Why is adiabatic process isentropic?

Why is dry adiabatic cooling a perfectly adiabatic process?

• An adiabatic process is often referred to as perfectly reversible. That is, an adiabatic process involved in a change in temperatures from T1 to T2 can be reversed by an adiabatic change in temperature from T2 to T1 (i.e. dq = 0 throughout).Explain why: b. Dry adiabatic cooling is a perfectly adiabatic process d. Saturated adiabatic cooling without precipitation is a pseudo-­‐adiabatic process (that is, it’s effectively adiabatic). f. Saturated adiabatic cooling with precipitation is actually a diabatic process.

• Answer:

  Karlene, these questions sound like they belong in some freshman homework. For example if the adiabatic process is dry then by definition it is perfectly adiabatic which is to say the changes in temperature are constant and due only to pressure changes on the parcel. When there is moisture in the air the process becomes a pseudo adiabatic that is a "false" adiabatic because of possible changes in state which will add to or subtract from the effects of the adiabatic process. If liquid precips during adiabatic lifting of the parcel it will not be available to evaporate and take in heat from its surroundings when the parcel descends.