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What is METAR?

METAR is a handy data format that allows one to record, disseminate, and read weather reports in a very efficient manner. It is the standard weather report format for aviation, but is useful even if you don't fly airplanes.

From the NOAA:
METAR is the international standard code format for hourly surface weather observations which is analogous to the SA coding currently used in the US. ... SPECI is merely the code name given to METAR formatted products which are issued on a special non-routine basis as dictated by changing meteorological conditions. The SPECI acronym roughly translates as Aviation Selected Special Weather Report.

Here is an example METAR:

METAR KOMA 250952Z 35012G20KT 10SM OVC021 M11/M16 A3023 RMK AO2 PK WND 35026/0923 SLP252 T11111161

In that small amount of space, one can determine the station (location), timehack, wind direction, windspeed in knots, gusts, visibility, weather, obscurations, sky condition, temperature, dewpoint, altimeter/barometer, and other misc. remarks at the discretion of the reporting agencies.


Decoding METAR

Reading METAR is actually quite simple, once you know the format and nomenclature involved. The METAR itself is composed of 10 blocks, like so:

METAR KOMA 250952Z 35012G20KT 10SM -BLSN OVC021 M11/M16 A3023 RMK AO2 PK WND 35026/0923 SLP252 T11111161

The "type" in a METAR will always be... METAR.

The ID block shows the Station ID of the reporting agent. They are often airport codes, or based on airport codes, as airports have all of the necessary gear to make METAR reports. In this case, KOMA is Eppley Field, in Omaha Nebraska.

This is the date/time hack, given in the format DDHHHH Zulu, where DD is the day of the month, and HHHH is a 24-hour time. In this example, the report was generated at 9:52 AM Zulu, on the 25th day of the month.

This is the windspeed/direction block, given in the format DDDSSG##KT, where DDD is the direction in degrees of the wind, SS is the windspeed in knots, and G## indicates the gust speed (again in knots). The "KT" on the end of the string is fixed, and indicates that all speeds given are in knots. Variable directions for gusts are indicated in the format XXXVYYY, where XXX and YYY are the boundary directions in degrees for the gust directions. For example, 35012G20KT 050V110 would indicate that gusts of up to 20 knots can have a direction of anywhere from 50 to 110 degrees.

This is the visibility, in Statute Miles.

This block indicates weather conditions. There is an enormous table to decode the notation in this block, but a basic knowledge of a few will help you in most situations. + or - in the beginning of the block indicate "heavy" or "light". The first two letters are a "Descriptor" such as "BL" for blowing, "TS" for thunderstorm, "PR" for partial, "FR" for freezing, etc. with the following two-letter combinations indicating precipitation types, like DZ-drizzle, RN-rain, SN-snow.

This is a general "what does the sky look like" report. BKN-broken, OVC-overcast, etc. with the altitude of the reported layer given as a three-digit altitude block. In this example, we have OVC021, meaning there is an Overcast layer at 2100 feet. There can be multiple layers reported as well: OVC030 BKN100 would indicate an Overcast layer at 3000 feet with a Broken layer at 10000 feet. The criteria for Few/Scattered/Broken/Overcast are based on partially subjective criteria.

This block indicates the temperature and dewpoint in degrees Celcius, with M indicating a negative reading. The example shows a temperature of -11C and a dewpoint of -16C.

This block indicates the altitude of the station with the four digit followon showing inches of mercury on the barometer. In this example it is reading 30.23 in/Hg

The remarks section has a huge set of tables all its own, and will always have RMK A01 or RMK A02 as the first entry. "A01" means no automated precipitation sensor, "A02" means the opposite.

Thoughts on METAR, and sources

So what use is METAR to you? Well, perhaps you need a weather update via text message when camping or on the road? Perhaps you want to build a script to scrape data for a desktop app? Perhaps you are worried that your unmet soulmate is a meteorologist and you want to be ready to snag them at the first opportunity?

  • "METAR" requires no further explanation, such as "METAR report" - it is an acronym derived from a French term that basically means "aerodrome weather"
  • Why they chose Statute Miles, and not Nautical Miles (which are used in aircraft navigation and charts) I will never understand.
  • There are redundancies in METAR format that could be eliminated for a bit more efficiency. The unit designators for measurements of speed and visibility, for example, are wasted characters.
  • The amount of possible "remarks" and the nonstandard order of coding defeats the purpose of a unified reporting method

For extended reading or for full lookup tables, check out The Federal Meteorological Handbook Number 1.

tanktop suggests that you might be interested in looking up your own METAR: NOAA.gov METAR data access (by station code)