Generally its easy to implement constant volumetrics, when amount of "water" in the air is aproximatelly the same evrywhere, for example exponential fog (you can google it out to find some formulas, they are quite easy). But if you want to do something more complex like clouds, where amount of "water" in the air isnt the same at any point, then you should do some sampling and aproximations.

This video might be really helpful with understanding these concepts: https://www.youtube.com/watch?v=y4KdxaMC69w&t