Which cloud type is indicative of unstable air?

Cumulus clouds are indicative of unstable air due to their characteristic formation process. These clouds typically develop in conditions where warm, moist air rises rapidly, leading to the formation of tall, puffy structures. The rising air cools, causing condensation to occur, which contributes to the cloud’s fluffy appearance. This process is often associated with thermal updrafts and can indicate instability in the atmosphere, especially when they grow into larger cumulonimbus clouds, which are capable of producing thunderstorms.

In contrast, stratus clouds are generally associated with stable air and often result in overcast conditions without strong vertical development. Cirrus clouds, being high-altitude and wispy, indicate fair weather and are formed in stable conditions high in the atmosphere. Nimbus clouds are primarily associated with precipitation, but they can form in both stable and unstable environments depending on their type. However, when considering which type is most indicative of instability, cumulus clouds stand out as the clear answer.