Which of the following is NOT one of the six common sky conditions?

In meteorological terms, "STR" is not recognized as one of the six common sky conditions used in aviation and weather reporting. The six standard sky conditions include "FEW" for a few clouds, "BKN" for broken clouds, "OVC" for overcast skies, and others like "CLR" for clear skies and "SCT" for scattered clouds.

To briefly outline the recognized conditions:

• FEW refers to a small number of clouds covering a small portion of the sky,

• BKN indicates a significant amount of cloud coverage where broken clouds are present,

• OVC signifies that the sky is completely covered by clouds.

Each of these terms provides crucial information for pilots and meteorologists for flight planning and weather observation. Since "STR" does not align with any of these defined conditions, identifying it as the option that is not part of the common sky conditions is accurate.