as the ceiling. FEW and SCT do not form a ceiling — the sky there counts as sufficiently open. The flight category depends on this.

Reading weather phenomena

Present weather is built from up to three parts: intensity, descriptor, phenomenon. So -SHRA is “light rain shower”, +TSRA is “heavy thunderstorm with rain”.

Change groups

The heart of a TAF is its time groups. They say when what changes:

The hazard types

These are the types we show on the map and in the SIGMET list — in exactly these colours:

• TS
  ThunderstormActive thunderstorms, often embedded or in a line. They combine turbulence, icing, hail and wind shear.
• CONVECTIVE
  ConvectiveWidespread convective areas — in US products the common umbrella category for thunderstorm complexes.
• TURB
  TurbulenceSevere turbulence, often clear-air turbulence (CAT) near the jet stream — with no cloud to warn you.
• ICE
  IcingSevere ice accretion on wings and sensors, typically in supercooled droplets between 0 and −20 °C.
• MTW
  Mountain waveMountain waves in the lee of high ranges: strong, large-scale up- and downdraughts that make altitude hard to hold.
• VA
  Volcanic ashAsh clouds destroy engines and sensors. Validity is exceptionally 6 hours, WMO identifier WV.
• TC
  Tropical cycloneHurricane, typhoon or cyclone with mean surface winds of 34 knots or more. Also valid 6 hours, identifier WC.

Glossary

The abbreviations that most often raise questions.

QNH

Pressure reduced to mean sea level. Set on the altimeter it reads aerodrome elevation on the ground.

Ceiling

The lowest layer reported as BKN or OVC.

FIR

Flight Information Region — the airspace block a SIGMET refers to.

KT

Knot, 1 kt = 1.852 km/h. The standard aviation wind unit.

SM

Statute mile, 1 SM ≈ 1.609 km. The US visibility unit.

Z

Zulu — aviation weather times are always UTC.