Definition
In METAR and TAF weather reports, SN is the standard contraction for snow — frozen precipitation in the form of ice crystals, usually clustered into flakes, falling from clouds when temperatures aloft and at the surface are at or below freezing.
Plain English
SN on a weather report means it is snowing at that station, or snow is forecast.
Context Anchor
Seen in aviation weather reports, especially in the present weather part of an airport weather observation.
Derivation
A simple two-letter contraction of the English word snow, used because METARs are kept short and standardized worldwide.
Why Pilots Care
Snow reduces visibility, contaminates runways, and requires de-icing or runway condition checks before departure.
Intuition Check
SN does not mean “serial number” in an aviation weather report. In this context, it means snow.
Example Sentence 1
The METAR showed -SN BR, so the pilot expected light snow with reduced visibility from mist.
Example Sentence 2
Preflight review of the TAF revealed periods of SN, prompting the crew to verify de-icing availability.