Which of the following cloud types would NOT typically be reported in a METAR?

In the context of METAR reporting, the identification of cloud types is based on their significance to aviation safety and weather observation. Cumulonimbus, stratus, and cirrus are all defined cloud types that can directly affect flight operations or are relevant in the context of weather reporting.

Cumulonimbus clouds are notable for their association with severe weather, including thunderstorms and turbulence, making them a critical observation for pilots and meteorologists. Stratus clouds can indicate low ceilings and reduced visibility, while cirrus clouds are often associated with high-altitude weather patterns that could influence weather forecasts.

Fog, however, is not classified as a type of cloud in the same way as the others listed. Fog is defined as a suspension of tiny water droplets in the air at or near the Earth's surface, causing reduced visibility, but it doesn't meet the criteria for cloud classification. In METAR reports, fog is usually noted as a weather phenomenon affecting visibility rather than a cloud type. Therefore, while fog is certainly important for aviation safety and is mentioned in METAR, it is not categorized as a cloud type and is why it would typically not be reported in conjunction with specific cloud formations.