What type of clouds typically signify an unstable atmosphere?

Cumulus clouds are typically indicative of an unstable atmosphere due to their formation process and characteristics. These clouds are often puffy, white, and associated with vertical development, which occurs when warm air rises and cools, leading to condensation. This vertical motion is a clear sign of instability, as it suggests that the air parcels are buoyant and able to rise rapidly through the atmosphere.

In contrast, stratus clouds generally form in stable conditions, as they are characterized by a horizontal layering and usually cover the sky in a uniform sheet. Nimbus clouds can be related to precipitation but do not specifically indicate instability on their own. Cirrus clouds, which are high-altitude, wispy clouds, typically signify stable atmospheric conditions and the presence of moisture at higher altitudes. Thus, the presence of cumulus clouds points towards an unstable atmosphere where convective activity is likely.