Which of the following is a characteristic of an unstable air mass?

An unstable air mass is characterized by vertical movement of air, leading to the development of cumulus clouds. These clouds form when warm, moist air rises and cools, leading to condensation. As the air continues to rise, it can develop into larger, vertically-developed clouds, such as cumulonimbus clouds, which are often associated with thunderstorms.

Uniform cloud cover and fog typically occur in stable conditions where the air is not rising significantly, resulting in widespread, layered clouds or low-visibility situations at the surface. Calm winds are also indicative of a stable atmosphere, where there is little to no vertical mixing of air. In contrast, the presence of cumulus clouds is a clear sign of atmospheric instability, as it suggests that warm air is rising and that significant weather changes may occur.