Definition
In aviation weather reporting, a surface wind condition in which the wind speed is less than one knot, reported as 00000KT in a METAR. Direction is not assigned because the air movement is too slight to determine.
Plain English
The air at the surface is essentially still — not moving fast enough to give it a direction or a usable speed.
Context Anchor
Heard in airport weather broadcasts and tower wind reports, and seen in written aviation weather reports.
Derivation
From the Latin 'calma,' meaning the heat of the day — historically the time when sea breezes died and ships sat motionless. The aviation use keeps that core idea: air at rest.
Why Pilots Care
Affects takeoff and landing performance calculations since there is no wind assistance or crosswind component.
Grounding Statement
Picture a windsock hanging limp beside the runway; that is the practical picture of calm wind.
Intuition Check
Calm does not mean the pilot is relaxed. Here it means the wind at the reporting point is essentially still.
Example Sentence 1
ATIS reported wind calm, so the tower assigned runway 27 as the calm wind runway for arrivals.
Example Sentence 2
During the preflight briefing, the pilot noted calm winds at the destination airport.