AO1
Indicates an automated weather observation station that lacks a precipitation discriminator. AO1 stations can detect that precipitation is occurring but cannot distinguish between liquid (rain) and frozen (snow) types. These stations often report UP (unknown precipitation) when precipitation is detected.
RMK AO1
RMK AO1 SLP982
RMK AO1 T02170183
AO1 stands for "Automated Station Type 1" in the remarks section of a METAR report. Indicates an automated weather observation station that lacks a precipitation discriminator. AO1 stations can detect that precipitation is occurring but cannot distinguish between liquid (rain) and frozen (snow) types. These stations often report UP (unknown precipitation) when precipitation is detected.
AO1 appears in the remarks (RMK) section at the end of a METAR observation. The remarks section follows the main body of the METAR and provides additional details about the observation that don't fit into the standard coded format.
Here is an example: "RMK AO1". This appears in the remarks section and indicates automated station type 1 information as part of the weather observation.