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I occasionally see AO2 in the remarks of an automated weather report. What does the AO2 mean?

METAR KSLC 280453Z 00000KT 10SM FEW042 SCT055 BKN080 04/02 A2981 RMK AO2 CIG 037 NW RNWY SLP087 T00440022

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The AO2 notation means the station is automated and has a precipitation discriminator. AO1 means automated but no precipitation discriminator. The precipitation discriminator will be able to sense the difference between rain and snow.

The METAR format is documented in the Federal Meterological Handbook No. 1 in Chapter 12. This particular notation is:

12.7.1 c. Type of Automated Station (AO1 or AO2). AO1 or AO2 shall be coded in all METAR/SPECI from automated stations. Automated stations without a precipitation discriminator shall be identified as AO1 ; automated station with a precipitation discriminator shall be identified as AO2 .

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    $\begingroup$ I'd like to suggest that the first paragraph of this answer is not entirely correct. An automated station denoted AO1 in the remarks section of the METAR would still have a precipitation "sensor" but would not be able to discriminate between the types of precipitation (ex.: rain or snow). $\endgroup$ – Porcupine911 Feb 4 '16 at 15:28

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