A deep, vertically developed convective storm cloud extending to the tropopause, producing squalls, lightning, heavy rain, and microbursts.
Cumulonimbus (CB) is the most energetic cloud type in the atmosphere, characterised by massive vertical development from a low base to the tropopause, where the anvil-shaped cirrus top spreads in the direction of the upper-level wind. It is classified in the WMO International Cloud Atlas (WMO-No. 407) as a cumuliform cloud of great vertical extent, generating heavy precipitation, lightning, severe turbulence, hail, and microbursts — a rapid downdraught that can produce a sudden and dramatic wind shift at the surface within a very short time.
For a watchkeeping officer, a cumulonimbus is a direct threat requiring immediate action. The squalls associated with CB cells can produce wind speeds far in excess of the ambient conditions — sometimes doubling or tripling within minutes — causing an unprepared vessel to heel sharply or part deck lashings. Lightning presents a risk to electronic equipment; officers should note that GMDSS antennas and navigation systems may be temporarily affected during a CB overhead. Microbursts are particularly hazardous during port operations involving cranes or tall cargo.
On radar, CB cells appear as areas of intense reflectivity, often with bright banding at the periphery. Officers should maintain a broader radar scale when CB activity is present to give adequate warning distance. SIGMET advisories from meteorological authorities identify regions with severe CB activity. SOLAS voyage planning under Regulation V/34 requires that CB-associated severe weather fronts be considered when planning a passage across any ocean.