Reading METARs and TAFs

METARs

METAR is also known as Meteorological Terminal Aviation Routine Weather Report or Meteorological Aerodrome Report.

ME - Meteorlogical

T - Terminal 

A - Aviation

R - Routine

METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting.

Raw METAR is the most common format in the world for the transmission of observational weather data.  It is highly standardized through the International Civil Aviation Organization (ICAO), which allows it to be understood throughout most of the world.

METARs typically come from airports or permanent weather observation stations. Reports are generated once an hour or half-hour, but if conditions change significantly, a report known as a special (SPECI) may be issued. Some METARs are encoded by automated airport weather stations located at airports, military bases, and other sites. Some locations still use augmented observations, which are recorded by digital sensors, encoded via software, and then reviewed by certified weather observers or forecasters prior to being transmitted. Observations may also be taken by trained observers or forecasters who manually observe and encode their observations prior to transmission. 

A typical METAR contains data for the temperature, dew point, wind speed and direction, precipitation, cloud cover and heights, visibility, and barometric pressure. A METAR may also contain information on precipitation amounts, lightning, and other information that would be of interest to pilots or meteorologists such as a pilot report or PIREP, colour states and runway visual range (RVR).

In addition, a short period forecast called a TREND may be added at the end of the METAR covering likely changes in weather conditions in the two hours following the observation. These are in the same format as a Terminal Aerodrome Forecast (TAF).

The complement to METARs, reporting forecast weather rather than current weather, are TAFs. METARs and TAFs are used in VOLMET broadcasts.

Some abbreviations are intuitive - KT for knots, BKN for broken, SM for statue miles, RA for rain.  But, since the original METAR system was developed by the French you will find some abbreviations to be not quite so obvious - BR for mist (French: Brume),   FU for smoke (French: Fume'e).  You can get the current METAR (along with other weather information) here: METAR Data/Decoder   - note that you can get the raw data form of a METAR or have this site decode it for you. 

North American METAR codes

 

North American METARs deviate from the WMO (who write the code on behalf of ICAO) FM 15-XII code. Details are listed in the FAA's Aeronautical Information Manual (AIM), but the non-compliant elements are mostly based on the use of non-standard units of measurement. This METAR example courtesy of Wikipedia (remember a contribution is always appreciated) is from Trenton-Mercer Airport near Trenton, New Jersey, and was taken on 5 December 2003 at 18:53 UTC.

METAR KTTN 051853Z 04011KT 1/2SM VCTS SN FZFG BKN003 OVC010 M02/M02 A3006 RMK AO2 TSB40 SLP176 P0002 T10171017

  • METAR indicates that the following is a standard surface weather observation

  • KTTN is the ICAO identifier for the Trenton-Mercer Airport.

  • 051853Z indicates the day of the month is the 5th and the time of day is 1853 Zulu/UTC, 6:53PM GMT, or 1:53PM Eastern Standard Time.

  • 04011KT indicates the wind is from 040° true (north east) at 11 knots (20 km/h; 13 mph). In the United States, the wind direction must have a 60° or greater variance for variable wind direction to be reported and the wind speed must be greater than 3 knots (5.6 km/h; 3.5 mph).  Note that METARs give winds from the TRUE heading.  ATIS, AWOS and other information and weather broadcasts give winds from the MAGNETIC heading.  (If you read it, it is TRUE; if you hear it, it is MAGNETIC)

  • 1/2SM indicates the prevailing visibility is 1⁄2 mi (800 m) SM = statute mile.

  • VCTS indicates a thunderstorm (TS) in the vicinity (VC), which means from 5–10 mi (8–16 km).

  • SN indicates snow is falling at a moderate intensity; a preceding plus or minus sign (+/-) indicates heavy or light precipitation. Without a +/- sign, moderate precipitation is assumed.

  • FZFG indicates the presence of freezing fog.

  • BKN003 OVC010 indicates a broken (?5⁄8 to ?7⁄8 of the sky covered) cloud layer at 300 ft (91 m) above ground level (AGL) and an overcast (8/8 of the sky covered) layer at 1,000 ft (300 m).

  • M02/M02 indicates the temperature is −2 °C (28 °F) and the dew point is −2 °C (28 °F). An M in front of the number indicates a negative Celsius temperature/dew point ("minus").

  • A3006 indicates the altimeter setting is 30.06 inHg (1,018 hPa).

  • RMK indicates the remarks section follows.
     

Note that what follows are not part of standard observations outside of the United States and can vary significantly.

  • AO2 indicates that the station is automated with a precipitation discriminator (rain/snow) sensor.  Stations that aren't equipped with a rain/snow sensor are designated AO1.[11]

  • TSB40 indicates the thunderstorm began at 40 minutes past the hour at 1840 Zulu/UTC, 6:40 p.m. GMT, or 1:40 p.m. Eastern Standard Time.

  • SLP176 indicates the current barometric pressure extrapolated to sea level is 1,017.6 hPa (30.05 inHg).

  • P0002 indicates that 0.02 inches (0.5 mm) of liquid-equivalent precipitation accumulated during the last hour.

  • T10171017 is a breakdown of the temperature and dew point in eight digits separated into two groups of four. The first four digits (1017) indicate the temperature. The first digit (1) designates above or below zero Celsius (0=above zero 1=below zero). The next three digits in the group "017" give the temperature in degrees and tenths of a degree Celsius, −1.7 °C (28.9 °F). The last four digits "1017" indicate the dew point, −1.7 °C (28.9 °F). Note: ASOS software, as of this update, uses whole degrees in °F to compute the °C values in this group.

  • = indicates the end of the METAR.

Terminal Area Forecast (TAF)

Terminal area forecasts (also known as Terminal Aerodrome Forecasts) are similar to METARs but instead of the actual weather observation at the time given they give the forecast for various time periods in the future.  For some parameters there may be differences when compared to METARs (e.g., the maximum visibility given is 6 SM - indicating that visibility is 6 SM or greater - vs. a METAR that gives a maximum visibility value of 10 SM to indicated visibility of 10 SM or greater).  Detailed information about TAFs can be found at the NOAA Site for TAF Decoding.

This example from Wikipedia shows a TAF which also shows the use of 'Probability (of)' and 'Temporary' conditions within the forecast time period from (FM) one forecast time period until the next forecast time period with the the time period when this probability or temporary change in conditions are expected.

TAF

KXYZ 051730Z 0518/0624 31008KT 3SM -SHRA BKN020

FM052300 30006KT 5SM -SHRA OVC030

PROB30 0604/0606 VRB20G35KT 1SM TSRA BKN015CB

FM060600 25010KT 4SM -SHRA OVC050

TEMPO 0608/0611 2SM -SHRA OVC030

RMK NXT FCST BY 00Z=

The first line contains identification and validity times.

  • TAF indicates that the following is a terminal aerodrome forecast.

  • KXYZ indicates the airport to which the forecast applies (ICAO airport code).

  • 051730Z indicates that the report was issued on the 5th of the month at 1730 UTC (also known as Zulu, hence the Z).

  • 0518/0624 indicates that the report is valid from the 5th at 1800 UTC until the 6th at 2400 UTC.

 

The remainder of the first line contain the initial forecast conditions. Variations of the codes used for various weather conditions are many.[3]

  • 31008KT indicates that the wind will be from 310° true at 8 knots (9 mph; 4 m/s).

  • 3SM -SHRA BKN020 indicates that visibility will be 3 statute miles (5 km) in light (-) showers (SH) of rain (RA), with a broken ceiling (between ​5⁄8 and ​7⁄8 of the sky covered) at 2,000 feet (600 m) above ground level (AGL).

 

Each line beginning with FM indicates a rapid change in the weather over a period of less than an hour.

  • FM052300 indicates the next period lasts from (FM) the 5th at 2300 UTC to the 6th at 0600 UTC (the effective time on the next FM line). The remainder of the line has similar formatting to the other forecast lines: 30006KT wind from 300° at 6 knots (11 km/h; 6.9 mph), 5SM visibility 5 statute miles (8 km), -SHRA light rain showers, OVC030 overcast at 3,000 feet (900 m).

    • PROB30 indicates a 30% probability of the following temporary conditions on the 6th between 0400 UTC and 0600 UTC: VRB20G35KT wind of variable direction at 20 knots (35 km/h; 25 mph) with gusts up to 35 knots (65 km/h; 40 mph), 1SM visibility 1 statute mile (1.6 km), TSRA thunderstorms and rain, BKN015CB broken cumulonimbus clouds at 1,500 feet (450 m).

  • FM060600 indicates that the next period lasts from the 6th at 0600 UTC until 2400 UTC (the end of the forecast period, as there is no following FM): 25010KT wind from 250° at 10 knots (20 km/h; 10 mph), 4SM visibility 4 statute miles (6.5 km), -SHRA light rain showers, OVC050 overcast at 5,000 feet (1,500 m).

    • TEMPO means that these conditions are temporarily replaced between 0800 UTC and 1100 UTC with: 2SM visibility 2 statute miles (3 km), -SHRA light rain showers, OVC030 overcast at 3,000 feet (900 m). (The wind speed remains the same as it is not mentioned.)

The final line is for errata, comments, and remarks.

  • RMK NXT FCST BY 00Z indicates a remark that the next forecast will be issued by 0000 UTC.