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December 30, 2022
Before you take off or land, knowing the airport’s weather is critical to making smart choices. Learn things like:
A lot of good general weather information is available through websites, apps, and even your local newscast. But knowing what’s happening currently on or close to the ground at your airport is essential to aviation safety, and you can access this information several ways.
Two important tools for making good aeronautical decisions are Meteorological Terminal Air Reports (METARs)--surface weather observations made available through websites and smart device apps--and recorded audio reporting weather information available by radio or phone through an Automatic Terminal Information Service (ATIS).
While you’ll find similar information from both sources, they are not identical. So it’s important to know the differences between them and how to use them together to capture a more full picture of the weather situation at an airport.
The information contained in METARs largely comes from automated weather stations at airports and other locations. It’s generally updated at 55 minutes past the hour.
Measurements of precipitation, barometric pressure, temperature, and other variables are captured by sensors and encoded by software before being relayed to the public via aviation weather Websites like the National Oceanic and Atmospheric Administration (NOAA)’s AviationWeather.gov and FltPlan.com and aviation apps like Foreflight and WingX Pro.
Some airports offer augmented observations, with the digital data reviewed and supplemented by weather observers or forecasters.
METARs are relayed in a code consisting of abbreviations and numbers that is also available through some sites and apps “translated” into standard English.
The information contained in a METAR depends on the weather observation equipment available at the airport but typically follows this sequence: four-letter International Civil Aviation Organization (ICAO) location indicator, day of the month and time (in Zulu) of the latest update, three-digit wind direction and speed (plus a “G” and additional wind speed if gusts are reported), horizontal visibility in statute miles, sky conditions, temperature and dew point in Celsius, altimeter setting, and remarks (if any), which may include density altitude and other information.
Here’s one example of a METAR in the default code, followed by an explanation in plain English.
KDSM 202354Z 34005KT 10SM FEW050 FEW300 29/18 A3008 RMK AO2 SLP178 T02890178 10306 20289 58008
Here’s what it all means:
METARs can provide other data as well, including lightning, Pilot Reports (PIREPs), and runway conditions, each with their own abbreviations. The National Weather Service has a table explaining all of the information that may be contained in a METAR.
In addition to METARs, pilots can get current information about conditions at an airport using one of several automated audio services that can be heard by contacting the appropriate frequency or calling a phone number. Some of these stations provide not only weather but also supplemental information about conditions or activities at the airport which pilots might find helpful.
ATIS stands for Automatic Terminal Information Service and is usually found only at Class B, C, and sometimes D towered airports and is typically updated once an hour or when conditions change significantly (in which case the word “special” is used after the Zulu time of the report is given).
Each new broadcast includes a phonetic alphabet letter, starting with Information “Alpha” for the first broadcast of the day.
Pilots are expected to report which phonetic alphabet letter is in effect to assure ATC that they have the most current information.
The general format of an ATIS report is:
The report may also include information about frequencies currently in use, the tower’s hours of operation, and important Notices to Airmen (NOTAMs). NOTE: Automated services like ATIS should not be relied on for NOTAMs; often there are many more NOTAMs in effect at and near airports than can be covered in a broadcast.
ASOS stations, usually found at non-towered airports, are owned by the National Weather Service, Federal Aviation Administration, and the U.S. Department of Defense and provide current weather information that’s updated every minute. They’re used to gather weather data for the entire country and not just for aviation purposes.
ASOS does not have the ability to report NOTAMs or any other information and does not include a phonetic alphabet designator.
Interesting fact: when a tower that offers ATIS closes for the night, the service typically reverts to ASOS.
Almost all AWOS stations are owned by the FAA, though local and state agencies will sometimes maintain them.
Like ASOS, AWOS broadcasts are refreshed every minute.
AWOS provides most of the same information as an ASOS, which can range from altimeter setting and winds to current precipitation and runway conditions. Newer stations even have the ability to add voice information to the broadcast regarding NOTAMs.
You can never have too much weather and airport information before departing or approaching an airport.
METARs and ATIS (or ASOS or AWOS) provide critical data that should be consulted as a matter of routine.
But don’t forget to use other resources that are readily available, including your own five senses. A METAR updated 35 minutes before your arrival at an airport may have reported calm winds, but if the wind is shoving you around five miles from your destination, be sure to listen in on the local weather frequency, consider overflying the field (at the appropriate altitude) to check the windsock, and watch and listen to the traffic frequency to determine which runway is being favored.
Having the most current information can mean the difference between a good day of flying, a bad day, or a day when you just decide to stay home and mow the lawn.
Want some practice listening to ATIS or decoding and understanding METARs? Take a look at the ATIS&METAR module in ARSim.
A series of lessons, provided for free, will provide instruction and practice on the weather codes, decoding METARs, and listening and understanding ATIS.
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PlaneEnglish created this blog to provide aspiring and current pilots a resource for all things related to aviation radio communication.
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