METARs typically come from airports or other permanent weather observation stations. Reports are generated once an hour or half-hour at most stations, but if conditions change significantly at a staffed location, a report known as a special (SPECI) may be issued.[4]:2[note 1] Some stations make regular reports more frequently, such as Pierce County Airport (ICAO code: KPLU) which issues reports three times per hour.[5] In addition to METARs and SPECIs, ASOS One-Minute Observations (OMO) are updated once a minute. OMOs can be in various formats, including the METAR format.[4]:3
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. In the United States, prior to mid-1990s, most observations are made manually, but today the vast majority are automated or augmented observations.[4]:2
History
The METAR format was introduced internationally on 1 January 1968, and has been modified a number of times since. North American countries continued to use a Surface Aviation Observation (SAO) for current weather conditions until 1 June 1996, when this report was replaced with an approved variant of the METAR agreed upon in a 1989 Geneva agreement. The WMO's publication No. 782 "Aerodrome Reports and Forecasts" contains the base METAR code as adopted by the WMO member countries.[6]
Information contained in a METAR
A typical METAR contains data for the airport identifier, time of observation, wind direction and speed, visibility, current weather phenomena such as precipitation, cloud cover and heights, temperature, dew point, and barometric pressure. This information forms the body of the report, consisting a maximum of 11 groups of information.[4]:5 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). These may be provided in coded or plain language and appended to the end of the METAR as remarks.[4]:6
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.
Cloud reporting
Cloud coverage is reported by the number of "oktas" (eighths) of the sky that is occupied by cloud. Automated substation substitutes time averaging of sensor data gathered during 30-minute period prior to reporting.[4]:2,4
"No cloud/Sky clear" used worldwide but in North America is used to indicate a human generated report[8][9]
NCD
"Nil Cloud detected" automated METAR station has not detected any cloud, either due to a lack of it, or due to an error in the sensors
CLR
"No clouds below 12,000ft (3,700m) (US) or 25,000ft (7,600m) (Canada)", used mainly within North America and indicates a station that is at least partly automated[8][9]
NSC
"No (nil) significant cloud", i.e., none below 5,000ft (1,500m) and no TCU or CB. Not used in North America.
Wind observation measures the horizontal vector component of the wind, which includes both direction and speed. These are determined by evaluating the measurement over a 2-minute period.[4]:24–10
The wind direction is coded with the first three digits in tens of degrees relative to the true north. If wind speed is less than or equal to 6kn (11km/h; 6.9mph), the wind direction will be displayed as variable or "VRB". If the wind speed is greater than 6 knots, but the direction varies more than 60° in the past 2 minutes, METAR will report the range of wind direction. For example, 21010KT 180V240 suggests the wind was variable from 180° to 240° at 10 knots.[4]:10
Immediately after the wind direction is the wind speed, coded in two or three digits measured in knots, km/h or m/s. If during past 10 minutes, the weather station detects more than 10kn (19km/h; 12mph) between minimum and maximum windspeed, METAR determines a wind gust exists and reports the maximum instantaneous windspeed.[4]:10
If the air is motionless, the wind will be reported as calm and coded as 00000KT.[4]:10
Visibility and runway visual range
Visibility measures the atmospheric opacity. It is the greatest distance where at least half of the horizon circle can be seen from the surface.[4]:11
Runway visual range (RVR) is an instrument-derived measurement that suggests the horizontal distance an observer may see down the runway. In the US, for stations with RVR reporting capacity, this information is omitted from the METAR unless the visibility is at or below 1mi (1.6km), or the designated instrument runway's RVR is at or below 6,000ft (1,800m). RVR of up to four designated runways may be reported, depending on the country.[4]:11
Regulations and conventions
METAR code is regulated by the World Meteorological Organization in consort with the International Civil Aviation Organization. In the United States, the code is given authority (with some US national differences from the WMO/ICAO model) under the Federal Meteorological Handbook No. 1 (FMH-1), which paved the way for the US Air Force Manual 15-111[11] on Surface Weather Observations, being the authoritative document for the US Armed Forces. A very similar code form to the METAR is the SPECI. Both codes are defined at the technical regulation level in WMO Technical Regulation No. 49, Vol II, which is copied over to the WMO Manual No. 306 and to ICAO Annex III.
Although the general format of METARs is a global standard, the specific fields used within that format vary somewhat between general international usage and usage within North America. Note that there may be minor differences between countries using the international codes as there are between those using the North American conventions — ICAO allows member countries to modify METAR code for use in their particular countries, as long as ICAO is notified.[4]:5
Examples
The two examples which follow illustrate the primary differences between the international and the North American METAR variations.[10][12][pageneeded]
041600Z indicates the time of the observation. It is the day of the month (04) followed by the time of day (1600 Zulu time, which equals 4:00pm Greenwich Mean Time or 6:00pm local time).
12012MPS indicates the wind direction is from 120° (east-southeast) at a speed of 12m/s (23knots; 27mph; 44km/h). Speed measurements can be in knots (abbreviated KT) or metres per second (abbreviated MPS).
090V150 indicates the wind direction is varying from 90° true (east) to 150° true (south-southeast).
R04/P1500N indicates the Runway Visual Range (RVR) along runway 04 is 1,500m (4,900ft) and not changing significantly.
R22/P1500U indicates RVR along runway 22 is 1,500m (4,900ft) and rising.
+SN indicates snow is falling at a heavy intensity. If any precipitation begins with a minus or plus (-/+), it's either light or heavy.
BKN022 indicates a broken (over half the sky) cloud layer with its base at 2,200ft (670m)above ground level (AGL). The lowest "BKN" or "OVC" layer specifies the cloud ceiling.
OVC050 indicates an unbroken cloud layer (overcast) with its base at 5,000ft (1,500m) above ground level (AGL).
M04/M07 indicates the temperature is −4°C (25°F) and the dew point is −7°C (19°F). An M in front of the number indicates that the temperature/dew point is below zero Celsius.
NOSIG is an example of a TREND forecast which is appended to METARs at stations while a forecaster is on watch. NOSIG means that no significant change is expected to the reported conditions within the next 2 hours.
8849//91 indicates the condition of the runway.
The first two characters indicate which runway is being described.
If there are two or more runways with the same number, some locations will use three characters (e.g. 25L and 25R). Otherwise, the left runway will use just its number and the right runway will add 50 (e.g. 25 = 25L and 75 = 25R).
88 indicates all the airport's runways.
99 indicates repetition of the last message as no new information received.
4 means the runway is coated with dry snow
9 means 51% to 100% of the runway is covered
// means the thickness of the coating was either not measurable or not affecting usage of the runway
91 means the braking index is bad, in other words the tires have bad grip on the runway
CAVOK is an abbreviation for Ceiling And Visibility OK, indicating no cloud below 5,000ft (1,500m) or the highest minimum sector altitude and no cumulonimbus or towering cumulus at any level, a visibility of 10km (6mi) or more and no significant weather change.[13]
= indicates the end of the METAR
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 is from Trenton-Mercer Airport near Trenton, New Jersey, and was taken on 5 December 2003 at 18:53 UTC.
051853Z indicates the day of the month is the 5th and the time of day is 1853 Zulu/UTC, or 1:53PM Eastern Standard Time.
04011KT indicates the wind is from 040° true (north east) at 11 knots (20km/h; 13mph). 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 (5.6km/h; 3.5mph).
1/2SM indicates the prevailing visibility is 1⁄2mi (800m) SM = statute mile.
VCTS indicates a thunderstorm (TS) in the vicinity (VC), which means from 5–10mi (8–16km).
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.
BKN003 OVC010 indicates a broken (5⁄8 to 7⁄8 of the sky covered) cloud layer at 300ft (91m)above ground level (AGL) and an overcast (8/8 of the sky covered) layer at 1,000ft (300m).
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 30.06inHg (1,018hPa).
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.[note 2] Stations that aren't equipped with a rain/snow sensor are designated AO1.[15]
TSB40 indicates the thunderstorm began at 40 minutes past the hour at 1840 Zulu/UTC, or 1:40p.m. Eastern Standard Time.
SLP176 indicates the current barometric pressure extrapolated to sea level is 1,017.6hPa (30.05inHg).
P0002 indicates that 0.02 inches (0.5mm) 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.[citation needed]
= indicates the end of the METAR.
In Canada, RMK is followed by a description of the cloud layers and opacities, in eighths (oktas). For example, CU5 would indicate a cumulus layer with 5⁄8 opacity.[16]
Flight categories in the US
METARs can be expressed concisely using so-called aviation flight categories, which indicates what classes of flight can operate at each airport by referring to the visibility and ceiling in each METAR. Four categories are used in the US:[17]
METAR abbreviations used in the weather and events section. Remarks section will also include begin and end times of the weather events.[10] Codes before remarks will be listed as "-RA" for "light rain". Codes listed after remarks may be listed as "RAB15E25" for "Rain began at 15 minutes after the top of the last hour and ended at 25 minutes after the top of the last hour."
Combinations of two precipitation types are accepted; e.g., RASN (rain and snow mixed), SHGSSN etc. If more than one type of weather is present, METAR will report in the following order:[4]:13
Tornadic activity
Thunderstorm
Most dominating weather
Precipitation
Obscuration
Type
Abbr.
Meaning
Intensity
-
Light intensity
Intensity
(blank)
Moderate intensity
Intensity
+
Heavy intensity
Descriptor
VC
In the vicinity (5–10 mi / 8–16 km from station); visible phenomena:
TS, SH, FG, DS, SS, VA, PO, FC, BLSN, BLDU, BLSA
Descriptor
RE
Recent hour's most important past phenomenon with residues:
Graupel[French: Grésil], snow pellets and/or small hail (not in the US).[note 3][18] Elsewhere hail is GR when it is 5mm or greater[19] Outside of the US when the hail is less than 5mm the code GS is used.[19])
Precipitation
GR
Hail[French: Grêle] (in the US includes small hail) [note 3]
Additional METAR numeric codes listed after RMK.[10]
Code
Description
11234
6-hour maximum temperature. Follows RMK with five digits starting with 1. Second digit is 0 for positive and 1 for negative. The last 3 digits equal the temperature in tenths.
This example value equals −23.4°C (−10°F).
20123
6-hour minimum temperature. Follows RMK with five digits starting with 2. Second digit is 0 for positive and 1 for negative. The last 3 digits equal the temperature in tenths.
This example value equals 12.3°C (54°F).
4/012
Total snow depth in inches. Follows RMK starting with 4/ and followed by 3 digit number that equals snow depth in inches.
This example value equals 12 inches of snow currently on the ground.
402340123
24-hour maximum and minimum temperature. Follows RMK with nine digits starting with 4. The second and sixth digit equals 0 for positive for 1 for negative. Digits 3–5 equal the maximum temperature in tenths and the digits 7–9 equals the minimum temperature in tenths.
This example value equals 23.4°C (74°F) and 12.3°C (54°F).
52006
3-hour pressure tendency. Follows RMK with 5 digits starting with 5. The second digit gives the tendency. In general 0–3 is rising, 4 is steady and 5–8 is falling. The last 3 digits give the pressure change in tenths of a millibar in the last 3 hours.
This example indicates a rising tendency of 0.6 millibars (0.018inHg).[20]
60123
3- or 6-hour precipitation amount. Follows RMK with 5 digits starting with 6. The last 4 digits are the inches of rain in hundredths. If used for the observation nearest to 00:00, 06:00, 12:00, or 18:00 UTC, it represents a 6-hour precipitation amount. If used in the observation nearest to 03:00, 09:00, 15:00 or 21:00 UTC, it represents a 3-hour precipitation amount.
This example shows 1.23 inches (31mm) of rain.
70246
24-hour precipitation amount. Follows RMK with 5 digits starting with 7. The last 4 digits are the inches of rain in hundredths.
This example shows 2.46 inches (62mm) of rain.
8/765
Cloud cover using WMO code. Follows RMK starting with 8/ followed by a 3 digit number representing WMO cloud codes.
98060
Duration of sunshine in minutes. Follows RMK with 5 digits starting with 98. The last 3 digits are the total minutes of sunshine.
This example indicates 60 minutes of sunshine.
931222
Snowfall in the last 6 hours. Follows RMK with 6 digits starting with 931. The last 3 digits are the total snowfall in inches and tenths.
This example indicates 22.2 inches (560mm) of snowfall.
933021
Liquid water equivalent of the snow (SWE). Follows RMK with 6 digits starting with 933. The last 3 digits are the total inches in tenths.
↑ Criteria for issuing a SPECI includes beginning or ending of hazardous weather, aircraft mishap, decreasing visibility or ceiling, or any other condition deemed critical by the observer of the responsible agency.[1]
↑ Precipitation discriminators are electrically heated at sub-freezing temperatures to calculate the water equivalent of frozen precipitation and snow accumulation.
1 2 In the US small hail is included with regular hail and the Remarks section is used saying "GR LESS THAN 1/4".
1 2 If the difference between temperature and dewpoint is less than or equal to 2°C, the station reports the condition as haze; else mist is reported.
OFCM.gov— US Federal Meteorological Handbook No. 1— Surface Weather Observations and Reports (September 2005). Complete documentation on the METAR format, PDF.
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