Up to now, we've been evalutating the stability of the atmosphere and the subsequent behavior of perturbed parcels by starting with an air parcel at the ssame temperature as the environment. In real life, it is possible for an air parcel to start off not in equilibrium; i.e., it will initially be positively or negatively bouyant. This parcel is NOT "unstable" if it is positively buoyant, nor is it called "stable" if it is negatively buoyant, since it will sink if negatively buoyant, and something that is "stable" would tend to stay put. A classic example of this is an air parcel forming over blacktop (which gets very hot under the sun) surrounded by white sand, which does not get as hot as the blacktop. The air parcel over the blacktop is positively buoyant compared to the rest of the environment, not because the environment is unstable, and not because the parcel is "unstable", but because the parcel is positively buoyant.

In this example illustrated here, the atmosphere is stable, based on the slopes of the environmental temperature profile (the red line) and the adiabats (the red and green lines). Suppose the parcel at the ground was heated up relative to the surrounding air, as illustrated in the diagram. Now that it is positively buoyant, it will rise freely (as in the convection mechanism of cloud formation). Because of the difference between the environmental and adiabatic lapse rates, the parcel temperature and environmental temperature will converge while the parcel rises. Eventually, the parcel stops rising when it reaches the level of neutral buoyancy, when the two temperatures coincide (the lines cross). This is how a stable atmosphere suppresses the vertical motion of positively buoyant air parcels.

In the example illustrated here, the parcel happens to reach its lifting condensation level before it becomes neutrally buoyant. We switch lapse rates because the parcel is now saturated; the parcel keeps rising since it is still positively buoyant. Because the atmosphere in this example is absolutely stable, the parcel still stops rising after a while--ending at a higher altitude than if it stayed unsaturated (follow the dashed red temperature trajectory).

Suppose we heat atmosphere at the ground to higher temperatures, such as in the upper plot of this diagram. Re-plotting the environmental temperature profile in the region where the air was superheated (by the ground, usually) results in a line that is sloped very much to the left, signifying an unstable region of atmosphere. Heating the atmosphere at the ground makes the atmosphere less stable.

This means that frequently, the atmosphere becomes less stable during the daytime, particularly in the afternoon, when the ground temperature is highest. In the Midwest, summers are often characterized by the formation of tall thunderstorms due to convection that occurs when the atmosphere becomes unstable.

In the lower diagram, the air at the ground is cooled (as it might be at night, due to radiative cooling of the ground). This results in a shallow temperature inversion (if at night, a nocturnal radiation inversion), which is absolutely stable. Cooling the atmosphere from below (or at the ground) results in a more stable atmosphere.