Beginning in July 1996, the United States will undergo the mostsignificant change for observing, reporting, and coding surfaceweather observations and terminal forecasts in the past fortyyears. Not since the early 1950s, when the present airways code(commonly known as Surface Aviation Observation or SA code), andTerminal Forecast (FT) codes were adopted, has there been sucha major code change for weather observations and forecasts.
On July 1, 1996, at 0800 hours Coordinated Universal Time, theNational Weather Service (NWS), Federal Aviation Administration(FAA), and Department of Defense will implement, for domesticdissemination, the international standard code for hourly andspecial surface weather observations, METAR/SPECI.
The METAR acronym stands for Aviation Routine Weather Report. A special report, SPECI, is merely a METAR formatted report whichis issued on a non-routine basis as dictated by changing meteorologicalconditions. The SPECI acronym stands for Aviation Selected SpecialWeather Report.
Meanwhile, the international standard code format for terminalforecasts issued for airports, TAF, will fully take effect atthat time. TAF stands for Aerodrome Forecast.
The U.S. METAR code is described in Federal Meteorological Handbook(FMH) No. 1 "Surface Observations and Reports", whilethe U.S. TAF code used by the NWS is described in Weather ServiceOperations Manual Chapter D-31. Both of these standards are tailoredto reflect existing longstanding U.S. national practices. Forexample, in order to lessen the burden on the U.S. aviation community,a number of exceptions to metric reporting units have been filedby the U.S. Winds will continue to be reported in knots (as opposedto meters per second), cloud layer heights, and runway visualrange (RVR) will continue to be reported in feet (as opposed tometers), visibility will continue to be reported in statute miles(as opposed to meters), and altimeter settings will continue tobe reported in inches of mercury (as opposed to hectoPascals).
The only METAR element that will be converted to metric unitsis the temperature/dewpoint field which will be reported in wholedegrees Celsius. In order to facilitate the conversion betweenCelsius and Fahrenheit for climatological and public forecastingpurposes, the hourly temperature/dewpoint will be in tenths ofdegrees Celsius in the additive data remarks section of the METARreport from selected stations in the U.S. All public and climatologicalproducts that are issued by the NWS will continue to utilize theFahrenheit scale for reporting temperatures and dewpoints.
Some of the significant changes in METAR from the SA code areas follows:
the order of elements has been changed;
changes in the codes for reporting present weather (e.g.,RA for rain, TS for thunderstorm, FG for fog, and SQ for squalls);
individual elements shall not be reported if they are missing;
METAR requires the use of four letter International CivilAviation Organization (ICAO) station identifiers (e.g., KBOS-Boston,MA; PAFA-Fairbanks, AK; and PHTO-Hilo, HI); reporting stationsthat presently have identifiers containing digits have been assigned new all-alphabetic station identifiers;
METAR has no explicit ceiling designator; the first brokenor overcast layer aloft is inferred to be the ceiling;
the reporting and evaluating units for sky cover will be ineighths or oktas rather than tenths; and
sea-level pressure in hectoPascals will move from the bodyof the report to the remarks section.
A sample observation in the SAO code would appear as follows:
IAD SA 1055 A02A 11 SCT E15 OVC 1/2S-F045/33/29/2119G27/945/R04VR30 PK WND 1929/16
The same observation in the 1996 U.S. METAR code would appearas follows:
METAR KIAD 081055Z COR 21019G27KT 1/2SM R04R/3000FT-SN FG SCT011 OVC015 01/M02 A2945 RMK A02 PK WND 19029/16 SLP045 T00081016
The changes to the TAF code are not significantly different fromthe current FT format. However, the NWS currently issues TAFsfor 102 airports and there is some more user familiarity withTAFs than with the hourly METAR reports.
A list of abbreviations used in METAR/TAF reportsis available here on AVweb, as is a list of answers to frequently-asked questions.
In addition to changes to the hourly surface observations (METAR)and aerodrome forecasts (TAFs), several aviation weather productsprepared by the NWS will also be impacted on July 1, 1996. Theyare:
Area Forecasts (FAs)
Significant Meteorological Information (SIGMETs)
Airmen’s Meteorological Information (AIRMETs)
Pilot Reports (PIREPS)
Transcribed Weather Route Forecasts (TWEBs)
Meteorological Impact Statements and
Center Weather Advisories
The changes to these products, which are fairly minor, include:
the use of new METAR weather coding conventions (e.g., "-TSRA"rather than "TRW-" to forecast thunderstorms with lightrain and cloud heights following the sky cover contraction ratherthan preceding it);
the use of 4-letter ICAO identifiers for airports used inthe main portion, or body, of the forecast; and
the use of standardized ICAO and FAA contractions.
A quick reference card summarizing the main featuresof the METAR and TAF codes is available from NWS. It is a 5"x 8" light blue card, with holes punched across the top anddown the left side to fit into flight planners. The NOAA documentnumber for the card is: NOAA/PA 96052. A digital copy of thecard (in .GIF format) is available here on AVweb. Organizations are free to download the card, utilize their ownlogo if they so choose, and print and distribute the card.
For more information on the METAR/TAF implementation, the NWShas established a home page on the World Wide Web at http://www.nws.noaa.gov/oso/oso1/oso12/metar.htm.
Any questions regarding this implementation can be addressed toHoward Diamond, the NWS METAR/TAF Implementation Manager, viae-mail at: email@example.com.