What is the general appearance of clouds classified as 'cumulus'?

Cumulus clouds are characterized by their distinctive appearance, which is often described as puffy or fluffy, resembling cotton balls in the sky. These clouds typically have a flat base that forms at a specific altitude, while the upper portions can appear rounded and billowy. This structure is the result of rising warm air, which cools as it ascends, leading to condensation and cloud formation.

In contrast, clouds that are grey and uniform represent a different classification, often associated with stratus clouds, which tend to cover the sky in a more even layer. Thin and wispy clouds are indicative of cirrus clouds, which form at higher altitudes and have a more delicate appearance. Heavy and dark clouds, typically associated with storm systems, describe cumulonimbus or nimbostratus clouds, which are characterized by their thickness and potential for precipitation. Each of these cloud types has distinct features that allow for classification based on their appearance, thus making 'puffy with a flat base' the correct description for cumulus clouds.