How is "cloud base" reported in a METAR?

In a METAR report, "cloud base" is indicated by specifying the height of the cloud layer above the ground level. This height is usually expressed in hundreds of feet. For instance, a report might indicate cloud layers at various altitudes, such as "BKN015" which means broken clouds at 1,500 feet above ground level.

Measuring the cloud base height is critical for pilots and meteorologists as it helps assess weather conditions, visibility, and potential flight safety. Other options like cloud density, relative humidity, or type of cloud do not provide the necessary information regarding the specific altitude of the cloud layers, which is essential for understanding atmospheric conditions in aviation and meteorology. Thus, indicating the height of the cloud layer is the standard method used in METAR reports to communicate cloud base information effectively.