Which type of cloud typically signifies unstable atmospheric conditions?

Cumulus clouds are often indicators of unstable atmospheric conditions. These clouds typically form as a result of rising warm, moist air which cools and condenses into visible cloud formations. The presence of cumulus clouds suggests that there is significant vertical movement in the atmosphere, which is a hallmark of instability. As these clouds grow in size, particularly when they develop into towering cumulonimbus clouds, they can lead to thunderstorms and other severe weather due to the vigorous updrafts associated with unstable air masses.

In contrast, other cloud types such as stratus, nimbostratus, and cirrus do not signify the same level of instability. Stratus clouds are generally associated with stable air and can produce light rain or drizzle but indicate a more uniform layer of moisture in the atmosphere. Nimbostratus clouds also imply steady, organized precipitation instead of turbulent conditions. Cirrus clouds, while they can signal changes in the weather, are high-altitude clouds that typically form in stable conditions and do not indicate atmospheric instability.

Thus, the nature of cumulus clouds, along with their association with upward momentum in the atmosphere, makes them the type that confidently signifies unstable atmospheric conditions.