Adiabatic Process

When discussing atmospheric stability we typically think about air parcels that can expand and contract freely but do not mix with the air around them. Furthermore, we can assume temperature changes within the air parcels occur adiabatically. Adiabatic processes are those in which there is no net heat transfer between a system and its surrounding environment.

For example, increasing pressure adiabatically heats air masses, falling pressures allow air to expand and cool. Adiabatic heating and cooling is common in convective atmospheric currents. In adiabatic heating and cooling, there is no net transfer of mass or thermal exchange between the system the external or surrounding environment, the change in temperature of the air mass is due to internal changes.

Imagine you have a parcel of air at the Earth’s surface. The air parcel has the same temperature and pressure as the surrounding air, which we will call the environment. If you were to lift the air parcel, it would find itself in a place where the surrounding environmental air pressure is lower, because the pressure of the surrounding environment decreases with height. Since the environmental air pressure outside the parcel is lower than the pressure inside the parcel, the air molecules inside the parcel will effectively push outward on the walls of the parcel and expand adiabatically. The air molecules inside the parcel must use some of their own energy in order to expand the air parcel’s walls, so the temperature inside the parcel decreases as the internal energy decreases. To summarize, rising air parcels expand and cool adiabatically without exchanging heat with the environment.

Now imagine that you move the same air parcel back to Earth’s surface. The air parcel is moving into an environment with higher air pressure. The higher environmental pressure will push inward on the parcel walls, causing them to compress, and raise the inside temperature. The process is adiabatic, so again, no heat is exchanged with the environment. However, temperature changes in the air parcel can still occur, but it is not due to mixing, it is due to changes in the internal energy of the air parcel.