Definition
A code appearing in a METAR report that identifies the weather observation as coming from an automated station equipped with a precipitation discriminator — a sensor capable of distinguishing between liquid precipitation (rain) and frozen precipitation (snow).
Plain English
AO2 tells you the weather report was generated automatically by a station that can tell the difference between rain and snow. If you see AO2 in a METAR, no human observer was involved, but the equipment can identify what kind of precipitation is falling.
Context Anchor
Seen in the remarks section of a METAR, usually near other station-type or sensor information.
Derivation
AO stands for Automated Observation. The '2' indicates the second-generation type of automated station — one with a precipitation discriminator. AO1 stations exist too, but they cannot tell rain from snow. The number simply marks the capability level of the equipment.
Why Pilots Care
It tells the pilot how detailed and reliable the automated precipitation information is for flight planning and safety decisions.
Intuition Check
AO2 does not mean “second-class” or “better weather.” It only tells you what kind of automated weather equipment produced the report.
Example Sentence 1
The METAR ended with 'AO2 SLP180', confirming the report came from an automated station with a precipitation discriminator.
Example Sentence 2
Pilots reviewed the AO2 designation to decide whether to trust the automated rain versus snow report for their approach.