The formation of clouds by simple convection requires air parcels to rise due to their positive buoyancy. This requires the rising air parcel to be warmer than the surrounding environmental air. The air parcel can be in this state either by

• being diabatically heated (for example, by conduction from being in contact with the sunlight-heated ground);
• not losing temperature as quickly as the environment as altitude increases and the air parcel expands adiabatically.

The second case will be examined here, because it also has an effect on how parcels rise in the first case. What we need to look at is the difference between the lapse rate for air parcels (adiabatic lapse rate, either dry or moist) and the lapse rate for the environment. The relationship between the two lapse rates (less than, greater than, or equal) will characterize the stability of the atmosphere (stable, unstable, neutral), while the difference between the air parcel temperature and the temperature of the surrounding air (less than, greater than, or equal) will characterize the buoyancy of the air parcel (negative, positive, or neutral buoyancy). The atmosphere's stability will then control the motion of air parcels by affecting the air parcel buoyancy, and therefore the shapes of clouds formed if condensation of cloud droplets occurs.

In the stable atmosphere case, an air parcel loses temperature faster than the environment as the air parcel rises. This means the air parcel, if it started out warmer than the environment, will quickly become the same temperature or cooler than the environment as the parcel and environmental temperatures converge.

If the air parcel started at the same temperature as the environment (so it is in a state of equilibrium, as shown in the diagram), nudging it upward would cause its temperature to become lower than the surrounding air temperature, since the air parcel loses temperature faster than the surrounding air in this particular situation. The air parcel sinks back down to its original position, since air parcels that are cooler than the environment will sink. This resembles stable equilibrium.

When the atmosphere is stable, vertical motion of air parcels is suppressed (buoyant air parcels will rise for a while then stop when its temperature equals the environmental temperature at the neutral buoyancy altitude).

In the unstable atmosphere case, we have the opposite relationship between the adiabatic lapse rate and the environmental lapse rate as we had with the stable atmosphere. Now, adiabatically expanding and rising air parcels lose temperature more slowly than the surrounding air, so the air parcels will tend to be warmer than the environment when they rise (or cooler than the environment when they sink). Thus, the initial motion of air parcels will be enhanced: rising air parcels will continue to rise or sinking air parcels will continue to sink. This resembles the case of unstable equilibrium, where nudging an unstable system causes it to accelerate away from the original equilibrium configuration.

In a neutral atmosphere, the environmental and adiabatic lapse rates are equal, so air parcels and the environment change temperature at the same rates and their temperatures will always be the same (if their initial temperatures were equal). The air parcel will always be in neutral buoyancy, and will neither rise by itself nor sink back down if nudged upward. This resembles a case of neutral equilibrium, where perturbing the equilibrium system causes a new equilibrium configuration to be established.

If the air parcel started out as positively buoyant (initial parcel temperature greater than the environmental temperature), it will remain buoyant but the difference in temperature between it and the environment will not change, so the upward buoyancy force will be constant (in the unstable atmosphere, the upward buoyance force will increase as the parcel rises).