Decoding Lesson #1

Austin-Bergstrom International Airport (KAUS) is located in the capitol of Texas. Austin is typically a relatively dry place compared to most of the state to the south, yet conditions can be stormy, or as is the case today, foggy! Here's the current surface observation:

KAUS 221253Z 00000KT 1/4SM R17L/1200V1600FT FG OVC001 17/17 A3000

At 1253Z (7:53 AM CDT) winds were calm (00000KT) and visibility was a quarter of a mile (1/4SM). At this hour Austin is also reporting Runway Visual Range (RVR). The code follows the visibility token because RVR is calculated from visibility, ambient light level, and runway light intensity, and is the maximum distance at which the runway, or the runway lights can be seen.

In this case, the RVR is for Runway 17L, the only runway at Austin that has CAT II/III approaches. The 1200V1600FT part of the RVR token in this METAR indicates that the runway is visibile from between 1200 to 1600 feet (V=variable).

If you were wondering what was causing the poor visibility in the Lone Star State's capitol this morning, just refer to the next token: "FG" - it's the symbol used for fog. Also, the ceiling is reported overcast at 100 feet above ground level.

Finally, the temperature-dew point spread definitely indicates fog. Fog is probable when the temperature-dew point spread is 10°C (50°F) or less and decreasing. Fog usually forms when the dew point and the temperature are within a few degrees of each other, and will start to lift when the temperature-dew point spread begins to increase.

Finally, the surface pressure is reported at 30.00 inches of Mercury (A3000).

I'm listening to Austin Departure and they are reporting that the RVR is currently 6000 feet, so it sounds like conditions are improving - likely that temperature-dew point spread is helping out.

Lots of Abbreviations But Only One Format

In 1996, with the beginning of airline deregulation, the U.S. started using a new format to disseminate current weather and forecasts. Prior to that time the U.S. (and Canada and Mexico) used Surface Aviation Observations (SAO) for current conditions and Terminal Forecasts (TF), while the rest of the world used METARS and TAFS. METAR is an abbrevaition of the French phrase "message d’observation météorologique régulière pour l’aviation", literally "aviation weather observation". A typical METAR includes the date and time the report was sent (in UTC), the winds, could cover and height above ground, temperature and dew point, and barometric pressure. For example, the METAR for this hour for Pontiac, Michigan is:

KPTK 202349Z 29008KT 3SM -RA BR OVC008 11/09 A3008

The four-letter identifer is an ICAO-format desginator for Oakland-Pontiac Airport. 202349Z is interpreted as the 20th day of the current month at 2349 Zulu (Zulu is slang for UTC 'Universal Time Coordinated' or GMT 'Greenwich Mean Time'). The wind direction is presented followdd by the wind speed; here's it's reported as coming from 290 degrees at 8 knots. The next token is the visibility - here's it's 3 statute miles. RA means rain, but the minus in front mean light rain (and +RA means heavy rain). BR is a token for "mist". The cloud cover is overcast (OVC) at 800 feet above ground level (AGL). The temerature is 11 degrees Celsius, and the dewpoint is 9 degrees Celsius - not much of a spread and perhaps indicative of potential icing conditions. The final token "A3008" is interprested as "Altimieter 30.08 inches of Mercury - the surface pressure reading.

As you can see, reading METARS requires a good understanding of the conventions used to report surface conditions using the METAR format. This format, used all over the world now with only minor changes, as spelled out by exceptions for only a handful of airports, makes it easier for pilots to read the current weather at almost any station in the world.

European METARS

This week I'm exploring European METARS. Generally they are very similar to North American weather reports, but have metric celings, sometimes metric winds, and some different weather codes. Stay tuned.