Cumulus (Cu) clouds are the quintessential "fair-weather clouds" — white, puffy, flat-bottomed heaps that form during daytime as the sun heats the surface and triggers convective updrafts. Their name comes from the Latin for "heap" or "pile". The flat base marks the lifting condensation level — the altitude at which rising air cools to its dew point and water vapour condenses into visible droplets.
Cumulus are classified by vertical extent: cumulus humilis (wider than tall — the classic fair-weather puff, indicating stable conditions), cumulus mediocris (roughly as tall as wide — moderate development), and cumulus congestus (towering cumulus, significantly taller than wide, capable of producing showers). If a congestus continues growing and glaciates (its top turns icy and fibrous), it transitions into a cumulonimbus.
For pilots, glider pilots, and paragliders, cumulus clouds are essential markers of thermals — the rising columns of warm air that provide lift. The base height can be estimated from the surface temperature and dew point: roughly (T − Td) × 125 metres. Cumulus typically form by late morning, grow through the afternoon, and dissipate at sunset as surface heating ceases. In satellite imagery, they appear as fields of bright white dots, often organised in "cloud streets" aligned with the wind direction.