Plain English Meaning
SN is the code used in a METAR weather report to say that it is snowing at the airport.
Full Definition
In a METAR, SN is the contraction used to report snow as the present precipitation. It is a form of frozen precipitation made up of ice crystals that have grown and clumped together as they fall through the atmosphere. SN is reported in the present weather group and may be preceded by an intensity sign (- for light, no sign for moderate, + for heavy) and/or a proximity or descriptor code such as VC (in the vicinity), SH (showers), or BL (blowing).
Context Anchor
Seen in the present weather part of a METAR, where short codes describe what is happening at the airport.