Plain English Meaning
The classic puffy, cotton-ball clouds with flat bottoms and bumpy tops. They form when warm air rises, and the air inside and around them is usually bumpy to fly through.
Full Definition
A cloud type with a flat base and puffy, dome-shaped or cauliflower-like upper portions, formed by rising columns of warm, moist air (convective currents). Cumulus clouds indicate unstable air and are associated with turbulence, gusty winds, and — when developed vertically — showers and thunderstorms.
Context Anchor
Seen in weather briefings, cloud descriptions, and turbulence discussions, especially when deciding whether clouds may mark rough air.
Derivation
From the Latin cumulus, meaning 'a heap' or 'pile.' The name fits — these clouds look like heaps of cotton stacked in the sky, which helps you remember they are built upward by rising air rather than spread out in flat layers.
Why Pilots Care
Building cumulus signals areas of vertical air movement that can produce bumps, altitude deviations, or stronger turbulence if the clouds continue to grow.
Grounding Statement
Picture warm air rising from the ground; as it rises and cools, its moisture becomes a visible puffy cloud.
Intuition Check
Do not assume “cumulus” always means harmless fair-weather cloud. In this context, it is a clue that air is rising, and rising air can mean turbulence.
Example Sentence 1
Scattered cumulus clouds were forming over the ridge by mid-morning, so we expected a bumpy ride on the climb out.
Example Sentence 2
Scattered fair-weather cumulus produced only gentle bumps while the pilot maintained visual separation.