May 21, 1996 METAR/TAF Update

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by	Howard Diamond
	NWS METAR/TAF Implementation Manager Office of Systems Operations

Beginning in July 1996, the United States will undergo the most significant change for observing, reporting, and coding surface weather observations and terminal forecasts in the past forty years. Not since the early 1950s, when the present airways code (commonly known as Surface Aviation Observation or SA code), and Terminal Forecast (FT) codes were adopted, has there been such a major code change for weather observations and forecasts.

On July 1, 1996, at 0800 hours Coordinated Universal Time, the National Weather Service (NWS), Federal Aviation Administration (FAA), and Department of Defense will implement, for domestic dissemination, the international standard code for hourly and special surface weather observations, METAR/SPECI.

The METAR acronym stands for Aviation Routine Weather Report. A special report, SPECI, is merely a METAR formatted report which is issued on a non-routine basis as dictated by changing meteorological conditions. The SPECI acronym stands for Aviation Selected Special Weather Report.

Meanwhile, the international standard code format for terminal forecasts issued for airports, TAF, will fully take effect at that time. TAF stands for Aerodrome Forecast.

The U.S. METAR code is described in Federal Meteorological Handbook (FMH) No. 1 "Surface Observations and Reports", while the U.S. TAF code used by the NWS is described in Weather Service Operations Manual Chapter D-31. Both of these standards are tailored to reflect existing longstanding U.S. national practices. For example, in order to lessen the burden on the U.S. aviation community, a number of exceptions to metric reporting units have been filed by the U.S. Winds will continue to be reported in knots (as opposed to meters per second), cloud layer heights, and runway visual range (RVR) will continue to be reported in feet (as opposed to meters), visibility will continue to be reported in statute miles (as opposed to meters), and altimeter settings will continue to be reported in inches of mercury (as opposed to hectoPascals).

The only METAR element that will be converted to metric units is the temperature/dewpoint field which will be reported in whole degrees Celsius. In order to facilitate the conversion between Celsius and Fahrenheit for climatological and public forecasting purposes, the hourly temperature/dewpoint will be in tenths of degrees Celsius in the additive data remarks section of the METAR report from selected stations in the U.S. All public and climatological products that are issued by the NWS will continue to utilize the Fahrenheit scale for reporting temperatures and dewpoints.

Some of the significant changes in METAR from the SA code are as follows:

1. the order of elements has been changed;

2. changes in the codes for reporting present weather (e.g., RA for rain, TS for thunderstorm, FG for fog, and SQ for squalls);

3. individual elements shall not be reported if they are missing;

4. METAR requires the use of four letter International Civil Aviation Organization (ICAO) station identifiers (e.g., KBOS-Boston, MA; PAFA-Fairbanks, AK; and PHTO-Hilo, HI); reporting stations that presently have identifiers containing digits have been assigned new all-alphabetic station identifiers;

5. METAR has no explicit ceiling designator; the first broken or overcast layer aloft is inferred to be the ceiling;

6. the reporting and evaluating units for sky cover will be in eighths or oktas rather than tenths; and

7. sea-level pressure in hectoPascals will move from the body of 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-F
045/33/29/2119G27/945/R04VR30 PK WND 1929/16

The same observation in the 1996 U.S. METAR code would appear as 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 from the current FT format. However, the NWS currently issues TAFs for 102 airports and there is some more user familiarity with TAFs than with the hourly METAR reports.

A list of abbreviations used in METAR/TAF reports is 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 products prepared by the NWS will also be impacted on July 1, 1996. They are:

• 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:

1. the use of new METAR weather coding conventions (e.g., "-TSRA" rather than "TRW-" to forecast thunderstorms with light rain and cloud heights following the sky cover contraction rather than preceding it);

2. the use of 4-letter ICAO identifiers for airports used in the main portion, or body, of the forecast; and

3. the use of standardized ICAO and FAA contractions.

A quick reference card summarizing the main features of the METAR and TAF codes is available from NWS. It is a 5" x 8" light blue card, with holes punched across the top and down the left side to fit into flight planners. The NOAA document number for the card is: NOAA/PA 96052. A digital copy of the card (in .GIF format) is available here on AVweb. Organizations are free to download the card, utilize their own logo if they so choose, and print and distribute the card.

For more information on the METAR/TAF implementation, the NWS has 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 to Howard Diamond, the NWS METAR/TAF Implementation Manager, via e-mail at: hdiamond@smtpgate.ssmc.noaa.gov.