We continue our construction of a METAR report in this edition of the Training Notes. You'll remember that we had discussed the coding of the first six (6) parameters in the observation, viz., type of report, station identifier, date and time, report modifier, wind, and visibility and runway visual range. Let's pick up where we left off with parameter 7, present weather.

PARAMETER 7: Present Weather.

DFW SA 1954 W5 X 3/4TRW+A 108/82/72/3319G26/981/R17LVR10V20 TB27 OVHD MOVG E FQT LTGICCG AB44 HLSTO1/2 PRESRR

Reporting this parameter can get rather involved. Not only are there a number of reportable phenomena, but the observer must also consider intensity and proximity, as well any appropriate descriptor terms.

The reportable phenomena can be grouped into three (3) classes: precipitation, obscuration, and other. In the interest of space and time, the reader is referred to Federal Meteorological Handbook No. 1: Surface Weather Observations and Reports (FMH1) for a listing of reportable phenomena and their associated two-letter codes. For the purposes of our example, the two (2) reportable phenomena are rain and hail, denoted by the codes "RA" and "GR", respectively.

The observer must add an intensity and descriptor term to match the detail provided in the SAO. So, to accurately report the weather occurring at DFW at the time of the observation, he/she must include "+" , "TS", and "GR" to the coding. Notice that "SH" is not allowed in association with "TS". I suppose it is implied.

How does it all link together? Well, let's look at what we have constructed so far:

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

Pretty strange looking isn't it? Rather than trying to figure it all out, I guess we just need to teach pilots that if they see "TSRAGR" in an observation, run for the hills!

It is important to note that no more than three (3) precipitation types may be coded in a single present weather group, and they shall be coded in order of decreasing predominance (according to Webster, that means most frequent or prevailing). Further, the observer shall not code more than three (3) present weather groups in a METAR/SPECI report.

A couple of other points should be brought out. While thunderstorm and rain can include an intensity qualifier, hail (GR) cannot. A remark can be included, however, that indicates hail size (e.g. "GR 1 3/4"). This, of course, is not any different than in a SAO report. By the way, hailstones that measure less than 1/4 inch in diameter are reported in the body of a METAR observation as "GS" with no associated size remark. We'll look closer at remarks shortly.

A thunderstorm can, as in an SAO, be reported without any associated precipitation. In addition, it can be coded along with the proximity qualifier for vicinity, "VC", as can fog, showers, dust/sand whirls, blowing dust, blowing sand, sandstorm, and duststorm. In this case, it would be coded as "VCTS".

There are many more specifics regarding present weather reporting in METAR observations. However, unless our desire is to write a Training Note that rivals War and Peace in size, we must refer the reader to FMH1 for these details.

PARAMETER 8: Sky Condition.

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

Let's begin this section by quelling any rumors that may be floating around. No, the U.S. is not going to report cloud amount specifically in oktas (nor hexadecimals). Instead, we will report only a qualitative term for sky cover that is based on okta coverage (i.e. SKC, FEW, SCT, BKN, and OVC). By the way, the transition point from SCT to BKN conditions is 5/8 coverage. That being said, let's look closer at sky condition reporting.

Sky condition will be reported in all observations using the following format: NsNsNshshshs or VVhshshs or SKC/CLR. The NsNsNs is the amount of sky cover (i.e. the SCT, BKN, etc.) and the hshshs is the height of the layer. The U.S., as mentioned in Training Note #2, will report cloud heights in hundreds of feet above the surface using three (3) digits. For example, a scattered deck of clouds at 2500 feet would be coded as "SCT025".

Vertical visibility is coded using the VV to indicate an indefinite ceiling and hshshs to indicate the vertical visibility into the indefinite ceiling, also in hundreds of feet. Note that an indefinite ceiling would indicate 8/8 coverage. This applies to the DFW observation, so let's see how it would be coded:

METAR KDFW 241954Z 33019G26KT 3/4SM R17L/1000V2000FT +TSRAGR VV005...

Now the question arises, "Can a partial obscuration be coded in a METAR observation?". Yes, using a qualitative sky cover term and "000". For example, a partial obscuration occurring in a sky that otherwise contains only scattered clouds would be coded as "SCT000". In the remarks section of the observation, expect to see the "SCT000" precede by the obscuring phenomena, e.g. "FG SCT000".

The term "SKC" deserves some mention. It will replace the old SAO denotation of "CLR" when no clouds are reported in manual METAR observations. In automated observations, "CLR" will continue to be used.

The observer can report a maximum of six (6) layers of clouds in a manual observation (three (3) in an automated observation). These layers are to be reported in ascending order up to the first overcast layer. Finally, the term "CB" or "TCU" shall immediately follow a SCT, BKN or OVC deck, whenever cumulonimbus or towering cumulus clouds are observed (e.g. "SCT010CB").

PARAMETER 9: Temperature and Dew Point.

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

Temperature and dew point reporting will represent a dramatic departure from our current way of doing things. In other words, it is out with the Fahrenheit scale and in with the Celsius temperature scale. Temperature will be coded in a METAR observation as two (2) digits rounded to the nearest whole degree Celsius. If either one goes below zero, it will be preceded by an "M" (e.g. 03/M04).

If the temperature is missing in the observation, expect to see the dew point missing as well. In contrast, a missing dew point will not necessarily mean that the temperature will be missing. The coding in this case will include a temperature followed by a solidus (/), but no dew point.

For many purposes, including climatological and research, the accuracy of the whole degree Celsius report is not acceptable. The NWS has recognized this and has proposed the inclusion of a remark that gives both temperature and dew point to a tenth of a degree.

The DFW observation in METAR format now has the following look:

METAR KDFW 241954Z 33019G26KT 3/4SM R17L/1000V2000FT +TSRAGR VV005 28/22...

PARAMETER 10: Altimeter.

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

Thanks to the generous hearts of our NWS policy-makers, they have been able to negotiate with the international community to maintain our current reporting practice of including altimeter setting in inches of Mercury. The prospect of decoding a hectopascal reading just doesn't sit right with most individuals. I think it is important to point out, however, that readings in hectopascals aren't as foreign to you as you might think. It turns out that the conversion from hectopascals to inches of Mercury is essentially the same as from millibars to inches of Mercury (I mb = 33.86 in Hg; 1 hectopascal = 33.80 in Hg). The difference is insignificant.

I digress, though. Altimeter settings will always be reported in inches of Mercury for domestic sites, and to four (4) digits preceded by an "A". So, for the DFW example:

METAR KDFW 241954Z 33019G26KT 3/4SM R17L/1000V2000FT +TSRAGR VV005 28/22 A2981...

You will notice that sea level pressure is not reported in the main body of the METAR report. Instead, it is included as a remark, which we will look at in the next section.

PARAMETER 11: Remarks.

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 information that can be encoded as a remark is enough to comprise a Training Note by itself. With this in mind, we will limit our discussion to some general comments and address specifically those items that concern our observation example. The reader is urged to consult the FMH1 for a complete listing and explanation of the full range of permissible remarks.

Remarks are, of course, separated from the body of the observation and are included as necessary to give a complete picture of the weather conditions. Their inclusion is preceded by the contraction "RMK". If no remarks are provided, then "RMK" is omitted. It is important to point out that all contractions used within the remarks section will follow the FAA Order 7340 Series, Contractions, but will adhere to ICAO usage, not NWS.

Regarding those remarks included in our example observation that describe the thunderstorm presently occurring, beginning and ending time of a thunderstorm shall be encoded in the form TSB(hh)mmE(hh)mm. The form, with the exception of the "TS" for thunderstorm is no different from the format used in the SAO. Thunderstorm location follows the form: TS LOC (MOV DIR), where again similarity is noted between the METAR and SAO. Lightning occurrence, too, follows a similar coding convention to that used in current domestic observations, viz. Frequency LTG(type) [LOC].

Putting all together for our DFW example,

METAR KDFW 241954Z 33019G26KT 3/4SM R17L/1000V2000FT +TSRAGR VV005 28/22 A2981 RMK TSB27 TS OHD MOV E FRQ LTGICCG...

Notice that there are some minor differences involving the contractions for thunderstorm location (OHD) and movement (MOV), and lightning frequency (FRQ).

The beginning and ending of hailfall follows essentially the same coding as that used in domestic observations, i.e. wwB(hh)mmE(hh)mm. Size coding, however, does represent somewhat of a departure from domestic practices. The format for reporting hailstone size is GR [size], where GR is the identifier for hail greater than 1/4 inch in diameter and size is the actual diameter of the stones. Hailstone size is reported in 1/4 inch increments. If the diameters of the falling stones are less than 1/4 inches, then the identifier GS is used with no associated size given.

Let's see what we have then:

METAR KDFW 241954Z 33019G26KT 3/4SM R17L/1000V2000FT +TSRAGR VV005 28/22 A2981 RMK TSB27 TS OHD MOV E FRQ LTGICCG GRB44 GR 1/2...

Finally, we come to the issue of reporting the rate of pressure change and sea level pressure. You will be pleased to find that there is no difference in how rapidly rising or falling pressure is encoded in a METAR observation compared to that in a SAO, i.e. PRESRR is the same in both observation formats. The encoding of sea level pressure, too, is quite simple. It follows the format: SLPppp, where ppp is the sea level pressure in (ugh!) hectopascals (again, this is close to millibars). In our example, the conversion does increase the value by one hectopascal.

So, let's finish the METAR observation:

METAR KDFW 241954Z 33019G26KT 3/4SM R17L/1000V2000FT +TSRAGR VV005 28/22 A2981 RMK TSB27 TS OHD MOV E FRQ LTGICCG GRB44 GR 1/2 PRESRR SLP109

Congratulations! You have survived the encoding of a rather involved METAR observation and have graduated to the position of Chief Encoding Specialist. But, don't get too comfortable. There's more fun ahead as we tackle the dreaded TAF next time.

Sources:

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

FAA, 1994: Contractions, Order 7340.1N.

NWS, April 1995: METAR/TAF Training Aids.

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