Aviation Weather — Reading and Understanding METAR, TAF, and SIGMET

Reading Aviation Weather -- METAR, TAF, and SIGMET Explained

No single factor influences flight planning as fundamentally as weather. For VFR pilots, weather assessment is one of the most critical skills -- a clear blue sky can give way to a solid overcast within an hour. The standardized weather reports METAR, TAF, and SIGMET form the backbone of aviation weather information worldwide. Pilots who can read and interpret them make better decisions and fly more safely.

METAR -- The Current Weather Observation

METAR stands for Meteorological Aerodrome Report and is a standardized weather observation generated at airports with weather reporting capability, typically every 30 or 60 minutes. It describes the current weather at the time of observation and is generally valid for 30 to 60 minutes.

A METAR consists of a fixed sequence of groups. Here is a typical example:

METAR KJFK 151520Z 24012G22KT 10SM FEW040 SCT080 BKN250 18/09 A3021 RMK AO2

Let us decode this METAR group by group:

Wind Group in Detail

The wind group always follows the pattern: Direction (3 digits) + Speed (2-3 digits) + optional Gusts + Unit.

• VRB03KT: Variable direction, 3 knots (used when wind is light and direction unstable)
• 18008KT: From 180 degrees (south), 8 knots, no gusts
• 27015G28KT: From 270 degrees (west), 15 knots, gusting to 28 knots
• 24012KT 200V280: Primary direction 240 degrees, 12 knots, varying between 200 and 280 degrees

Wind direction in the METAR always references true north -- in contrast to runway designations, which reference magnetic north.

Decoding Cloud Groups

Cloud coverage is reported in oktas (eighths of the sky):

The number following the abbreviation gives the cloud base in hundreds of feet AGL. "SCT025" means scattered clouds at 2,500 ft above ground level. The ceiling is the lowest layer reported as BKN or OVC -- a critical value for VFR go/no-go decisions.

Additional descriptors appended to cloud groups:

• CB: Cumulonimbus -- thunderstorm clouds, warning of turbulence, hail, lightning
• TCU: Towering Cumulus -- rapidly building cumulus, precursor to CB

Weather Phenomena in the METAR

Between the visibility and cloud groups, weather phenomena may be listed. The most important abbreviations:

Intensity prefixes: - (light), no prefix (moderate), + (heavy). Thus: -RA = light rain, +TSRA = heavy thunderstorm with rain.

Temperature-Dewpoint Spread

The difference between temperature and dewpoint (spread) is a key indicator of fog potential. As a rule of thumb:

• Spread above 10 degrees C (18 degrees F): Low fog risk
• Spread 5-10 degrees C (9-18 degrees F): Caution during evening and morning hours
• Spread below 3 degrees C (5 degrees F): Elevated fog/mist risk
• Spread 0-1 degrees C (0-2 degrees F): Fog likely or already present

The dewpoint decreases by approximately 0.5 degrees C per 1,000 ft of altitude gain, while the temperature drops about 2 degrees C per 1,000 ft. From this, you can roughly estimate the condensation level (cloud base): divide the spread by 2.5 to get the cloud base in thousands of feet AGL. With a spread of 9 degrees C, the cloud base would be approximately 3,600 ft AGL.

TAF -- The Forecast

TAF stands for Terminal Aerodrome Forecast -- the aerodrome weather forecast for a specific airport. TAFs are typically updated every 6 hours and are valid for 24 or 30 hours.

TAF KJFK 150500Z 1506/1606 28008KT P6SM SCT030 BKN060
BECMG 1510/1512 18015G25KT 6SM -RA BKN020
TEMPO 1512/1518 3SM RA BKN010
PROB30 TEMPO 1514/1517 1SM +TSRA BKN008CB
BECMG 1520/1522 30010KT P6SM SCT040

The change groups in a TAF are crucial:

Pay special attention to the TEMPO group: it describes conditions that may temporarily occur, each episode lasting less than one hour. For flight planning, TEMPO means you must be prepared for these conditions even though they do not represent the prevailing state.

Interpreting the TAF for VFR

A TAF is relevant for VFR pilots when it forecasts conditions below VFR minimums. Critical values include:

• Visibility below 3 statute miles (5 km in EASA) in controlled airspace
• Ceiling (BKN/OVC) below the minimum safe altitude -- typically below 1,500-2,000 ft AGL
• Thunderstorms (TS, CB) -- even at PROB30, a reason for caution
• Freezing conditions (FZ) -- a no-go for aircraft not equipped for known icing

Rule of thumb: If a TEMPO or PROB40 value falls below your personal VFR minimums, plan an alternate airport or postpone the flight. A PROB30 TS may be statistically unlikely, but a thunderstorm is not a risk worth taking.

SIGMET -- Significant Weather Warnings

SIGMET stands for Significant Meteorological Information and warns of weather phenomena that represent a serious hazard to flight. SIGMETs are issued by Meteorological Watch Offices (MWO).

There are three types:

A SIGMET includes the affected area (as a coordinate polygon or FIR designation), the type of hazard, affected altitudes, and the expected trend (INTSFYG = intensifying, WKN = weakening, STNR = stationary, MOV = moving with direction and speed).

For VFR pilots: An active SIGMET in your planned flight area is a strong argument against flying. The described phenomena exceed the capabilities of most private pilots and the performance limits of single-engine piston aircraft.

AIRMET -- The SIGMET's Smaller Sibling

AIRMETs warn of weather phenomena that may be hazardous to general aviation (below FL100 / 10,000 ft MSL) but do not reach SIGMET thresholds. In the US, AIRMETs are issued for three categories: Sierra (IFR and mountain obscuration), Tango (turbulence), and Zulu (icing). Typical AIRMET contents include:

• Moderate turbulence
• Moderate icing
• Widespread areas with visibility below 3 statute miles (5 km)
• Widespread BKN/OVC below 1,000 ft AGL
• Moderate mountain waves

AIRMETs are often more relevant to VFR pilots than SIGMETs, as they cover exactly the altitude bands and weather phenomena encountered in general aviation operations.

Weather Information Sources

Reliable weather information is the cornerstone of every preflight briefing. The most important sources:

Flight Categories -- A Quick VFR Assessment

In the United States, weather conditions are classified into flight categories for quick assessment:

• VFR: Ceiling above 3,000 ft AND visibility greater than 5 statute miles
• MVFR (Marginal VFR): Ceiling 1,000 to 3,000 ft AND/OR visibility 3 to 5 statute miles
• IFR: Ceiling 500 to below 1,000 ft AND/OR visibility 1 to below 3 statute miles
• LIFR (Low IFR): Ceiling below 500 ft AND/OR visibility below 1 statute mile

In Europe, a comparable system is the GAFOR (General Aviation Forecast), used in Germany, which divides the country into numbered areas and classifies them as Open (O), Difficult (D), Marginal (M), or Closed (X) for VFR operations.

Practical Weather Assessment for a VFR Flight

A structured preflight weather assessment follows this sequence:

• Step 1: Check the synoptic situation -- frontal systems, pressure patterns, weather trends
• Step 2: Review flight category forecasts (GAFOR in Europe / flight categories at aviationweather.gov in the US) for a quick go/no-go assessment along the route
• Step 3: Read METARs for departure, destination, and alternate airports
• Step 4: Study TAFs for departure, destination, and alternate -- focus on trends and TEMPO groups
• Step 5: Check SIGMETs and AIRMETs for the FIR / region
• Step 6: Analyze winds aloft for the planned cruising altitude
• Step 7: Review current and forecast radar imagery
• Step 8: Apply personal minimums and compare against the data

The ability to read weather data and place it in context grows with experience. As a new pilot, set more conservative personal minimums than the regulatory requirements -- for example, 5 statute miles visibility instead of 3, and ceilings above 3,000 ft AGL instead of the legal minimums. These safety margins will naturally relax as your experience and competence grow, not the other way around.