Cumulus is a type of cloud that is characterized by its fluffy, cotton-like appearance and flat base. These clouds are often described as "pillow-shaped" or "cotton ball-like." They typically form at low altitudes and can be seen on sunny days. Cumulus clouds can indicate fair weather when they are small and scattered, but if they grow larger and taller, they may develop into towering cumulonimbus clouds that bring thunderstorms.