Definition
A station type identifier used in METAR reports indicating the observation was generated by an automated weather station that does not have a precipitation discriminator. The station can detect that precipitation is occurring but cannot distinguish between liquid (rain) and frozen (snow) precipitation.
Plain English
A code in a weather report that tells you the report came from an automatic weather station that can tell when precipitation is falling but cannot tell whether it is rain or snow.
Context Anchor
You will see AO1 in the remarks section of a METAR, usually near the end of the report.
Derivation
The 'A' stands for Automated and 'O' for Observation. The '1' indicates the simpler of the two automated station types — one without the extra sensor that distinguishes rain from snow. AO2 is the more capable version.
Why Pilots Care
It signals that any precipitation type listed in the report may need manual confirmation before flight planning decisions.
Intuition Check
Do not read AO1 as a quality rating or priority level. It identifies the kind of automated weather station and one of its sensing limits.
Example Sentence 1
The METAR ended with AO1, so I knew the station could detect precipitation but couldn't tell us if it was rain or snow.
Example Sentence 2
Because the report carried AO1, the pilot verified precipitation type with a nearby station before departure.