I first wanted to ask a very specific question: There is something called the Brunt-Vaisala frequency. It describes the frequency of oscillation in a fluid with a density gradient. Because if a parcel of fluid is pushed up or down from its stable state it will oscillate around it. What i don't get is why this can be applied to the oscillation of air behind mountains. One of the fundimental axioms you derive this from is that your fluid parcel has a constant density. And with air does not make any sense. A bit of air would simply expand (violating constant density) and slightly cool when going up and then pretty much stay where it is. Vice versa when going down. Wouldn't it ? But this got me thinking ... I actually don't really understand gravity driven waves.(Maybe waves in general ) (We didn't have them in school yet.) (Although we did have basic mechanical and electric oscillations.) Even the miraculously simple equations for wave speed in water dont really make sense to me. If somebody had nice derivations for mathematical desciptions of these things i would really appretiate it: Water/Air Interface (basically ocean) waves Atmospheric waves (stratified) Waves on a String or spring .... Posting some links with reasonably "easy" derivations would really help me out. I think this and many other questions about waves would solve themselves if i knew a little more about them in general. Sorry i know this question is a little dumb and the only person who finds it interesting or would be helped is probably me, but i am going to ask it anyways and hope for some replies.