Plain English Meaning

The early growing phase of a thunderstorm, when warm air is rising fast inside a building cloud and nothing has started falling out of it yet.

Full Definition

The first stage in the life cycle of a thunderstorm, characterized by strong, continuous updrafts of warm, moist air that build a cumulus cloud rapidly upward. During this stage the cloud grows vertically, no precipitation reaches the ground, and downdrafts have not yet developed.

Context Anchor

Seen in weather training, thunderstorm discussions, and decisions about avoiding growing convective clouds.

Derivation

From the Latin cumulus, meaning 'a heap' or 'pile.' The name fits because the cloud builds upward in piled, swelling shapes as warm air rises into it.