Why does air rush in from colder to hotter space and not vice versa?

jamesb100

When you open a window of a warm room, the air from the colder outside rushes in (you can see the curtains bend into the room, for example).

But shouldn't it be the opposite? The higher kinetic energy of the hotter air molecules inside the room should create more pressure than the colder air molecules outside, and so when a window is open the air should move from higher pressure to lower pressure, i.e. from inside to outside.

And if the explanation is that the hot air does indeed move to the outside, and the cold air rushes in to fill the created vacuum, why do we feel only the cold air coming in and why is the net force pushing in the inside the room direction?

davenn

hi there james
welcome to PF

how is the room being cooled ? ducted A/Con?
if so then it will be pumping air into the rooms and producing a higher pressure inside the building/room than the pressure outside.
the temperature difference is pretty much irrelevent

Dave

DrClaude

Isn't the wind the dominant factor here?

CWatters

See my diagram. I believe this is an illusion due to the fact that the curtain is fixed at the top.

When the window is opened the cold air outside will descend between the curtain and wall/window pushing the curtain into the room slightly. The hot air inside at ceiling level would flow over the top of the curtain and out of the window.

The top of the curtain is fixed, where as the bottom is free to move.

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CWatters

PS: If the room is otherwise sealed the hot and cold flow rates must be the same or the pressure in the room would change.

davenn

Duh...

I just realised I totally misread the question
Wish the edit feature lasted a little longer :(

Dave

xlsdx

i think the only reason the cold air comes in when you open a window is because of the pressure from the wind. if there was absolutely no wind then gravity would take over and the warm air in the room would escape at the top of the window (due to convection) and the cold outside air would be sucked in at the bottom of the window, until the temperatures equalized..

boneh3ad

The cold air would come in even without wind. It more dense than the warm air so there would be a vertical stratification between the two densities, which would be unstable. The slightest perturbation (for example, from opening the window) would cause the situation to break down and the cold air would flow into the room from the bottom portion of the window while warmer air would then get pushed out the top assuming only one window was open, much like CWatters drew. The presence of wind or any sort of over- or under-pressure in the house would complicate matters a bit, but the overall idea is the same.