Final answer:
Stratiform clouds are a characteristic of stable air due to the minimal vertical movement in the atmosphere. Cumulus clouds, in contrast, indicate unstable conditions and are not associated with stable air.
Step-by-step explanation:
A characteristic of stable air is the presence of stratiform clouds. Stable air occurs when there's little vertical movement, and the atmosphere tends to resist vertical motion. This stability is often a result of air that has flowed a long distance over water before encountering land, which tends to stabilize the atmosphere, minimizing turbulence and leading to the formation of layered clouds known as stratiform clouds. Additionally, such stable conditions can lead to the aggregation of particulates like fog and smoke, although it doesn't necessarily result in unlimited visibility. Cumulus clouds, on the other hand, are typically indicative of unstable air since they form because of convection; the rising of water vapor which doesn't align with the characteristics of stable air.