A cumulonimbus is the deep, precipitating convective cloud responsible for thunderstorms. It grows in strong updrafts through most or all of the troposphere, produces lightning and thunder by definition, and typically flattens into an anvil where the updraft encounters stable air near the tropopause.
How a cumulonimbus grows
The parent cell begins as a cumulus congestus tower rising from a moist, unstable boundary layer. Once the updraft has enough buoyancy to keep accelerating past the freezing level, ice forms, mixed-phase microphysics begins, and the tower is called a cumulonimbus.
The cloud carries a mix of hydrometeors stacked by temperature. Water droplets and rain dominate the warm lower half, ice crystals and snow dominate the upper half, and graupel and small hail form in the mixed-phase zone in between. Collisions among ice, graupel, and supercooled water separate electric charge, which is what makes a Cb, by definition, a thunderstorm cloud.
Reading the cloud visually
A young cumulonimbus whose top has not yet glaciated is called a calvus, meaning bald. Once the upper portion turns to ice and softens into a fibrous, striated appearance, it is a capillatus. When the top spreads horizontally into the classic flat cap, that feature is called incus, the anvil.
Cues about updraft strength show at the top and along the flanks. An overshooting top pushing a dome above an otherwise smooth anvil marks an updraft strong enough to punch briefly into the lower stratosphere. Sharp, crisp cauliflower texture on the flanks means the cloud is still accelerating upward, while a soft, ragged appearance means the updraft is weakening or dying.
Why it matters operationally
A convective forecast is essentially a forecast of cumulonimbus behavior. Parcel-based tools such as CAPE and the LCL describe how tall and how buoyant a Cb can become in a given environment, and wind shear across the updraft controls how organized and long-lived it will be.
A tall, thin CAPE profile in weak shear favors pulse cumulonimbi that live for less than an hour. Longer, more sheared CAPE favors organized multicell clusters, squall lines, and supercells that can persist for hours and produce every severe hazard in the book.
What a cumulonimbus alone does not tell you
Seeing a cumulonimbus does not by itself indicate severity or hazard type. A pulse Cb in a moist, low-shear environment can produce brief heavy rain and lightning and little else. The same visual shape in strong deep-layer shear can support long-lived hail and tornado producers.
Visual assessment also gets harder at night and behind rain. A high-based cumulonimbus over the Plains can hide a strong updraft, and an outer curtain of precipitation can mask the base entirely. Radar and satellite trends are needed to close those gaps.