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I seem to remember from high school, the state of equilibrium. Volumes of more molecules wanting to get to a volume of less molecules and causing a state of equilibrium.

From this I believe cold air is pushed in not sucked in. The force is behind the more molecules moving into a space of less molecules.

Most of my friends say the cold air is sucked in. Who is right??

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  • $\begingroup$ As Rod Vance says, it's the same thing. $\endgroup$ May 29, 2016 at 0:16

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I was going to make this a comment, but I thought of a phrase that I think intelligent non-physicists should grasp, so it might be worth keeping in your "teaching toolkit".

At one level it's wholly a matter of taste and English usage that tells which phrase is correct, but the phrase "rushes in" is probably more evocative of the true physics than "sucked in".

And this is for exactly the reason you state in your middle sentence: molecules drift stochastically and any density nonuniformities tend to be filled in by the "passive" drift. There's no force from the departed friends saying "hey guys, we're going up now so we're going to drag you along". Of course, if somehow there does suddenly arise a significant density nonuniformity owing to energy flows (supplied by the Sun), that inrush can seem violent, as in something like a tornado, so it can seem as though there is some unseen fiend gathering up all the air, which is where the English usage of "suck" (when said of wind) comes from. Indeed, the collective momentum of all the air molecules in a room, if they could somehow suddenly be turned in one direction, is of the same order of the momentum of a largeish truck travelling at highway speeds - work out an estimate to see what I mean!

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