What report is used when no clouds are visible?

The correct report for indicating that no clouds are visible is SKC, which stands for "Sky Clear." This term is used in aviation meteorology to specify that there are no clouds present in the sky, thus providing pilots with crucial information regarding visibility and weather conditions for flight operations.

Using SKC effectively communicates that the sky is completely clear, which is essential for flight planning and safety. When meteorologists or observers report SKC, it reassures pilots that visual flight rules (VFR) conditions are met since there are no obstructions to visibility from clouds.

The other options, while related to cloud conditions, do not correctly indicate a complete lack of clouds. CLR is often used informally but not as a standard meteorological term, No CLDS might imply no notable cloud formations but isn't an official code, and VV indicates visibility obscured by mist or haze, not a clear sky. Therefore, SKC is the appropriate term for when no clouds are present at all.