Which type of clouds indicates instability when present in the atmosphere?

Cumulonimbus clouds are a key indicator of atmospheric instability. These towering clouds can extend high into the atmosphere and are typically associated with severe weather phenomena such as thunderstorms, heavy rainfall, hail, and sometimes tornadoes. The vertical development of cumulonimbus clouds occurs due to strong updrafts that are a result of warm, moist air rising rapidly. This vertical growth signifies that the atmosphere is unstable, allowing for significant energy releases in the form of thunderstorms and other severe weather.

In contrast, the other types of clouds listed, such as stratus and stratocumulus, generally form in stable air conditions and do not exhibit the same level of vertical development or severe weather potential. While cumulus clouds can indicate some instability, they do not possess the same characteristics associated with serious weather as cumulonimbus clouds. Thus, the presence of cumulonimbus clouds is a strong indicator of instability in the atmosphere.