Weather, in the form of METARs (Aviation Routine Weather Reports) and TAFs (Terminal Aerodrome Forecasts), are available on your GPS or on iPad Apps (like ForeFlight) in coded format. They are difficult to read until you learn the structure and abbreviations. This article explains the METAR and TAF format by summarizing features from AC 00-45G, Aviation Weather Services. METARs are current weather for an airport, and TAFs are forecasts for a selected airport.
The METAR structure is shown in Figure 1 as taken from the advisory circular (as are all other figures and tables in this summary). This report for Oklahoma City on the first day (01) of the month, at 1955Z, is from a fully AUTOmated site. The wind is 220° @ 15 gusting to 25 knots, and is variable between 180° and 250°. If the wind is variable and below 6 knots, it would be listed for example as VRB, or VRB03KT. Calm is listed as 00000KT.
Visibility is ¾ statute miles – 3/4SM – and automated stations may put M in front of that. The runway visual range (RVR) for R17L is 2600 ft. Reportable values are in 100-ft increments up to 1000 ft, then 200 ft to 3000 ft, then 500 ft up to 6000 ft. A value is only given if the RVR is less than 1SM, and/or the RVR for the designated instrument runway is 6000 ft or less. If the RVR is variable (by more than one increment) over a 10-minute period it would say, for example, R32R/0600V1000FT. The lowest reportable manual RVR is 600 ft, so if it is less than that it would say R27/M600FT. If RVR is higher than the highest reportable value it is preceded by a P; R27/P6000FT.
The present weather group includes precipitation, obscurations, and other weather phenomena and is coded according to Table 1. In Fig 1, +TRSA means thunderstorms (TS) and heavy (+) rain (RA) with mist (BR). Note that TS is a descriptor and RA is a precip type. Qualifiers change a two-letter entry to a four-letter entry. For example, BLSN is blowing snow and VCDU means widespread dust in the vicinity (between 5 and 10 sm away). Light freezing rain and fog is
–LFRA FG, and +FC TSRAGR BR is tornado, thunderstorm, (moderate) rain, hail, and mist.
There can be up to three weather groups, listed from the most severe to the least. The order is tornadic activity (tornado, funnel cloud, or waterspout), thunderstorms with or without precip, and present weather in order of decreasing dominance.
The next portion is the Sky Condition group, which is coded in ascending order, ending at the first overcast layer. The Figure 1 report has an overcast sky (OVC) with a 1000-ft ceiling. The three numbers give the ceiling in hundreds of feet (010 = 1000ft). Manual stations attach CB (cumulonimbus) or TCU (towering cumulus) to each layer if present. Ceilings are reportable in 100-ft increments below 5000 ft, then 500 ft to 10,000 and 1000 ft above 10,000.
The codes VV (vertical visibility) or OVC indicate total sky coverage, with the difference illustrated in Figure 2. With an indefinite ceiling VV is used and vertical visibility (not ceiling) is given.
With clear skies you will see SKC (observer sees no layers) or CLR (automated station reports clear below 12,000 ft). Partial sky coverage is indicated by FEW (1/8 to 2/8), SCT (3/8 to 4/8), or BKN (5/8 to 7/8). If there are multiple layers there will be an entry for each as shown in Figure 3.
The final entries in the body of the report are the temperature/dew point spread in °C, and the altimeter setting (inches of mercury). Minus temperatures use M as a prefix (M02 = – 2°C).
If there are remarks they will be appended after the body of the METAR, starting with RMK. These fall into two categories: a) Automated, Manual, and Plain Language, or b) Additive Maintenance Data. In Fig 1 the remarks are from an Automated station with a precipitation discriminator (AO2). Without the discriminator it is an A01. The message here is that thunderstorms overhead are moving east. The remarks end with the sea level pressure (SLP) given in millibars, but with the decimal moved a bit (1013.2 mb is coded 132). I know, 1013.25 mb is the same as 29.92 in Hg.
The abbreviations and words used in remarks are numerous but should be readable. Refer to the advisory circular if there are portions that are uncertain.
TAFs and AFs (area forecasts) also have coded entries and require some practice to learn to read. There are some similarities in use of symbols, which helps in the interpretation. In addition to regular TAFs, it may be amended (TAF AMD) or corrected (TAF COR). It starts with the location identifier followed by the date and time of forecast (e.g. 061473Z), valid period (e.g. 2306/2412 = 0600 on the 23rd to 1200 on the 24th), and wind group (e.g. 28020G35KY).
This is followed by the Visibility Group, consisting of the significant weather group, cloud and vertical obscuration group, and Non-convective low-level wind shear (LLWS) group. These use the codes given in the tables and discussions earlier.
Visibility greater than six miles has the prefix P (remember the use of P in the RVR numbers), so is coded as P6SM. Wind shear is indicated by WS, as in this portion of a TAF: 13012KT WS020/27055KT. This means surface winds from 130° @ 12 kts change to 270° @ 55 kts at 2000 ft.
The report ends with the change group, and can start from a given time (FM group), be temporary changes (TEMPO group), or probable changes (PROB group). Here is an example:
KDSM 022336Z 0300/0324 20015KT P6SM BKN015
FM030230 29020G35KT 1SM +SHRA OVC005
TEMPO 0303/0304 30030G45KT 3/4SM -SHSN
Here, the KDSM forecast is made at 2336Z on the 2nd, and is valid from 0000 UTC on the 3rd of the month to 2400 UTC on the 3rd, with a 15-knot wind from 200°, visibility better than 6 miles, and a 1500-ft broken ceiling. But a change is expected from 0230 UTC on the 3rd with a 20-kt wind from 290° gusting to 35, visibility lowering to 1 mile, and heavy rain showers with an overcast at 500 ft. Temporarily, starting at the hour of 03 on the 3rd and ending at the hour 04 on the 3rd, the wind will increase to 30 kts @ 300, gusting to 45 kts, and the visibility lowers to ¾ mile with light snow showers.
Try reading reports using this guide and you should quickly become fairly proficient. When you come across something unusual (like VA for volcanic action) refer to the advisory circular for the above material and for much more about weather products and services.
Keith Thomassen is a pilot, instructor, and educator who specializes in teaching pilots how to get the most out of their GPS units. He has written several manuals on this and other subjects. For more see http://www.avionicswest.com/default.html