What characterizes an unstable atmosphere?

An unstable atmosphere is characterized by an environmental lapse rate that is steeper than both the wet and dry adiabatic lapse rates. This condition leads to an increase in temperature with altitude at a rate greater than that at which a rising air parcel cools as it ascends, resulting in a situation where the air parcel is warmer (and therefore less dense) than its surrounding environment. As a result, any parcel of air that is lifted can continue to rise freely, potentially leading to the development of turbulence and convection currents, which are fundamental to the formation of clouds and storms.

In contrast, a shallow environmental lapse rate suggests stability, as the surrounding air cools more slowly with altitude, making it more difficult for rising parcels to continue ascending. A negative lapse rate indicates that temperature increases with height, which is highly uncommon and suggests a thermal inversion, leading to stable conditions. A constant lapse rate would mean that the atmosphere does not exhibit the necessary variations in temperature to create the vertical motions required for instability. Thus, only an environmental lapse rate steeper than the adiabatic rates fulfills the criteria for an unstable atmosphere.