In this edition of METAR/TAF Training Notes, we begin a comparison of the new international codes with their domestic predecessors, the SAO and FT. The focus initially will be on comparing the SAO and METAR codes, with a comparison of the TAF and FT deferred until next edition.

While there are a number of differences between the observation codes, especially with regard to terms used and the order in which they occur, a number of similarities do exist. While a few of the similarities will be noted, our attention will center on the differences. In addition, we will note exceptions in the international codes that are unique to the observing practices in the U.S. These exceptions have been formally negotiated with the international community and are aimed at minimizing confusion and outcry among our users.

We will begin our comparison of the SAO and METAR codes by looking at general format variances. This will set the stage for a closer, yet still somewhat general, element-by-element comparison of the codes. Future issuances of this Training Note series will be devoted to exploring the particulars of each report element in complete detail.

The discussion will concentrate on manual recording practices. However, with the scheduled implementation of over 1000 Automated Surface Observation Systems (ASOS) nation-wide, the U.S. will soon be transitioning to a predominantly automated mode for taking and recording surface observations. Each ASOS will be programmed to accommodate the new METAR code. We will note, where appropriate, a few of the changes in current ASOS recording practices that will be required to satisfy METAR implementation.

All of us are naturally familiar with the order of elements in a Surface Airways Observation (SAO), i.e.

LOCID TP TM CLD VSBY WX SLP/TEMP/DEWPT/WND/ALT/RMKS

There will be a significant difference in the order in which these elements are reported in a METAR observation, especially at the front-end of the observation. The best way to remember the order of the elements is to think:

WHAT WHERE WHEN WIND

For example,

METAR KDFW 121955Z 14015G25KT etc...

Reports of cloud layers that follow the observation time in a SAO will take their place after the weather (WX) group and before the temperature/dew point (TEMP/DEWPT) group in a METAR observation. The remaining elements are essentially in the same order as in a SAO, with the exception that sea-level pressure (SLP) will not be reported in the body of a METAR observation, but may be included as a remark at specified observation sites.

Let's now examine the individual elements of a METAR observation and see how they compare to an SAO.

Type of Report. The term METAR will replace the routine reports coded as SA in the SAO. The comparable term for special SAOs (SP, RS) will be SPECI 1SPECI will contain all the data elements found in a METAR plus additional plain language information which elaborates on data in the body of the report. Thus, for the purposes of this Training Note, reference to a METAR observation will also be a reference to a SPECI, except where noted. in the international format. If specials criteria are met at the time of a routine report, the observation will be coded as METAR.

ASOS will include a few additional terms related to test mode operations. Currently, an ASOS SAO uses either TA, TP, UTP, or TS for denoting test operation. In METAR, TESTM will represent a test routine report and TESTS will represent a test special report.

Station Identifier. Our current SAO reports allow for a 3-character station identifier. This will change in the new METAR code. International Civil Aviation Organization (ICAO) requirements call for a 4-character ID. A complete listing pf all identifiers assigned to U.S. observation sites is given in ICAO Document 7910, "Location Indicators". For the most part, U.S. reporting sites will maintain their 3-character ID with a "K" placed at the beginning (e.g. LBB becomes KLBB). The "K" represents a "telecommunications service, called an Aeronautical Fixed Service (AFS), provided primarily for the safety of air navigation and for the regular, efficient and economical operation of air services" (excerpt from ICAO Document 7910). Sites in Alaska and several island areas, including Hawaii, are part of AFS "P" and thus will be prefixed as such.

The term AUTO will immediately follow the station identifier for ASOS sites. The type of automated site will be indicated in the remarks section of the observation. "A01" will denote an automated site without a present weather discriminator, while "A02" will denote one with a present weather discriminator.

Date/Time. This element will expand in a METAR observation to include the day of the month. Time of the observation will continue to be in UTC, but will be followed by "Z" in a METAR observation. For example, an observation recorded on the 26th day of the month at 1255 UTC would be coded as 261255Z in METAR.

Wind. The wind direction will be reported in 10's of degrees from true north using three digits (e.g. 220), instead of two as reported in a SAO. The speed will continue to be in knots in the U.S. METAR observation, but will include the contraction KT immediately following the speed (e.g. 25KT). A variable wind will be reported in the body of a METAR report, using "VRB" (for speeds below 6 knots) or "V" (separating the extreme directions). Examples of these are as follows: VRB05KT and 240V350.

Visibility/Runway Visual Range. There will be few changes between the two codes regarding visibility. The U.S. has negotiated the continued use of statute miles and fraction of miles (something that is making the FAA breathe a little easier since all of the printed approach plates reference statute miles). Two changes are particularly notable, however. In a METAR observation, "SM" will immediately follow the visibility value to denote statute miles. Also, coding for visibility in METAR observations will allow for a space between whole miles and fractions of whole miles (e.g. 1 1/2SM).

Regarding runway visual range, where reported, it will be included in the body of a METAR observation (in contrast to the SAO where it is a remark) whenever the prevailing visibility is one statute mile or less and/or RVR for the designated runway is 6000 feet or less. As in the SAO, the RVR group will begin with an "R" followed by a two-digit runway direction, a parallel runway designator (if applicable), and RVR values in feet. However, the METAR observation will append the contraction "FT" to denote that the runway visual range is in feet. It is our understanding that the RVR value in feet will be converted by software to meters before dissemination outside the U.S.

Present Weather. There will be some notable differences in the way METAR will handle present weather. Some of the weather codes presently employed in SAO coding will take on an expanded look in a METAR observation. For example, fog, which is coded as "F" in a SAO, will expand in METAR to "FG" (for visibilities less than 5/8 of a statute mile) or "BR" (for visibilities between 5/8 of a statute mile and 6 statute miles inclusive). Likewise, rain will be denoted as "RA" versus the present "R".

Some codes will change altogether. Rain showers, for example, will no longer be coded as "RW" in a METAR observation, but become "SHRA". Similarly, drizzle will change from "L" to the more logical "DZ". A heavy rainshower will become "+SHRA" (look for the intensity sign to precede the weather code in a METAR observation, not succeed it as is current practice in a SAO).

The SAO includes proximity qualifiers as a remark. Not so with the METAR. A thunderstorm in the vicinity of the observation site will be coded in the body of the report as "VCTS". A few other phenomena that can be reported in the vicinity of the observation site include blowing dust (VCBLDU), fog (VCFG), dust whirls (VCPO), and blowing snow (VCBLSN).

The U.S. will report no more than three (3) present weather and obstructions to visibility groups, and will add "blowing spray" (BLPY) to the list of reportable weather codes. Automated systems will be limited, too, in the number of weather codes they can report. The weather phenomena in a METAR observation will be reported in order of decreasing predominance, not the standard liquid, freezing, frozen, obstructions to vision order employed in the SAO.

Clouds and Vertical Obscurations. The METAR observation will allow a maximum of six (three for automated systems) reportable cloud layers, as opposed to the unlimited number reportable through a SAO. The U.S. has opted to report the clouds in hundreds of feet, and while maintaining the international standard of reporting amounts in oktas, using the more qualitative terms of FEW (less than 1 okta), SCT (1-4 oktas), BKN (5-7 oktas), and OVC to denote the range of coverage. The cloud height will follow immediately after the amount designator, without an intervening space (e.g. SCT015). If a thunderstorm or towering cumulus cloud is observed, CB or TCU will be appended to the cloud group, respectively. The contraction for clear skies will change from the CLR used in the SAO to SKC in a manual METAR observation (automated METAR observations will continue to use CLR).

With all clouds considered as opaque, ceiling designators will be understood to be the first reported BKN or OVC layer; no "E" or "M" designator will be allowed in a METAR observation. Obscurations will be handled differently from the SAO. Total obscurations will be reported in METAR as "VV" followed by the vertical visibility in feet. Partial obscurations, on the other hand, will be reported in the body of a METAR observation as FEW000, SCT000, or BKN000 and also in the remarks portion of the observation prefixed with the obscuring phenomenon (e.g. FG SCT000).

Temperature/Dew Point. This is a biggie! The U.S. will finally concede and report surface temperatures and dew points in whole degrees Celsius. Subzero values will be prefixed with an "M". The latest word is that a more definitive temperature and dew point, i.e. with accuracy to a tenth of a degree, will be included as a remark in U.S. METAR observations to satisfy various service, forecast and research requirements.

Altimeter Setting. In the U.S., expect to see all altimeter settings reported in inches of mercury prefixed with an "A" (be thankful for small favors - we could be reporting altimeter setting in hectopascals prefixed with "Q"). The altimeter setting will include four digits (A2992), as opposed to the three presently reported in the SAO.

Remarks. For the most part, the remarks allowed in a METAR observation will not differ significantly from those in a SAO. Remarks, however, may vary significantly between a METAR and SPECI observation, and between a manual and automated METAR observation. We will defer our discussion of the particulars of this section until a future issuance of METAR/TAF Training Notes.

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