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).
Plain English
SN is the code used in a METAR weather report to say that it is snowing at the airport.
Context Anchor
Seen in the present weather part of a METAR, where short codes describe what is happening at the airport.
Derivation
SN is a compact weather-report code formed from the word “snow.” The word “snow” comes from Old English “snāw,” meaning snow; in aviation weather reports, it is shortened so conditions can be reported quickly and consistently.
Why Pilots Care
Snow reduces visibility, creates the risk of airframe icing, and requires pilots to assess braking action and possible de-icing before takeoff.
Grounding Statement
If you see SN in the airport weather report, picture snow falling at the reporting location, not just snow on the ground.
Intuition Check
SN does not mean the runway is necessarily snow-covered. It means snow is being reported as falling weather at or near the airport observation point.
Example Sentence 1
The METAR showed -SN and three-quarters of a mile visibility, so we delayed departure to wait for better conditions.
Example Sentence 2
With SN expected later in the afternoon, the flight crew planned extra time for de-icing on the ramp.