I am modelling the atmosphere as a perfect, static gas subject to uniform gravity, assuming ideal gas equation, the density is found to follow: p=A*exp(-z/H) where A is a const, z is the heigh, and L is the scale height. I want to know when this approximation breaks down! at what density? i am thinking that as the density goes down the approximation for he idea gas law breaks, but not quite sure if there is a specific value at which it breaks! maybe a certain number of molecules?? thanks a million!