Plain English Meaning

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.

Full 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 , CB is appended to a cloud layer report to indicate that the reported layer contains clouds (for example, BKN040CB).