Which of the following is a sign of unstable air?

Cumuliform clouds are indeed a sign of unstable air. These clouds typically form due to rising air parcels that cool and condense as they ascend. In an unstable atmosphere, warm air near the surface can rise rapidly, leading to the development of cumulus clouds that can grow vertically into larger, more turbulent formations. This vertical growth indicates instability, as it is driven by the buoyancy of warm, moist air rising through cooler air aloft.

In contrast, stratus clouds are usually associated with stable air, where air moves horizontally rather than vertically. Good visibility typically suggests clear, stable air conditions rather than the instability seen with cumulus cloud development. Steady precipitation often occurs in stable air conditions characterized by stratiform clouds, rather than the turbulent and variable weather patterns associated with unstable air. Thus, cumulus clouds serve as a clear marker of atmospheric instability, indicating conditions that can lead to convective activity and more dynamic weather phenomena.