The cumulonimbus (Cb) is the most imposing and dangerous cloud in the atmosphere. It develops vertically from low altitudes (600–2,000 m) up to the tropopause, easily reaching 10,000 to 15,000 metres. Its top spreads into the characteristic anvil (incus) shape as it flattens against the ceiling of the troposphere, while its base is dark and turbulent.
Inside, violent updrafts (up to 30 m/s) and downdrafts are responsible for lightning, hail, torrential rain, microbursts and even tornadoes. The separation of electrical charges by collisions between ice crystals and supercooled droplets generates the electric fields that trigger lightning discharges. A single cumulonimbus can contain the energy equivalent of several nuclear weapons and process hundreds of thousands of tonnes of water.
Cumulonimbus clouds are classified into three species: calvus (rounded top, still growing), capillatus (fibrous, icy top — the classic anvil), and the special variety incus (fully spread anvil). When a cumulonimbus develops persistent internal rotation it becomes a supercell — the most severe type of thunderstorm, capable of producing large hail (>5 cm) and violent tornadoes. On radar, cumulonimbus appear as intense echoes (>50 dBZ), often with distinctive hook echoes indicating rotation.