Definition
A unit of distance equal to 5,280 feet, used in METAR and other aviation weather reports to express prevailing visibility.
Plain English
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.
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.
Why Pilots Care
Determines whether visual flight rules minima are met and whether the runway environment is visible during takeoff or landing.
Intuition Check
SM does not mean nautical miles. In this weather-report context, it means statute miles—the regular land miles used to state visibility.
Example Sentence 1
The METAR reported visibility as 10SM, meaning the pilot could expect to see 10 statute miles in any direction from the airport.