To this point, we have discussed the process that the United States will follow as we implement the new international coding requirements, and have briefly compared the new codes with their domestic counterparts. We are now ready to examine in more detail the METAR and TAF codes and try our hand at encoding an actual observation and forecast.

My original thought was to present this segment as a decoding exercise; however, our familiarity lies with the current domestic codes. Thus, I have chosen to encode rather than decode a METAR and TAF example. As was done previously, we will look first at the surface observation.

Consider for our encoding exercise the following nightmarish SAO:

DFW SA 1954 W5 X 3/4TRW+A 108/82/72/3319G26/981/R17LVR10V20 TB27 OVHD MOVG E FQT LTGICCG AB44 HLSTO 1/2 PRESRR

Each element of the observation will be discussed in some detail as we construct the corresponding METAR observation. So as to avoid a long and involved discussion of the observation parameters, we will examine only a few of the parameters in this edition of the Training Notes, leaving the rest for the next edition.

PARAMETER 1: Type of Report.

DFW SA 1954 W5 X 3/4TRW+A 108/82/72/3319G26/981/R17LVR10V20 TB27 OVHD MOVG E FQT LTGICCG AB44 HLSTO 1/2 PRESRR

In a METAR report, this parameter comes first. It will take one of two forms: METAR, for a routine (scheduled) observation, or SPECI, for a non-routine (unscheduled) report. Both METAR and SPECI reports consist of the following parameters: wind, visibility, runway visual range (where measured), present weather, sky condition, temperature, dew point, and altimeter setting in the "body" of the report. Coded and/or plain language remarks that elaborate on data within the body of the reports often follow.

PARAMETER 2: Station Identifier.

DFW SA 1954 W5 X 3/4TRW+A 108/82/72/3319G26/981/R17LVR10V20 TB27 OVHD MOVG E FQT LTGICCG AB44 HLSTO 1/2 PRESRR

Next, the station identifier is given. However, unlike current domestic practices, the identifier follows International Civil Aviation Organization (ICAO) requirements and consists of four alphabetic-only characters. A listing of acceptable identifiers is given in ICAO Document 7910/76. In a nutshell, the identifiers for sites in the contiguous 48 states are essentially the same in both the METAR and SAO. The only difference is the addition of a "K" prefix in a METAR observation.

The selection of the identifiers does follow a prescribed method, however. The first letter (K) represents the Aeronautical Fixed Service (AFS) routing area. The AFS is a telecommunication service between specified fixed points and is provided primarily for the safety of air navigation and the economics of air service. The second letter is the letter assigned to the State or territory within which the location is situated. The third and fourth letters are somewhat arbitrarily assigned by the State or territory, but are done so as to facilitate smooth and efficient routing of communication services. This is probably more than you wanted to know about identifier assignment, so let's move on.

PARAMETER 3: Date/Time.

DFW SA 1954 W5 X 3/4TRW+A 108/82/72/3319G26/981/R17LVR10V20 TB27 OVHD MOVG E FQT LTGICCG AB44 HLSTO 1/2 PRESRR

This parameter in a METAR observation is of the format YYGGggZ, where YY represents the issuance day of the month, GGgg represents the actual time of issuance (UTC) or the time when SPECI criteria are met, and "Z" denotes that the time is indeed in UTC. The day and time are included in all reports. If the report is a correction to one previously disseminated, the time of the corrected report shall be the same time as the report that is being corrected. Again, note that the issuance day is peculiar to METAR/SPECI and is not included in an SAO.

Let's begin the encoding process and see what we have to this point:

METAR KDFW 241954Z...

PARAMETER 4: Report Modifier.

A report modifier may follow the issuance time. If the observation is fully automated (no human intervention), "AUTO" will be entered to indicate this. The term "COR" will be entered in place of "AUTO" to indicate a corrected METAR/SPECI report.

PARAMETER 5: Wind.

DFW SA 1954 W5 X 3/4TRW+A 108/82/72/3319G26/981/R17LVR10V20 TB27 OVHD MOVG E FQT LTGICCG AB44 HLSTO 1/2 PRESRR

Now we get into the real "meat" of a METAR observation. Unlike the SAO, wind is the first weather parameter reported in a METAR observation. The wind group consists of a direction, speed, and, if recorded, a gust according to the following format: dddff(f)Gff(f)KT.

The wind direction is reported in tens of degrees using three digits, e.g. 050 for a northeast wind from 50 degrees. The speed immediately follows the direction and is coded in whole knots using two or three digits. A wind of 105 knots would be coded as 105 in the observation. If a gust is recorded, it will be denoted with a "G" followed by the two- or three-digit speed, again in whole knots. The contraction "KT" is included at the end of the wind group to indicate that the speeds are in knots.

Three other wind conditions can be addressed in a METAR observation. For a variable wind direction with a speed of 6 knots or less, the contraction "VRB" is substituted for the direction followed by the actual wind speed in whole knots. For example, a variable wind of 4 knots would be reported as "VRB04KT".

In the case of a variable wind direction with a speed in excess of 6 knots, the coding would take the form of "dndndnVdxdxdx", where the directions denote the extent of variability reported in a clockwise direction. This variable direction statement immediately follows the actual wind group that consists of both an average wind direction and an accompanying speed. Consider a wind of 15 knots that is varying in direction from 40 degrees to 100 degrees. It shall be coded as "07015KT 040V100".

Finally, a calm wind must be considered. It simply is coded in a METAR observation as "00000KT".

Let's see what our METAR observation looks like with the wind group included:

METAR KDFW 241954Z 33019G26KT...

PARAMETER 6: Visibility and Runway Visual Range.

DFW SA 1954 W5 X 3/4TRW+A 108/82/72/3319G26/981/R17LVR10V20 TB27 OVHD MOVG E FQT LTGICCG AB44 HLSTO 1/2 PRESRR

The visibility group is the next parameter to tackle. It follows the wind group in a METAR observation and is fairly straight-forward in its coding. The surface visibility is coded in the following format: vvvvSM. The "SM" appended to the visibility value indicates that the units are statute miles. Again, this deserves a collective sigh of relief, since we certainly are accustomed to reporting visibility in statute miles and FAA-established landing and takeoff minimums are tied to visibilities in these units.

Manually reported visibilities range from zero to greater than 50 miles. Values less than 1/2 mile (Low IFR conditions) are reported in 1/16-mile increments; from 1/2 to 2 miles, 1/8-mile increments; from 2 to 3 miles, 1/4-mile increments; from 3 miles to 15 miles, 1-mile increments; and greater than 15 miles, 5-mile increments. Automated observation platforms report visibilities below 1/4 of a mile as "M1/4SM".

Following the visibility group, where measured, is the runway visual range (RVR) group. You will note that its inclusion in the body of a METAR report is a drastic departure from an SAO, which reports this parameter as a remark. The runway visual range group is of the form RDRDR(DR)/VRVRVRVRFT, where DRDR is the runway number (the additional DR denotes an approach direction for parallel runways) and VRVRVRVR is the reportable runway visual range value in feet. Variable RVR can be reported by including a "V" between the minimum and maximum RVR values.

RVR is reported in increments of 200 feet at or below 3000 feet, and in increments of 500 feet above 3000 feet. Regulations state that, where instrumentation to measure RVR is available, it shall be reported whenever the prevailing visibility is 1 statue mile or less and/or the RVR for the designated runway is 6000 feet or less.

With the visibility and runway visual range included, our METAR observation, to this point, is as follows:

METAR KDFW 241954Z 33019G26KT 3/4SM R17L/1000V2000FT...

Well, this is enough to digest for the present time. We'll complete construction of the METAR observation in our next edition of Training Notes.

Sources:

FAA, 1995: New Aviation Weather Formats: METAR/TAF.

ICAO, September, 1993: Location Indicators, Document 7910/72.

NWS, April 1995: METAR/TAF Training Aids.

Office of the Federal Coordinator, 1995: Federal Meteorological Handbook No. 1 (DRAFT).