What cloud type does "CB" refer to in METAR reports?

"CB" in METAR reports refers to Cumulonimbus clouds. These clouds are significant in meteorology due to their association with severe weather phenomena. Cumulonimbus clouds are towering, vertical clouds often characterized by their anvil-shaped tops and can extend to great heights in the atmosphere. They are primarily formed through strong updrafts and can produce thunderstorms, heavy rain, lightning, hail, and even tornadoes.

In contrast, the other cloud types listed do not have the same severe weather implications as Cumulonimbus. Cumulus clouds are typically fluffy and indicate fair weather, Stratus clouds form in flat layers that often bring overcast skies and light precipitation, and Cirrus clouds are high-altitude, wispy clouds that generally signify fair weather. Therefore, "CB" specifically indicates the potential for severe weather, making it a crucial identifier in aviation and weather forecasting.