When is an air mass considered stable?

An air mass is considered stable when the vertical motion of rising air tends to become weaker and disappears. In a stable air mass, vertical movements are suppressed, which means that any air that rises will likely cool and then sink back down because it is denser than the surrounding air. This lack of significant upward motion tends to lead to clearer skies and less turbulent weather, as stability prevents the development of large convection currents that could lead to cloud formation and precipitation.

When air is stable, changes in temperature with height do not favor rising air, and this is reflected in the conditions described in the other choices, which do not characterize stability effectively. For instance, a rapidly decreasing temperature with height might indicate instability, generating more convection; constant pressure does not provide adequate data for determining stability; and a high environmental lapse rate with little vertical motion of air currents suggests potential instability rather than stability. Hence, the characteristics of stable air best align with the option given.