Plain English Meaning

SM stands for statute mile, the standard land mile most people are familiar with. In weather reports, it tells you how far you can see horizontally from the airport.

Full Definition

A unit of distance equal to 5,280 feet, used in METAR and other aviation weather reports to express prevailing visibility.

Context Anchor

Seen in routine airport weather reports, especially in visibility entries such as 10SM or 1/2SM.

Derivation

Statute comes from the Latin statutum, meaning 'a law or fixed rule.' The statute mile is the mile fixed by law (defined by English statute in 1593), as opposed to the nautical mile used for navigation. Aviation weather reports use the statute mile because the U.S. National Weather Service standardized on it for surface observations.