I recently went through the wikipedia page for Rain. It said that
There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling,….
It proceeds as
Adiabatic cooling occurs when air rises and expands.
Why is this a adiabatic process?
Best Answer
"Adiabatic" when it is not obvious (that is, when the process is not inside an insulator) means "so fast that it doesn't have time to exchange energy with the exterior". This is roughly what happens in the atmosphere, the airmass expansion happens in a time scale shorter than the mixture equilibrium.
As a side note, we use this approximation because it simplifies a lot the equations. For more realistic computations, we can use modified values of the adiabatic constant to take into account that the process is not purely adiabatic.