What is one of the key characteristics of a stable air mass when forced upward?

The key characteristic of a stable air mass when forced upward is that it retains horizontal flow, resulting in flat and stratified cloudiness. Stability in an air mass means that it resists vertical motion. When such an air mass is lifted, it does not ascend freely; instead, it tends to spread out horizontally. This creates a layer of clouds that is more broad and less towering, which is typical of stratiform clouds.

In contrast, when an unstable air mass is lifted, it allows for vertical development and can lead to the formation of towering clouds and potential thunderstorms, which is not a characteristic of stable air. Additionally, stable air typically does not produce significant turbulence nor does it encourage rapid changes in weather conditions. Thus, the flat, layered cloud formation associated with stable air embodies the nature of stability when forced upward.