What Does AO2 Mean in a METAR?
AO2 means automated station with a precipitation discriminator (can tell rain from snow). It stands for Automated Observation, type 2.
AO2 in a real report
RMK AO2
What it changes for you
AO2 is a capability flag, not a weather report. It tells you what kind of station produced everything else in the line.
That matters most in winter. An AO2 station can report -RA or -SN and mean
it. An AO1 station reports UP instead. It knows something is falling. It does
not know what.
What an automated station still cannot see
Both types share the same blind spots, and the flag is your reminder that they are there.
- Cloud is sampled by a ceilometer looking straight up, so isolated cloud that never crosses the sensor is never reported
- Visibility is measured at one point, not around the horizon
- Thunderstorms are detected by lightning sensors, so a storm with no detected strikes may not appear
A human observer would catch all three. AO2 with no augmentation remarks means nobody was standing outside.
Where you will see it
Always in the remarks, after RMK, usually first. Most US stations are
automated, so it is one of the most common groups you will see.
Related codes
Decode a report containing this code
Paste a METAR, TAF, PIREP or SIGMET into the WxDecoder decoder. Every group comes back in plain English. The raw code sits beside it, so you can check the decode rather than trust it.
Frequently asked questions
Does AO2 mean precipitation is happening?
No. AO2 describes what the station can do, not what the weather is. It says the sensor can tell rain from snow. Whether anything is falling is reported in the body of the METAR, not by this flag.
What is the difference between AO1 and AO2?
AO2 has a precipitation discriminator and can tell rain from snow. AO1 cannot, so it reports unknown precipitation as UP and leaves you to work out the type from the temperature.