Under what condition is unsaturated air considered unstable?

Unsaturated air is considered unstable when the environmental lapse rate exceeds the dry adiabatic lapse rate. This condition indicates that as air rises, it cools at a rate slower than the surrounding environment, which promotes further rising and leads to instability. Under these circumstances, any upward movement of the air will cause it to continue rising since it remains warmer and lighter than the surrounding air.

In contrast, if the environmental lapse rate were less than either the dry or wet adiabatic lapse rate, the air would be stable, meaning that it would resist vertical motion. If the environmental lapse rate equals the saturated adiabatic lapse rate, the air would be in neutral equilibrium, not unstable, hence not promoting further vertical motion. This understanding is essential for interpreting atmospheric stability and predicting weather phenomena related to unsaturated air.