How to Read METAR & TAF | Aviation Weather Guide

Before each flight, you must check the METAR and TAF for your departure airport, destination airport, alternate airport, and even en-route alternates.

I.2.9 Significant Change Indicators

II. What is a METAR?

A METAR is a coded report providing an actual observation of current weather conditions at an aerodrome. It is primarily used for real-time decision-making for takeoff, approach, and landing.

• Every 30 or 60 minutes (depending on the aerodrome and regulations);
• Valid at the time of observation; typically considered operationally valid for 30 to 60 minutes. TREND forecasts, when included, are valid for 2 hours.

METAR shall contain the following elements in the order indicated:

• Identification of the type of report;
• Location indicator;
• Time of the observation;
• Identification of an automated or missing report, when applicable;
• Surface wind direction and speed;
• Visibility;
• Runway visual range, when the reporting criteria are met;
• Present weather;
• Cloud amount, cloud type only for cumulonimbus and towering cumulus clouds and height of cloud base or, where measured, vertical visibility;
• Air temperature and dew-point temperature;
• Atmospheric pressure (QNH and, when applicable, in local routine and local special report, QFE);
• Supplementary information, when applicable.

For aerodromes, location indicators are four-letter codes listed in ‘ICAO Doc 7910 - Location Indicators’.

Indicates the day and time of the observation in Coordinated Universal Time (UTC), using the format DDTTTTZ, where:

• DD = day of the month;
• TTTT = time in hours and minutes (HHMM);
• Z = indicates UTC (Zulu time).

AUTO or NIL

The wind is reported in steps of 10 degrees true and 1 kt.

Variations from the mean wind direction during the past 10 min are reported as follows, if the total variation is ≥ 60°:

VRB = Variable wind.

Variations from the mean wind speed (gusts), during the past 10 minutes shall be reported when the maximum wind speed exceeds the mean speed by 5 KT or 10 KT:

G25KT = gusting to 25 knots. Variations from the mean wind speed, during the past 10 minutes shall be reported when the maximum wind speed exceeds the mean speed by 5 or 10 knots.

CALM = Calm = When a wind speed of less than 1 kt is reported, it shall be indicated as calm.

P99KT = When a wind speed of 100 KT or more is reported (e.g., 140P99KT).

Visibility = Visibility for aeronautical purposes, which is the greater of the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognised when observed against a bright background or; the greatest distance at which lights in the vicinity of 1000 candelas can be seen and identified against an unlit background.

The visibility shall be reported in steps of:

• 50 m when the visibility is less than 800 m;
• 100 m when it is 800 m or more, but less than 5 km, and;
• in kilometre steps when the visibility is 5 km or more, but less than 10 km;
• and it shall be given as 10 km (or 9999) when the visibility is 10 km or more, except when the conditions for the use of CAVOK apply.

In METAR visibility should be reported as prevailing visibility.
Prevailing visibility = the greatest visibility value, observed in accordance with the definition of ‘visibility’, which is reached within at least half the horizon circle or within at least half of the surface of the aerodrome. These areas could comprise contiguous or non-contiguous sectors.

When the visibility is not the same in different directions the lowest visibility observed should also be reported when the lowest visibility:

• < 1,500 m;
• < 5,000 m and < 50% of the prevailing visibility.

When possible, its general direction relative to the aerodrome reference point should be indicated using one of the eight points of the compass

RVR = Runway Visual Range means the range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line. In METAR, the RVR shall be reported throughout periods when either the visibility or the runway visual range is less than 1 500 m.

The RVR shall be reported in steps of:

• 25 m when it is less than 400 m;
• 50 m when it is between 400 and 800 m;
• 100 m when it is more than 800 m.

When the RVR is above or below the measurement limits of the system in use, it is reported with a prefix indicating the exceedance.
P = Plus = Greater than. Used when the RVR is above the maximum value that can be determined by the system in use. It is followed by the maximum value that can be determined by the system.
M = Minus = Less than. Used when the RVR is below the minimum value that can be determined by the system in use. It is followed by the minimum value that can be determined by the system.

When the variation of the RVR values shows an upward or downward tendency, this should be indicated by the abbreviation ‘U’ or ‘D’, respectively. In cases when actual fluctuations during the 10-minute period show no distinct tendency, this should be indicated using the abbreviation ‘N’.

• U = Upward trend (RVR is increasing)
  
• D = Downward trend (RVR is decreasing)
  
• N = No distinct tendency (fluctuations without clear trend)

Present weather shall be reported using up to three abbreviations, combining (see the table in the following section):

• Intensity or proximity (e.g., +, -, VC)
  
• Characteristic (e.g., TS, SH)
  
• Type (e.g., RA, SN)

‘MOD’ can be used in an D-ATIS message:

VCY = VC = Vicinity . Means between approximately 8 and 16 km of the aerodrome reference point.

SH = Shower. Used to report showers.

BL = Blowing. Used to report weather phenomena raised by the wind to 6 ft (2 m) or more above ground level.

DR = Low drifting. Used to report weather phenomena lifted by the wind to less than 6 ft (2 m) above ground level.

MI = Shallow. Used to report a very low layer of fog or other phenomena, less than 6 ft (2 m) above ground.

BC = Patches. Used to report fog patches randomly covering parts of the aerodrome.

PR = Partial. Used to report fog covering a substantial part of the aerodrome while the rest remains clear.

TS = Thunderstorm. Used to report a thunderstorm in progress.

FZ = Freezing. Used to report supercooled water droplets or precipitation, typically combined with other phenomena (e.g. FZRA).

DZ = Drizzle. Used to report drizzle.

RA = Rain. Used to report rain.

SN = Snow. Used to report snow.

SG = Snow grains. Used to report snow grains.

PL = Ice pellets. Used to report ice pellets.

GR = Hail. Used to report hail when the largest hailstones are 5 mm or more in diameter.

GS = Small hail or Snow pellets. Used to report small hail or snow pellets when the largest hailstones are less than 5 mm.

UP = Unidentified Precipitation. Used when the type of precipitation cannot be determined (typically in automated reports).

Hydrometeors = water-based particles suspended in the atmosphere, either liquid or solid, that affect visibility or fall as precipitation.
FG = Fog. Used to report fog when visibility is less than 1,000 meters.
BR = Mist. Used to report mist when visibility is between 1,000 and 5,000 meters.

Lithometeors = dry particles such as dust, sand, smoke, or haze suspended in the air that reduce visibility. They consist of non-water-based particles.
The following should be used only when the obscuration consists predominantly of lithometeors and the visibility is 5,000 m or less, except ‘SA’ when qualified by ‘DR’ and volcanic ash:
HZ = Haze. Used to report haze when visibility is 5,000 meters or less, mainly caused by dry particles (lithometeors).
SA = Sand. Used to report sand in suspension in the atmosphere.
DU = Dust. Used to report widespread dust in the air.
FU = Smoke. Used to report smoke affecting visibility.
VA = Volcanic ash. Used to report volcanic ash in the atmosphere.

// = Temporary failure of system/sensor. The present weather should be replaced by ‘//’ when it cannot be observed due to a temporary failure of the system/sensor.

PO = Dust/Sand Whirls. Used to report small, short-lived rotating columns of dust or sand (dust devils).

SQ = Squall. Used to report a sudden, strong, and brief increase in wind speed lasting at least one minute.

FC = Funnel Cloud. Used to report a tornado or waterspout (funnel-shaped cloud extending toward the ground or water).

DS = Dust Storm. Used to report a widespread dust storm that significantly reduces visibility.

SS = Sandstorm. Used to report a widespread sandstorm causing reduced visibility and surface conditions.

RE = Recent Weather Phenomena. Refers to weather phenomena observed at the aerodrome during the period since the last routine report or within the last hour, whichever is shorter, but not at the exact time of the current observation.

In a METAR, cloud type is reported only for cumulonimbus (CB) and towering cumulus (TCU) clouds. Other cloud types can be found on SIGWX charts. 

The cloud amount should be reported using the abbreviations ‘FEW’, ‘SCT’, ‘BKN’ or ‘OVC’.

Oktas are a unit used to describe cloud cover, representing how much of the sky is covered in eighths (1 okta = 1/8 of the sky).

NSC is used when there are no clouds of operational significance and no vertical visibility restriction, but ‘CAVOK’ is not applicable.

NCD = No Clouds Detected. Used when no clouds are detected by an automated observing system.

CAVOK = Ceiling And Visibility OK. Used in place of visibility, RVR, present weather, and cloud information when all of the following conditions are met:

• Visibility is 10 km or more (and the lowest visibility is not reported);
• no clouds of operational significance;
• no significant weather is observed.

‘Cloud of operational significance’ means a cloud with the height of cloud base below 5,000 ft or below the highest minimum sector altitude, whichever is greater, or a cumulonimbus cloud (CB) or a towering cumulus cloud (TCU) at any height.

Note: ‘ceiling’ means the height above the ground or water of the base of the lowest layer of cloud below 20,000 ft covering more than half of the sky (BKN, OVC or VV).

In a METAR, the height of the cloud base is reported, or, when measured, vertical visibility is provided instead.

VV = Vertical Visibility. Used when the sky is obscured and clouds cannot be observed or forecasted. If vertical visibility is available, it is reported using ‘VV’ followed by the measured value in feet.

/// = Unknown cloud type. Used when the cloud type cannot be observed; replaces the cloud type in each cloud group.

CB/TCU with /// = Unknown amount or height. Used when cumulonimbus (CB) or towering cumulus (TCU) clouds are detected, but their amount and/or the height of cloud base cannot be observed. The missing information is replaced by ‘///’.

VV/// = Vertical visibility unknown. Used when the sky is obscured but the vertical visibility value cannot be determined due to a temporary system or sensor failure.

In such cases, it’s worth deepening your weather analysis: check charts, satellite imagery, webcams, and other sources to get a clearer picture.

‘Significant meteorological conditions’ means any of the following conditions are present:

M = Minus. Used in METARs to indicate negative temperatures or values below zero. It precedes the numeric value.

Remember, the DP (dew point) is the temperature at which air becomes saturated with water vapor, causing the vapor to condense into liquid water. At this temperature, the air can no longer hold all of the water vapor it contains, and some of it condenses into dew, fog, or clouds. The dew point is a critical measure in meteorology as it indicates the moisture content of the air; a higher dew point means more moisture, while a lower dew point indicates drier air.

The spread, or the difference between the Outside Air Temperature (OAT) and the dew point, is closely related to cloud base height. A smaller spread indicates higher humidity levels and suggests that the air is closer to saturation. This condition often leads to cloud formation at lower altitudes, resulting in a lower cloud base height. Conversely, a larger spread indicates drier air and typically results in higher cloud base heights.

The QNH and QFE shall be computed in tenths of hectopascals and reported therein in steps of whole hectopascals, using four digits.

 QNH = Altimeter Pressure Setting to Sea Level. The atmospheric pressure reduced to mean sea level, used to calibrate the altimeter so it reads altitude above sea level.

QFE = Altimeter Pressure Setting to Airfield Elevation. The atmospheric pressure at the airfield reference level, used to set the altimeter to zero at the airfield elevation, indicating height above ground level.

See our conversion tables in the guide: Math for Pilots for more useful equivalents.

Also, remember to review our course on altimetry—covering topics such as QNH settings, transition altitudes, and flight levels

Although a METAR is an observation report, it may include a forecasts for landing, known as a TREND forecast.

This TREND must begin with one of the change indicators: ‘BECMG’ (becoming) or ‘TEMPO’ (temporary). When no change is expected to occur, this shall be indicated by the term ‘NOSIG’ (no significant change). To indicate the end of a weather phenomenon’s occurrence, the abbreviation ‘NSW’ (nil significant weather) shall be used. See our chapter on significant changes for more definitions.

III. What is a TAF?

A TAF is a coded forecast describing the expected weather conditions at an aerodrome over a specified period (9 hours, but typically 24 or 30 hours). It is essential for flight planning, alternate selection, and dispatch. See our topic on aerodrome selection policy.

Generally, the same elements as in a METAR are found, but the TAF is usually longer as it covers a greater forecast period. Significant changes sometimes include probabilities, which are not found in observation messages.

Only some important differences with the METAR section are covered in this chapter.

TX = Maximum Temperature. Indicates the highest air temperature expected during the forecast period, along with the time (UTC) when it is expected to occur.

TN = Minimum Temperature. Indicates the lowest air temperature expected during the forecast period, along with the time (UTC) when it is expected to occur.

Generally, the same elements as in a METAR are found, but the TAF is usually longer as it covers a greater forecast period. Significant changes sometimes include probabilities, which are not found in observation messages.

Only some important differences with the METAR section are covered in this chapter.

IV. Significant Change Indicators: BECMG, TEMPO, PROB, NOSIG, NSW

IV.1. Forecast

Although a METAR is primarily an observation, it may include a short-term landing forecast (TREND forecast), depending on the aerodrome — the provision of such forecasts is determined by the competent authority.

Significant changes affecting one or more of the following elements are indicated:

• Surface wind;
• Visibility;
• Weather phenomena;

NOSIG = No Significant Change. Used when no significant changes are expected during the TREND validity period (within the next 2 hours).

NOSIG is a term used in TREND forecasts only, so you should not find it in a TAF.

BECMG = Becoming. Used to describe forecast changes where the meteorological conditions are expected to reach or pass through specified values at a regular or irregular rate. The time period shall not exceed 4 hours.

TEMPO = Temporary. Used to describe forecast temporary fluctuations in the meteorological conditions which reach or pass specified values and last for a period of less than 1 hour in each instance and, in the aggregate, cover less than one half of the period during which the fluctuations are forecast to occur.

PROB = Probability. Used to indicate a 30% or 40% chance of alternative meteorological conditions occurring during a specified forecast time period.

PROB TEMPO = Probability of temporary fluctuations. Used to indicate the probability of temporary fluctuations, showing that there is a 30% or 40% chance of temporary meteorological conditions occurring during a specified forecast period.

The period during which, or the time at which, the change or temporary fluctuations are forecast to occur shall be indicated using the abbreviations  ‘FM’ (from), ‘TL’ (until), or ‘AT’ (at), as appropriate, each followed by a time group in hours and minutes.

When the change is forecast to commence at the beginning of the trend forecast period and be completed by the end of that period, or when the change is forecast to occur within the trend forecast period but the time is uncertain, the abbreviations ‘FM’, ‘TL’ or ‘AT’ and their associated time groups should be omitted and the change indicator ‘BECMG’ should be used alone.

When the period of the temporary fluctuations is forecast to commence at the beginning of the TREND forecast period and cease by the end of that period, both abbreviations ‘FM’ and ‘TL’ and their associated time groups should be omitted and the change indicator ‘TEMPO’ should be used alone.

If a temporary fluctuation is expected to last 1 hour or longer, the change group ‘BECMG’ shall be used, or the forecast’s validity period should be subdivided.

When a set of prevailing weather conditions is expected to change significantly and more or less completely to a different set, the validity period must be divided into self-contained periods.

This is done using the abbreviation ‘FM’ followed immediately by a six-digit UTC time group (day, hour, minutes) indicating the expected time of change.

The forecast conditions following ‘FM’ supersede all previous conditions given before it, making each subdivided period independent.

V. Change Groups and probability indicators: How to read forecast evolutions

Let’s analyse the visibility forecast.

Suppose we are operating under VFR and planning to land on the 6th at 09:00 UTC, with a minimum visibility requirement of 5 km. The following forecast details are included in the TAF for our destination:

How to interpret a probability:

• PROB30 = Low probability
• PROB40 = High probability

How you act on this depends on your environment — for example, whether you're flying in a valley with no escape options, or you have plenty of alternate aerodromes or diversion possibilities. It also depends on your experience level and how critical the flight is to you.

ICAO doesn’t define PROB30 or PROB40 as “low” or “high” officially — these are our operational interpretations. PROB40 still doesn’t guarantee the event will happen, but it does require greater caution.

It is better to be on the ground and regret not being in the air than to be in the air and regret not being on the ground.

See our article on interpreting TAFs in the context of Commercial Air Transport (CAT).

BECMG : 

Deterioration: Assume weather conditions worsen from the start of the BECMG period.

Improvement: Assume weather conditions improve only by the end of the BECMG period.

Always plan conservatively — never expect better conditions before they are fully established.

TEMPO / PROB30/40

Temporary improvements in weather conditions should generally be disregarded in flight planning.

Deteriorations depend on the weather type:

• Transient or showery conditions (e.g., thunderstorms, showers):

May be disregarded if you carry enough fuel to hold or divert upon arrival.

• Persistent conditions (e.g., haze, mist, fog, dust/sandstorms, continuous precipitation):

Should be taken into account as they are likely to impact flight operations.

For PROB30/40 combined with TEMPO:

This depends on your experience and risk mitigation capabilities. For example, in commercial aviation, the following recommendations apply:

• Deteriorations may be disregarded in flight planning.
• Improvements should be disregarded...

See our article on interpreting TAFs in the context of Commercial Air Transport (CAT).

VI. Other aviation reports

ATIS = Automatic Terminal Information Service. The automatic provision of current, routine information to arriving and departing aircraft, available 24 hours a day or during a specified period.

Voice-ATIS = Voice Automatic Terminal Information Service. Provision of ATIS through continuous and repetitive voice broadcasts.

D-ATIS = Data Link Automatic Terminal Information Service. Provision of ATIS information via data link.

ATIS can be provided via radio frequency, data link, or telephone.

In METAR and SPECI, surface wind direction is reported in degrees true.

In ATIS, surface wind direction is reported in degrees magnetic.

SPECI = Special Weather Report. A special report issued when significant changes occur between routine METAR observations, providing immediate updates for safety.

SIGMET = Significant Meteorological Information. It means information, issued by a meteorological watch office, concerning the occurrence or expected occurrence of specified en-route weather and other phenomena in the atmosphere which may affect the safety of aircraft operations and of the development of those phenomena in time and space;

OBSC = Obscured. Thunderstorms and cumulonimbus clouds obscured by haze, smoke or darkness.

EMBD = Embedded. Thunderstorms embedded within cloud layers not readily recognisable.

ISOL = Isolated. Individual features affecting less than 50% of the area concerned.

OCNL = Occasional. Features well separated affecting between 50% and 75% of the area concerned.

FRQ = Frequent. Little or no separation between adjacent thunderstorms affecting more than 75% of the area.

SQL = Squall Line. A line of thunderstorms with little or no space between clouds.

GR = Hail. Used to describe hail associated with a thunderstorm.

ICE = Icing. Refers to severe and moderate icing in non-convective clouds.

FZRA = Freezing Rain. Severe icing caused by freezing rain.

TURB = Turbulence. Refers to low level turbulence linked to strong surface winds, rotor streaming or CAT = clear air turbulence. Not used with convective clouds.

Moderate TURB. EDR peak value between 0.20 and 0.45.

Severe TURB. EDR peak value greater than or equal to 0.45.

EDR = Eddy Dissipation Rate. A quantitative measure of atmospheric turbulence intensity, representing the rate at which turbulent kinetic energy is dissipated by viscous forces in the atmosphere. It is used to classify turbulence severity, with higher values indicating stronger turbulence.

MTW = Mountain Wave.

Moderate: Downdraft between 350 and 600 ft per minute (1.75 to 3.0 m/s) and/or moderate turbulence.

Severe: Downdraft greater than or equal to 600 ft per minute (3.0 m/s) and/or severe turbulence.

AIRMET = Airmen’s Meteorological Information. Advisories for moderate weather phenomena of operational significance, typically intended for low-level flights.

VOLMET = Meteorological information for aircraft in flight.

It is a service providing, by continuous and repetitive voice broadcasts, current METAR, SPECI, TAF and SIGMET information for selected aerodromes. VOLMET broadcasts are intended to keep flight crews informed about en-route weather conditions relevant to their flight planning and in-flight decision making.

VOLMET broadcast = The provision, as appropriate, of current METAR, SPECI, TAF, and SIGMET by means of continuous and repetitive voice broadcasts.

D-VOLMET = Data link-VOLMET. This is provision of METAR, SPECI, TAF, SIGMET, special air-reports not covered by SIGMET, and, where available, AIRMET via data link.

When prescribed by the competent authority, HF or VHF VOLMET broadcasts, or D-VOLMET services, or all of these, shall be provided using standard radiotelephony phraseologies.

GAMET = General Aviation Meteorological Forecast. It provides concise forecasts of significant meteorological phenomena over specific flight information regions (FIRs) aimed primarily at general aviation pilots.

Local Routine Report = Meteorological report issued at fixed intervals for dissemination only at the originating aerodrome, providing detailed local observations.

Local Special Report = A meteorological report issued according to criteria established for special observations, intended only for dissemination at the aerodrome of origin where the observations were made.

AIREP = Special Air Report. Reports from aircraft in flight detailing encountered meteorological conditions, valuable for real-time data beyond ground observations.

VAR : Volcanic Activity Report. Issued to report volcanic eruptions or ash clouds, providing critical safety information about volcanic hazards.

The report on pre-eruption volcanic activity, volcanic eruptions, and volcanic ash clouds shall be made in the format of a Volcanic Activity Report including the following meteorological information in the order indicated:

• Message type, VOLCANIC ACTIVITY REPORT;
• Station identifier, location indicator, or station name;
• Date and time of the message;
• Location and name of the volcano, if known;

A concise description of the event, including as appropriate the level of volcanic activity intensity, occurrence of an eruption with date and time, and the presence of volcanic ash clouds in the area with their direction of movement and height.

NOTAM = Notice to Airmen. A notice containing essential information about the establishment, condition, or change of any aeronautical facility, service, procedure, or hazard, which is timely and crucial for flight operations.

ASHTAM = Ash Temporary Airmen’s Information Message. It is a special series of NOTAM notifying by means of a specific format of a change in the activity of a volcano, a volcanic eruption and/or volcanic ash cloud that is of significance to aircraft operation

SNOWTAM = Snow Temporary Airmen’s Information Message. It is special series NOTAM given in a standard format, which provides a surface condition report notifying the presence or cessation of hazardous conditions due to snow, ice, slush, frost, standing water or water associated with snow, slush, ice, or frost on the movement area

• See the Commission Implementing Regulation (EU) 2017/373
• MET.TR.200
• AMC2 MET.TR.200
• AMC2 MET.TR.205(e)(1)
• MET.TR.205
• AMC1 MET.TR.205
• MET.TR.210
• AMC1 MET.TR.210
• MET.TR.220
• MET.TR.225
• AMC1 MET.TR.225
• MET.TR.250
• ATS.TR.325
• SERA.9010