What does the term "ceiling" refer to in a METAR report?

In a METAR report, the term "ceiling" specifically refers to the lowest layer of clouds above ground level that covers at least half of the sky. This is an important measurement as it helps pilots determine the extent of cloud cover and the vertical visibility when planning approaches and landings. A defined ceiling is crucial for safe flight operations, especially under instrument flight rules (IFR), as it directly impacts whether the conditions allow for visual flight rules (VFR) operations or require instruments for navigation and control.

The other options, while they relate to cloud observations in some fashion, do not accurately represent what is defined as ceiling in METAR reports. Knowing the correct interpretation of these terms enhances safety and efficiency in air traffic operations.