Adiabatic Processes in the Atmosphere


The traditional definition of an adiabatic process is one in which heat is neither added to nor removed from the system. In the atmosphere, the system is an air parcel. This definition is patently incorrect because heat is not a quantity, but rather a process of energy transfer. The correct definition of an adiabatic process is one in which the system does not exchange energy with its surroundings by virtue of the temperature difference between them. In this regard, the term surroundings refers to the immediate environment of the air parcel (which of course has no defined boundaries but rather is a concept). In contrast, a diabatic process is one in which energy is added to or removed from the system (e.g., by radiation, latent heating due to phase change, turbulent mixing). I prefer to think of adiabatic processes as non-diabatic processes.

A reversible process is one in which the system can be returned exactly to its original state after having passed through various other states. Otherwise, the process is said to be irreversible. While real atmospheric processes are neither perfectly adiabatic nor reversible, these two concepts are important in developing a fundamental understanding of thermodynamics.

Dry Adiabatic Process - more accurately called an unsaturated adiabatic process, is one in which the vertical displacement of an air parcel lead to no changes of phase. Note that a dry adiabatic process also describes moist air parcel motions, so long as the air remains unsaturated. The vertical displacement of a dry or unsaturated parcel of air results in a parcel temperature change of approximately 10 deg/km. This is called the dry adiabatic lapse rate. Dry adiabatic processes are completely reversible.

Moist Adiabatic Process - more accurately called a saturated adiabatic process, is one in
which phase changes
in water substance, and the subsequent production of precipitation (e.g., snow, rain, hail, etc), occur as a parcel of air moves vertically. Two types of saturated adiabatic processes exist:

Moist (often called Saturated) Process - in this case, the condensation products (e.g., rain) are assumed (ideally) to never fall out of the air parcel. As a result, such processes are completely reversible (see example below).

Pseudoadiabatic Process - in this case, all condensation products are assumed to fall out of the parcel immediately after they form. Therefore, this process is strictly irreversible. The prefix pseudo is used here because the falling precipitation carries energy away from the parcel, which violates the definition of a strictly adiabatic process.

The moist adiabatic lapse rate is approximately 6 deg/km, and varies only slightly between the moist and pseudoadiabatic processes.

Example: If a parcel of air is lifted to saturation, and then lifted further, latent heating will occur as precipitation forms (the parcel will cool at the rate of 6 C/km). If we assume that the precipitation stays inside the parcel of air, and then return the parcel to its original level, the precipitation will evaporate during the descent and the temperature of the parcel will increase at the rate of 6 C/km. Upon arrival at its original level, the parcel will therefore have the same temperature with which it began. This is a moist adiabatic process. If we now perform the same experiment but assume that all precipitation falls out of the parcel, the parcel will warm at the dry adiabatic rate upon descent to its original level because all liquid water available for evaporation will have exited the parcel. As a result, the parcel will warm at the dry adiabatic rate (10 C/km) and arrive at its original location with a temperature higher than with which it started. This is a pseudoadiabatic process.