Which cloud type is most directly associated with thunderstorms and lightning?

Cumulonimbus is the cloud type most directly associated with thunderstorms and lightning.

Cumulonimbus clouds are tall, dense clouds produced by strong upward motion in unstable air. They can begin as growing cumulus clouds and develop vertically through much of the troposphere. Their tops may spread into a flat anvil shape when they reach a stable layer near the tropopause.

These clouds can produce heavy rain, hail, lightning, thunder, strong wind gusts, and sometimes tornadoes. Not every cumulonimbus cloud generates every hazard, but the cloud type signals deep convective activity rather than ordinary layered cloud cover.

Cumulonimbus is often confused with cumulus, which usually has smaller, detached heaps and may bring fair weather. Cirrus clouds are high and ice-rich, while stratus clouds form broad, relatively uniform layers. A mature thunderstorm may contain several cloud forms, but its main towering storm cloud is cumulonimbus.