An unstable air mass is characterized by which of the following?

An unstable air mass is typically associated with turbulence and cumuliform clouds. When an air mass is unstable, it means that it is prone to vertical movement. This vertical movement occurs because warmer air rises and cooler air sinks, leading to the development of clouds that can grow vertically. Cumuliform clouds, such as cumulus and cumulonimbus, are indicative of this instability, as they often form in conditions where warm, moist air is lifted, resulting in the potential for turbulence and even severe weather phenomena like thunderstorms.

In contrast, the other options describe characteristics typically associated with stable air masses. For instance, clear skies and high humidity indicate a lack of significant vertical air movement, which is the opposite of instability. Similarly, stable winds and low clouds suggest a more uniform atmospheric condition, while calm weather and nimbostratus clouds indicate a stable environment that supports precipitation over a broad area without the strong vertical development seen in unstable conditions.