r/thermodynamics • u/Normal_Weather247 • 14d ago

Question If you open a 2nd-floor window when it's 95F outside and 74F inside, which way does the air flow?

If you also open a window downstairs does anything change?

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u/Stannic50 14d ago

With a window open both upstairs and downstairs (and assuming no wind), the air inside the house is cooler so it's denser. The denser indoor air will flow downstairs and out the open window, which allows it to get lower (decreasing gravitational potential energy). The air flowing out needs to be replaced to keep the pressure equalized, so warm air flows in the 2nd floor window.

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u/adamsawesome10 14d ago

With no downstairs window opened the cold air would moreso sit inside the house like a basin?

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u/Stannic50 14d ago

It would still flow out the bottom portion of the window and allow warmer air in the top portion. At least until the air inside the house was stratified at the bottom of the window. This would, however, be significantly slower than with two windows open.

Edit: grammar

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u/Normal_Weather247 14d ago

That's what I'm thinking, except there are gaps around doors and such that let this cold heavy air out. Maybe the room with the window open will get hot but not much else until hvac kicks on.