Every flight begins with a weather assessment. These four tools decode the standardised aviation weather formats that pilots, dispatchers, and flight operations personnel rely on daily — METAR, TAF, SNOWTAM, and surface observations — translating raw coded reports into the actionable numbers that determine whether a flight is legal, safe, and operationally sensible.
Free Decoding Tools
All tools are free, browser-based, and require no account or installation. Each decoder accepts standard ICAO-format reports issued by airports worldwide.
Paste any raw METAR string and receive a complete plain-English breakdown of every field: wind, visibility, ceiling, present weather, temperature, dew point, and altimeter setting. Colour-coded by VFR, MVFR, IFR, and LIFR flight category.
METAR DecoderDecode Terminal Aerodrome Forecasts into plain English. Understand forecast wind, visibility, ceiling, and present weather up to 30 hours ahead. Interprets change groups including TEMPO, BECMG, FM, and PROB automatically.
TAF DecoderDecode SNOWTAM runway condition reports and NOTAM notices into clear operational language. Identifies runway contamination type, depth, braking action by RWYCC code, and associated aircraft performance implications.
SNOWTAM & NOTAM DecoderEstimate cloud base height from surface temperature and dew point spread using the standard 400 ft per 1°C formula. Cross-check reported METAR ceilings and predict convective cloud base for departure and destination.
Cloud Base CalculatorFoundational Knowledge
Aviation weather is communicated in a globally standardised code format designed to be unambiguous across language barriers. Understanding how to decode these formats accurately is a prerequisite for safe flight planning at every level of aviation.
The International Civil Aviation Organisation (ICAO) defines the format and content of all aerodrome weather reports and forecasts used in international aviation. This standardisation means a pilot flying in Australia can decode a METAR from Iceland using exactly the same method they learned for their home country. The format encodes wind, visibility, cloud cover, present weather phenomena, temperature, dew point, and altimeter setting into a compact string that can be transmitted efficiently over text-based communication systems.
The ability to decode these reports accurately — not just read the numbers but understand their operational implications — is examined in every PPL, CPL, and ATPL written examination worldwide. It is also a daily operational skill that professional pilots exercise every time they review pre-flight weather packages.
Aviation weather reports use abbreviated codes rather than natural language for three reasons: brevity (a complete weather observation is transmitted in under 100 characters), precision (coded formats eliminate the ambiguity of natural language descriptions), and interoperability (a single format works across all languages, communication systems, and display technologies from cockpit datalink to paper ATIS strips).
The challenge is that this efficiency comes at the cost of readability for anyone
who has not trained extensively with the format. A METAR such as
EGLL 221520Z 24018G28KT 210V270 2500 RASN BKN006 04/02 Q0998
is instantly interpretable by an experienced pilot but opaque to a student or
non-aviator. Decoding tools bridge that gap by presenting every field in clear,
unambiguous English while preserving the operational accuracy of the original data.
The Four Formats
Each weather format serves a distinct role in the preflight and in-flight weather picture. Understanding what each one contains — and what it does not contain — is as important as knowing how to decode it.
The METAR is the primary source of current aerodrome weather information. It is issued at least once per hour and reports conditions at the moment of observation, not an average or forecast. Each field in the METAR follows a strict positional order defined by ICAO Annex 3.
A complete METAR contains: the station ICAO identifier (4 letters); the observation date and time in UTC (DDHHmmZ); the report type modifier (AUTO for automated, COR for corrected); wind direction in degrees magnetic and speed in knots with gusts and variable direction range; prevailing visibility in metres or statute miles; runway visual range (RVR) when visibility is below 1,500 metres; present weather using standard two-letter codes (RA = rain, SN = snow, FG = fog, TS = thunderstorm); sky condition layers in FEW, SCT, BKN, or OVC with height in hundreds of feet; temperature and dew point in degrees Celsius; and altimeter setting in hPa (Q) or inches of mercury (A).
KJFK 221451Z 24015G22KT 10SM FEW025 BKN080 18/07 A2992
The TAF provides the forecast weather for a specific airport over a period of 9 to 30 hours. It is issued by qualified meteorological forecasters (not automated systems) four times per day at 0000, 0600, 1200, and 1800 UTC and covers a 5 nautical mile radius around the aerodrome.
A TAF begins with a base forecast covering the full period, using the same wind, visibility, and sky condition format as a METAR. The critical difference is the inclusion of change groups that indicate when conditions are expected to change. FM (from) indicates a complete change of all conditions from a specified time. BECMG (becoming) indicates a gradual change to new conditions that will persist. TEMPO (temporary) indicates conditions that will last less than 60 minutes in any one occurrence and less than half of the forecast period in total. PROB30 or PROB40 indicates conditions with a 30% or 40% probability of occurring.
BECMG 1416 18012KT 5000 SHRA BKN020 TEMPO 1618 2000 TSRA
A SNOWTAM is a special series NOTAM that provides runway surface condition information following contamination by snow, ice, slush, or standing water. Since November 2021, the global standard has been the GRF (Global Reporting Format), which uses RWYCC (Runway Condition Codes) on a scale from 0 to 6 to describe friction and braking action by runway third.
The RWYCC codes map directly to aircraft performance: code 6 represents a dry surface; code 5 is wet or damp; code 4 indicates wet snow or dry snow over compacted snow; codes 3, 2, and 1 represent progressively more contaminated and slippery conditions; and code 0 (nil braking action) is the most hazardous condition, requiring specific aircraft certification for operations. Airlines and operators are required to crosscheck RWYCC codes against performance data in their aircraft operations manual before every winter landing.
NOTAMs (Notices to Airmen) are the broader system within which SNOWTAMs sit. A NOTAM can cover any operationally relevant information: runway closures, ILS out of service, airspace restrictions, laser activity, parachute operations, construction on taxiways, or changes to published instrument procedures. Every professional preflight briefing includes a NOTAM check for departure, route, destination, and alternates.
The cloud base calculator uses the relationship between surface temperature and dew point to estimate the height at which a rising parcel of air will reach saturation and form cloud. This height is the lifting condensation level (LCL), and for convective cumulus clouds it closely approximates the visible cloud base.
The formula is derived from the difference between the dry adiabatic lapse rate (DALR) and the dew point lapse rate. As an air parcel rises, it cools at the DALR of approximately 3°C per 1,000 feet. The dew point decreases more slowly, at approximately 0.5°C per 1,000 feet. The parcel reaches saturation when temperature equals dew point — a convergence rate of 2.5°C per 1,000 feet. Inverting this gives approximately 400 feet per 1°C of surface temperature-dew point spread.
This calculation is most reliable for convective (cumulus) cloud development. For stratiform (layered) cloud, stratus, or radiation fog, the formula is less applicable because these clouds form through different mechanisms. In those cases, the METAR ceiling should be treated as the authoritative source.
Flight Category Reference
Flight categories translate raw METAR ceiling and visibility data into an immediate operational assessment. Each category corresponds to a specific range of conditions and determines which pilot certificates, aircraft equipment, and ATC clearances are required to fly legally.
| Category | Ceiling | Visibility | Operational Implication |
|---|---|---|---|
| VFR | Above 3,000 ft AGL | Greater than 5 SM | Standard visual conditions. VFR flight permitted without ATC clearance in most airspace classes. |
| MVFR | 1,000 – 3,000 ft AGL | 3 – 5 SM | Marginal conditions. Technically legal for VFR but requires heightened awareness. Instrument currency strongly recommended. |
| IFR | 500 – 999 ft AGL | 1 – <3 SM | Instrument conditions. Instrument rating and equipped aircraft required. ATC IFR clearance mandatory in controlled airspace. |
| LIFR | Below 500 ft AGL | Below 1 SM | Low IFR conditions. Precision instrument approaches required. Many airports become operationally closed. High situational risk. |
The ceiling for flight category purposes is the lowest broken (BKN) or overcast (OVC) layer in the METAR sky condition group. Scattered (SCT) and few (FEW) layers are not ceilings and do not determine the flight category, even if they are very low. This distinction is critical: a METAR reporting FEW003 SCT008 BKN025 has a ceiling of 2,500 feet (BKN025) and is classified MVFR, not IFR, despite the low few and scattered layers.
Vertical visibility (VV) is reported instead of sky condition when the sky is obscured — typically by dense fog, heavy precipitation, or smoke. Vertical visibility acts as the ceiling for flight category determination: VV004 indicates the pilot can see only 400 feet straight up into the obscuration, placing conditions firmly in the LIFR category.
Flight category is determined by whichever of ceiling or visibility produces the lower classification. If ceiling is above 3,000 feet (VFR) but visibility is only 2 statute miles (IFR), the airport is classified as IFR. If visibility is 8 SM (VFR) but the ceiling is 700 feet (IFR), the airport is IFR. The most restrictive element always determines the applicable category.
This matters particularly in conditions involving widespread low stratus with good horizontal visibility below the cloud base — a common winter scenario where pilots may be tempted to fly VFR below a very low but technically scattered layer. The ceiling and visibility must both meet VFR minimums for the applicable airspace class, not just one of them.
Operational Workflow
A structured weather briefing follows a logical sequence from macro to micro — starting with the big picture and progressively narrowing to the specific conditions at each phase of the planned flight. The sequence below reflects standard professional practice for VFR and IFR operations.
Begin with the surface analysis chart and significant weather prognostic charts. Identify the position and movement of pressure systems, fronts, and associated weather. This sets the context for all specific reports — a METAR reporting 5 SM in mist means something different ahead of an approaching warm front than it does in a post-frontal air mass.
Decode the current METAR for the departure airport. Identify the flight category, crosswind component for the active runway (using the crosswind calculator), altimeter setting, and any significant present weather. Check the TAF to confirm conditions during the planned departure window are above your minimums.
Check SIGMET advisories for severe turbulence, severe icing, volcanic ash, and tropical cyclone activity along and within 50 NM of the route. Check AIRMET advisories for moderate icing, moderate turbulence, extensive mountain obscuration, and low-level wind shear. Review available PIREPs for pilot-reported conditions at your planned altitude.
Decode the destination METAR and check the TAF for conditions during the ETA window. For IFR planning, the destination TAF must show conditions above alternate minima for the period 1 hour before to 1 hour after the planned ETA. If the destination TAF is not available, treat the destination as requiring an alternate regardless of actual forecast confidence.
For IFR flights requiring an alternate, verify the alternate TAF shows conditions at or above alternate minima (typically ceiling 600 ft and visibility 2 SM for a precision approach alternate, or ceiling 800 ft and 2 SM for a non-precision approach alternate, under FAA regulations). Check the alternate’s METAR to confirm current conditions are reasonable.
Review NOTAMs for all airports and the route of flight. In winter or following precipitation, check the SNOWTAM for departure and destination runways. Verify that runway friction codes are within your aircraft’s approved operating envelope and that no nav-aids, approaches, or runways you are counting on are out of service.
Beyond METAR and TAF
METARs and TAFs form the core of aerodrome weather information but are supplemented by a broader suite of products that cover en route conditions, upper-level weather, and hazardous phenomena at scales beyond the individual airport.
Significant Meteorological Information. Issued for severe conditions hazardous to all aircraft: severe or extreme turbulence, severe icing, widespread dust or sandstorm, volcanic ash, or tropical cyclone. Convective SIGMETs (WST in the US) are issued for lines of thunderstorms, embedded thunderstorms, and areas of severe or extreme turbulence associated with convection.
Airmen’s Meteorological Information. Covers conditions significant to light aircraft but less severe than SIGMET thresholds: moderate icing (AIRMET Sierra/Zulu), moderate turbulence (AIRMET Tango), IFR conditions and mountain obscuration (AIRMET Sierra), and low-level wind shear (AIRMET Sierra). Valid for 6 hours, or 12 hours over mountainous terrain.
Pilot Report. Voluntary or mandatory pilot-generated reports of in-flight conditions including turbulence, icing, cloud tops, and significant weather encounters. PIREPs are the only real-time source of conditions aloft and are invaluable for validating or questioning forecast products. Mandatory PIREPs are required in many jurisdictions when encountering moderate or greater icing or turbulence.
Forecast of wind direction, speed, and temperature at specific pressure altitudes from 3,000 to 45,000 feet. Used for fuel planning, TAS and groundspeed calculation, optimal cruise altitude selection, and determining the most efficient routing for long-distance flights. In the US, the product is the FB (Winds and Temperatures Aloft Forecast); internationally, the ICAO equivalent is the GAMET or gridded forecast products.
Automatic Terminal Information Service. A continuous broadcast of current airport weather and operational information on a dedicated frequency. Each new broadcast is identified by a sequential letter of the ICAO phonetic alphabet (Information Alpha, Bravo, Charlie...). Pilots must listen to the current ATIS and report the information identifier on initial contact with ATC. D-ATIS (Digital ATIS) delivers the same information via datalink to ACARS-equipped aircraft.
A forecast of visual conditions and non-convective clouds for a broad geographic area, typically valid for 18 hours and covering multiple states or regions. Used for general situational awareness and VFR route planning. The area forecast is supplementary to aerodrome TAFs and does not replace individual airport forecasts for flight planning purposes.
Common Questions
A METAR (Meteorological Aerodrome Report) is a standardised aviation weather observation issued at regular intervals by certified weather observers or automated surface observation systems (ASOS/AWOS). At most controlled airports it is issued every hour, on the hour (METAR), and an updated report called a SPECI is issued whenever significant changes in wind, visibility, ceiling, or present weather occur between scheduled observations. In ICAO format, METARs are valid for one hour from the observation time stamped in the report. Pilots use the METAR to determine the current flight category, assess compliance with VFR or IFR minima, and extract the altimeter setting for their altimeter subscale.
A METAR is an observation — it reports what the weather actually is at a specific airport at a specific moment in time, based on measurements taken by instruments or trained observers. A TAF (Terminal Aerodrome Forecast) is a prediction — it forecasts what the weather is expected to be at the same airport over the next 9 to 30 hours. METARs are updated hourly or when conditions change significantly. TAFs are issued four times per day (every 6 hours) and are expressed in a similar coded format to METARs but include change groups such as TEMPO (temporary conditions lasting less than 60 minutes), BECMG (conditions becoming and remaining), FM (from a specific time), and PROB (probability of specified conditions).
Aviation weather flight categories classify the severity of weather conditions at an airport relative to standard VFR minimums. VFR (Visual Flight Rules) applies when ceiling is above 3,000 feet AGL and visibility is greater than 5 statute miles — good conditions for visual flight. MVFR (Marginal VFR) applies when ceiling is 1,000 to 3,000 feet AGL or visibility is 3 to 5 statute miles — legal for VFR but requiring caution. IFR (Instrument Flight Rules) applies when ceiling is 500 to 999 feet AGL or visibility is 1 to less than 3 statute miles — instrument conditions. LIFR (Low IFR) is the most severe category, when ceiling is below 500 feet AGL or visibility is below 1 statute mile — extremely limited conditions requiring precision instrument approaches.
Wind in a METAR is reported in the format DDDffKT, where DDD is the wind direction in degrees magnetic from which the wind is blowing (using three digits, so 090 means easterly), ff is the wind speed in knots (two or three digits), and KT indicates the units are knots. Gusts are appended as GXX, so 27018G26KT means wind from 270° at 18 knots, gusting to 26 knots. Variable wind direction is reported as VRB when wind speed is 6 knots or less and direction is changing, or as a range such as 240V310 when speed exceeds 6 knots and direction varies by 60° or more. Calm wind (less than 3 knots) is reported as 00000KT.
CAVOK (Ceiling and Visibility OK) is a METAR and TAF abbreviation used when all three of the following conditions are simultaneously met: visibility is 10 kilometres or greater; there are no clouds below 5,000 feet (1,500 metres) AGL or below the minimum sector altitude (whichever is higher), and no cumulonimbus or towering cumulus at any level; and there are no significant present weather phenomena. When CAVOK is reported, the visibility, cloud, and present weather groups are replaced by the single term CAVOK. It is used in ICAO-format METARs and TAFs worldwide but is not used in US domestic METARs, which instead use SKC (sky clear) or CLR.
A SNOWTAM is a special series NOTAM issued by an aerodrome authority to notify pilots and airlines about runway surface conditions caused by snow, ice, slush, or standing water. It is issued after each inspection of the runway and provides information about the type and depth of contaminant on each third of each runway, the friction coefficient or braking action measured, and any operational restrictions resulting from the surface condition. The global standard for reporting runway condition information since 2021 is the GRF (Global Reporting Format), which uses RWYCC (Runway Condition Codes) from 0 (nil braking action) to 6 (dry). SNOWTAMs are part of the NOTAM system and are time-limited, typically valid for 8 hours from the observation time.
Cloud base height is estimated using the surface temperature-dew point spread — the difference in degrees Celsius between the two values. The standard formula used in general aviation is: cloud base (in feet AGL) = temperature-dew point spread × 400. This relationship is based on the moist adiabatic lapse rate — as a rising air parcel cools at approximately 3°C per 1,000 feet and the dew point decreases at approximately 0.5°C per 1,000 feet, they converge at a rate of 2.5°C per 1,000 feet, or roughly 400 feet per 1°C of spread. For example, a temperature of 20°C and a dew point of 12°C gives a spread of 8°C, estimating cloud base at 3,200 feet AGL. This formula is most accurate for convective cumulus clouds and is a useful cross-check against reported METAR ceiling data.
Cloud cover in aviation is described using oktas (eighths of sky cover): FEW (1-2 oktas), SCT or scattered (3-4 oktas), BKN or broken (5-7 oktas), and OVC or overcast (8 oktas, or full sky cover). The ceiling is specifically defined as the lowest layer of clouds reported as broken or overcast, or the vertical visibility into an obscuration such as fog or precipitation. A ceiling is what matters for flight category determination and instrument approach minimums — not scattered or few cloud layers, which do not constitute a ceiling. A METAR reporting SCT015 BKN035 has a ceiling of 3,500 feet (the broken layer), not 1,500 feet, because only broken and overcast layers define the ceiling.
A SPECI (Special Meteorological Report) is an unscheduled METAR issued when weather conditions change rapidly enough to require immediate notification to pilots and ATC. A SPECI is mandated when any of the following changes occur: wind direction changes by 60° or more with mean speed before or after change of 10 knots or more; wind speed changes by 10 knots or more; visibility crosses a threshold of 800 metres, 1,500 metres, 3,000 metres, or 5,000 metres; present weather changes to or from thunderstorm, freezing precipitation, or low drifting snow; cloud amount or height of lowest broken or overcast layer changes across 300 feet, 600 feet, 1,500 feet, or 3,000 feet thresholds; or temperature changes significantly. Pilots do not request SPECIs — they are issued automatically by the aerodrome meteorological office. However, pilots should request a fresh ATIS or check for updated METARs before committing to an approach when weather is near minimums.
A complete preflight weather briefing for any flight should include: the current METAR for the departure airport (wind, visibility, ceiling, present weather, altimeter setting); the TAF for the departure airport covering the planned departure window; METARs and TAFs for the destination airport and any planned alternates; SIGMET and AIRMET advisories for the route of flight (covering significant weather, icing, and turbulence); PIREPs from pilots who have recently flown the route or nearby airspace; winds aloft forecasts for the planned cruise altitude; and NOTAM checks for departure, destination, alternates, and any IAP changes. In winter, SNOWTAMs for all relevant airports should be included. The standard for a complete briefing includes departure, en route, and destination weather — not just the origin and destination.