How is a ceiling defined in a METAR report?

The definition of a ceiling in a METAR report specifically refers to the height of the lowest layer of clouds that is below 20,000 feet above ground level. This measurement is crucial for pilots and meteorologists because it indicates the vertical visibility in the atmosphere and helps assess whether flight conditions meet the necessary criteria for visual flight rules (VFR) or if instrument flight rules (IFR) apply.

When discussing cloud layers, a ceiling is established when any portion of a cloud layer is present at or below this altitude threshold. It's important for safety and navigation in aviation; thus, this definition is central to understanding METAR reports accurately. The other options address different aspects of aviation and weather but do not directly pertain to how ceilings are technically defined in a METAR context.