Definition
A dense, vertically developed cloud that produces thunderstorms, characterized by strong updrafts and downdrafts and associated with heavy precipitation, lightning, hail, severe turbulence, icing, and possible tornadoes. In a METAR, CB is appended to a cloud layer report to indicate that the reported layer contains cumulonimbus clouds (for example, BKN040CB).
Plain English
CB means a thunderstorm cloud is in the area. When you see CB in a weather report, it's telling you that the cloud layer mentioned is the kind that brings thunderstorms — the towering, anvil-topped clouds with rain, lightning, and dangerous flying conditions inside and around them.
Context Anchor
Seen in METAR sky condition reports, often attached to a cloud layer to show that the reported clouds include cumulonimbus clouds.
Derivation
From Latin cumulus meaning 'heap' or 'pile' and nimbus meaning 'rain cloud.' So cumulonimbus literally means 'heaped rain cloud' — which fits, because these are the tall, piled-up clouds that produce heavy rain and storms.
Why Pilots Care
CB clouds signal severe hazards including extreme turbulence, icing, and lightning that can lead to aircraft damage or accidents.
Grounding Statement
Picture a tall, dark storm cloud building upward like a tower; that is the kind of cloud CB is warning you about.
Intuition Check
CB does not mean just any big or dark cloud. It specifically means cumulonimbus, the thunderstorm cloud type pilots treat as hazardous.
Example Sentence 1
The METAR reported BKN035CB, so the pilot delayed departure until the thunderstorm cells moved east of the airport.
Example Sentence 2
Pilots avoid flying under or near cumulonimbus clouds because of the risk of severe turbulence.