What format is used to report wind direction and speed in METAR when the speed is calm?

The format used to report wind direction and speed in METAR when the speed is calm is represented as "00000KT." This indicates that the wind is effectively calm, with a direction of 000 degrees and a speed of 00 knots. In aviation meteorology, reporting wind in this format allows pilots and meteorologists to easily interpret the wind conditions at a glance, adhering to standardized practices in METAR documentation.

While "Calm" is a term that can be used informally to describe still wind conditions, it does not conform to the specific format required in METAR reports. Similarly, "VRB," which indicates variable winds, is not applicable when winds are calm, as it suggests some degree of variability rather than stillness. The term "No Wind" is not standard in METAR reporting; thus, the correct and formal representation of calm winds is "00000KT."