What Does AO1 Mean in a METAR?
AO1 means automated station without a precipitation discriminator (cannot tell rain from snow). It stands for Automated Observation, type 1.
AO1 in a real report
RMK AO1
What it changes for you
AO1 means the station knows something is falling and cannot tell you what.
In summer this rarely matters. In winter it matters a lot. Rain and snow at the same temperature mean a wet runway or a contaminated one. The report will not settle which.
Reading around the limitation
When you see AO1 with UP in the body, work the type out yourself.
- Temperature and dew point near or below zero point to snow or ice pellets
- Temperature comfortably above zero points to rain
- A temperature near zero over a colder surface is the freezing rain case, and the one to treat most carefully
A nearby AO2 station settles it directly, if you have one.
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
What does AO1 mean in a METAR?
The station is automated and has no precipitation discriminator. It can detect that precipitation is falling but cannot tell rain from snow, so it reports UP for unknown precipitation. You work the type out from the temperature.
Is AO1 worse than AO2?
For winter operations, yes. An AO1 station cannot distinguish rain from snow, so you have to infer the type from the temperature and dew point rather than reading it off the report.