Which action would decrease the stability of an air mass?

Warming from below is a key factor that decreases the stability of an air mass. When the surface air is warmed, it becomes lighter and more buoyant compared to the surrounding cooler air. As this warm air rises, it can create vertical motion, leading to unstable atmospheric conditions. In contrast, if an air mass is cooled from below, it tends to become more stable, as the cooler surface air is denser and resists upward movement.

Increasing humidity can also contribute to instability, as moist air is less dense and can promote rising currents, but the primary action that directly causes destabilization is warming from below. Decreasing altitude does not inherently affect the stability of an air mass; it is more related to the raise or lowering of pressure systems and other atmospheric conditions.