Gases in the Air

[Vix134]

I was just wondering...if each gas has certain densities, why don't we have layers of gas in the air?

[molly]

Probably because the air is never still. There are always high and low pressures, wind, air currents. Not to mention the whole carbon dioxide/ oxygen/ plant/ animal thing.

[incitatus]

even without convection (all those weather type things) the gases would not settle out. This is because the molecules of a gas have no real attraction to each other, and so no fixed relationship to each other. thus the motion induced by rising temperature translates into a free , random motion rather than a vibration about a fixed mean point. this means that the only damper on free motion is the gravitational force on each individual atom. this is so tiny in comparison to the kinetic energy of the atom that it has almost no effect. There is a slight effect over very large distances: the air at the top of a mountain has a very slight difference in the balance between the gases compared to the air at ground level, but to really separate layers out like this you need to observe tens of kilometres of gas at the least.

[blackfive]

While everthing you say is true incitatus, there are circumstances where gases can separate into layers eg in a blind heading in an underground coal mine you can have a distinct layer of carbon dioxide at the floor (you can feel it as you walk through it!) and a distinct layer of methane at the roof.

[blackfive]

Ps. A "blind heading" is a dead-end roadway.

[incitatus]

yeeeesss but you are talking about a specialised set of circumstances. i.e. a totally enclosed isothermal area with nonturbulent gas sources. even so if you closed the mine up and left it for 30 years the likelihood is high that simple diffusion would mix the gases for you.