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

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Discussion Overview

The discussion centers around the behavior of air movement when a window is opened in a warm room, specifically why colder air from outside rushes in rather than warmer air moving out. Participants explore concepts related to pressure differences, temperature effects, and the role of wind and convection in this phenomenon.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions why colder air enters a warm room instead of warmer air escaping, suggesting that higher kinetic energy in warmer air should create higher pressure inside.
  • Another participant points out that if the room is cooled by air conditioning, it may create higher pressure inside, making temperature differences less relevant.
  • A different participant introduces the idea that wind may be a significant factor influencing air movement when a window is opened.
  • One participant presents a diagram to illustrate that the movement of the curtain is an illusion caused by the fixed position of the curtain's top, suggesting that cold air descends while warm air escapes from the top of the window.
  • Another participant notes that if the room is sealed, the flow rates of hot and cold air must be equal to prevent pressure changes.
  • A participant reflects on their misunderstanding of the question, indicating a moment of confusion in the discussion.
  • One participant argues that without wind, gravity would cause warm air to escape from the top of the window, allowing cold air to enter from the bottom due to convection.
  • Another participant counters that cold air would still enter without wind, citing its higher density and the resulting instability that would lead to cold air flowing in and warm air flowing out.

Areas of Agreement / Disagreement

Participants express differing views on the primary factors influencing air movement when a window is opened, including pressure differences, wind effects, and convection. No consensus is reached regarding the dominant explanation.

Contextual Notes

Some assumptions about the room's sealing and the presence of wind are not fully explored, and the discussion includes various interpretations of pressure dynamics and air density effects.

jamesb100
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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 into 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?
 
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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
 


jamesb100 said:
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).
Isn't the wind the dominant factor here?
 


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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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.
 


Duh...

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

Dave
 


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..
 


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.
 

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