HABYTIME MINI LECTURE 34:
(IR)REVERSIBLE ADIABATIC PROCESSES
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METEOROLOGIST JEFF HABY
A reversible adiabatic process is a process that can go back in reverse to get back to the same initial starting position of thermodynamic
values. An example of a reversible adiabatic process is a rising and sinking of air which occurs at only the dry adiabatic lapse rate
and the parcel is assumed to be adiabatic, meaning no heat, mass and momentum are exchanged between the parcel of air and the environment.
An example of a reversible adiabatic process is shown below. The parcel has an initial condition of a temperature of 20 C and a dewpoint
of 10 C. The parcel rises adiabatically with temperature decreasing at the dry adiabatic lapse rate and dewpoint decreasing at the dewpoint
lapse rate. At a certain height, the temperature of the parcel is now 10 C with a dewpoint of 8 C. The parcel has stayed unsaturated while
it was rising. Next, the parcel sinks back down to the original elevation. As to parcel sinks it warms at the dry adiabatic lapse
rate and the dewpoint increases at the dewpoint lapse rate. The parcel has the exact same temperature and dewpoint at the end of its
journey back to its initial elevation. Next, we will examine an irreversible adiabatic process.
An irreversible adiabatic process is a process in which the parcel CAN NOT return to the same initial conditions after undergoing adiabatic
transformation. An example of this (shown below) is a parcel of air that undergoes both dry and wet adiabatic transformation before
sinking in elevation. In the example below, the parcel starts with a temperature of 20 C and a dewpoint of 12 C. The parcel rises high
enough that it becomes saturated and then it continues to rise when it is saturated at the wet adiabatic lapse rate. At the top of the
mountain, the temperature and dewpoint are both 4 C. The parcel is next shown on the diagram as sinking down on the other side of the
mountain. After sinking to the other side of the mountain the temperature is 24 C with a dewpoint of 8. Notice the dewpoint at the
low elevation has dropped from 12 C to 8 C after going all the way over the mountain. This loss of moisture is the precipitation
that occurred as the parcel rose at the wet adiabatic lapse rate. This process is irreversible since the loss of the moisture can
not be replaced except by a non-adiabatic process. The parcel of air also warmed on the other side of the mountain from 20 to 24 C
at the lower elevation. The windward side of the mountain tends to be cooler and wetter while the leeward side of the mountain tends
to be warmer and drier.
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