Friday, December 4, 2009

The METAR reader

I am certainly not a meteorologist. Indeed, my knowledge of the weather is minimal. This will change over time and with study, as knowing how to read and predict the weather is critical for a pilot. I've been told that regulations require a full-on weather briefing from a Flight Service Station prior to any cross-country flights, and even if you are only flying locally, knowing the weather is vital for safety.

All controlled fields have a weather report on a specific radio frequency (known as an ATIS, or Automated Terminal Information Service) that should be monitored before and during flights. Many uncontrolled fields have automated weather observation stations (AWOS) that will broadcast current weather conditions to radio-equipped pilots within range of the field. Even the airport I flew out of in Auburn, Maine had and still has an AWOS station at 118.025 on your radio dial.

Since my flight for the day was cancelled today, I decided to practice reading weather reports. Aviation weather reports come in many forms, but one form that is accessible and standardized worldwide is the METAR. What does METAR stand for? I have no idea; I understand it's an acronym for a phrase in French that has to do with the weather. I think of it as a METeorological Aviation Report, which is not "technically" right but it's close enough.

METAR's come in a standard format. Current METAR for the airport I will be flying from, KGIF, is reported as:

KGIF 041959Z AUTO 33005KT 1 3/4SM -RA =5/8sm)" style="cursor: pointer; ">BR BKN004 BKN024 OVC031 14/13A3002 RMK AO2 P0006

Wow. What a jumble of nonsense, eh? But with a closer look it can make perfect sense. All of these letters and numbers have meaning and come in a standard format that makes interpretation simple and easy. Let's go through the list.

KGIF. This is the International Civil Aviation Organization's four-letter designation for Winter Haven's Gilbert Field. A METAR for my hometown of Auburn, Maine would have KLEW listed here, Orlando International would be KMCO, and so on. Most fields have an ICAO designator.

041959Z. This is a timestamp to let you know what time the observation was taken from. the 04 means this is the 4th of the month; 1959Z means that the observation was recorded at 1959 Zulu (Greenwich Mean) time. Zulu time is used to give a standard time across the make planning and communication during long-distance flights across time zones easier. We in the Eastern time zone are 5 hours behind GMT or Z-time; this observation was sampled at 1459 local time.

AUTO. This means that the report was generated automatically by an ASOS, an Automated Surface Observation System.

33005KT. This is a wind observation: the wind is from heading 330 at 05 knots. Sometimes you will see this amended with a G and a number, such as G15; that indicates gusting.

1 3/4 SM -RA BR. This indicated that visibility is 1 3/4 Statute Miles in light rain and mist.

BKN004 BKN 024 OVC031. This portion of the METAR is reporting clouds. There is a broken layer at 400 feet above ground level, another at 2400 feet, and an overcast layer at 3100 feet.

14/13. This is the temperature and dewpoint in celsius. This not only tells you if it is warm or cold, but the Temperature-Dew Point Spread can reveal whether fog is likely. The dew point is the point at which dew condenses from the air and causes fog. A low spread between temperature and dew point indicates that fog is likely; a wider spread means fog is less likely. Current conditions are conducive to fog.

A3002. This is the altimeter setting. Altimeters work on pressure differentials, and if the atmospheric pressure changes it can cause a faulty reading on your altimeter, which can cause you to fly too high or too low. It is a good idea on cross-country flights to check weather stations along your route and adjust your altimeter accordingly. Current altimeter setting at KGIF is 3002, adjustable in a little window on the altimeter gauge.

RMK AO2 P0006. Remarks: Automated Observation, Total Precipitation .06 of an inch this hour.

And there you have it: the METAR in a nutshell. It is confusing and bulky at first, but once you read a few of them and practice, it gets easier. AOPA members can get their free weather reports at; non-members can still get METARs and aviation weather courtesy of NOAA's Aviation Weather Center.

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