Understanding ATIS and METARs

Before you take off or land, knowing the airport’s weather is critical to making smart choices. Learn things like:

• which runway to use,
• what kind of crosswind to expect,
• how much runway you may need,
• and even whether you should cancel, delay, or divert your flight

 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.

Table of Contents

• How to Read METARS
• ATIS, ASOS, and AWOS
• Putting it all together

How to Read METARS

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.

Decoding METARs

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:

• Des Moines International Airport, on the 20th day of the month (in this case, July), at 11:54 p.m. Zulu time, is reporting wind from 340 true at 5 knots, 10 statute miles of horizontal visibility, few clouds at 5,000 feet and again at 30,000 feet, temperature 29 degrees Celsius, dew point 18 degrees Celsius, and altimeter setting of 30.08.
• Remarks (RMK): the reporting weather station is coded A02, which means it’s automated (A) and capable of capturing precipitation data. Sea-level pressure is 1,017.8 millibars (the 1 and 0 before the 178 in the coded METAR are assumed).
• The numbers after the capital T provide the hourly temperature and dew point to the nearest tenth of a degree Celsius, in this case 28.9 degrees and 17.8 degrees Celsius, respectively.
• The number 20289 indicates the date and, in tenths of a degree Celsius, the highest temperature during the most recent six-hour period reported. Since this data is updated at 0000, 0600, 1200, and 1800 Zulu time, this METAR indicates that between 1200 and 1800 Zulu on May 20 a high temperature of 28.9 degrees Celsius (a 0 before the temperature reading means the temperature is higher than 0 degrees Celsius, and a 1 indicates a temperature below 0 degrees Celsius).
• The number 58008 indicates the pressure behavior over the previous three hours. The 5 is a group code that includes a range from 0 to 8 to describe the pressure trend. In this case, the 8 after the 5 indicates “steady or increasing, then decreasing; or decreasing then decreasing more rapidly.” The last three digits, 008, represent the amount of pressure change in tenths of a millibar, in this case a decrease of 0.8 mb in the three-hour period ending at 1800Z.

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.

ATIS, ASOS, and AWOS

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.

What is ATIS?

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:

• Airport name
• Phonetic alphabet designation (Alpha, Bravo, Charlie, etc.)
• Zulu time of the report
• Wind direction and speed
• Visibility and weather
• Cloud conditions
• Temperature and dew point
• Altimeter setting
• Runway(s) in use
• Comments or special information
• Designator (repeated at the end of the broadcast)

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.

Automated Surface Observation System (ASOS)

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.

Automated Weather Observation System (AWOS)

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.

Putting it all together

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.

Need help understanding METARs or Listening to ATIS?

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.

Download the app from the AppStore or Google Play or access it on the web and take advantage of these free lessons to become comfortable and proficient in understanding this important information.

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