# Does it make sense to open one window all the way when the other window is much smaller?

I can't wrap my head around this idea because I don't know much about air flows.

Say we have this imaginary apartment with two windows, one of which is two times smaller than the other:

Will the first one let more air through than the second? Or is it the same because of the smallest window?

How does this work?

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Is air (the breeze) blowing in one and out the other (series), or are they on the same wall (with a large third window involved), in parallel? – Phil Perry Apr 17 '14 at 18:51
This example shows a top-down view. The actual situation is too complex to describe, but I just want to get to know the basics here. – user1306322 Apr 17 '14 at 18:55
OK. By the way, before someone offers the tired old advice about opening up all the windows (or the downwind ones) if a tornado approaches -- DON'T. Use the time to take shelter. Unless you have heavy steel shutters, the windows will be shattered anyway by a direct hit (along with the walls and roof...). Even a near miss will still have lots of water damage from rain blowing in, and there's the issue of wind blowing in overpressurizing the house. Don't bother. – Phil Perry Apr 17 '14 at 19:03

Each window represents a restriction to the air flow. The greater the pressure difference across the aperture, the greater the flow.

An electrical analogy: each window is a resistor. The current through the resistor is proportional to the voltage across it - but when you have two resistors in series they must carry the same current (air that enters through one window must exit through the other). If the total voltage across the two resistors is constant (the "force of the wind"), then if one resistor is smaller (the "big window") it leaves more of the voltage available across the second resistor (the "small window") - thus the over all air flow will be greater. See this diagram:

So yes - it makes sense to open the larger window fully. The pressure drop across it will be smaller, the pressure drop across the small window will be larger, and there will be more air flow through the apartment.

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+1 for graphic. – Antonio Ragagnin Apr 17 '14 at 14:26

If you are familiar with electric circuits in a quite loose analogy you can look at your windows as impedances for the air current. Since current is turbulent (take a look at the Reynolds number if you do not know it) the air does not directly go all the way down through all the windows, but each of them will create some impedance to the flow. The bigger the window the less the impedance so yes, fully open also the second one! You will also get more diffusion which is not included in the previous discussion.

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What do you mean - "you get more diffusion"? – Floris Apr 17 '14 at 13:59
@Floris even if there is no an average air flow, air molecules can go from inside to outside and viceversa just because of their thermal motion. If you increase the opening surface, you increase this effect (diffusion). – DarioP Apr 17 '14 at 14:17
I see what you mean. Yes you are right. Even when there is no wind, leaving the window open will help keeping the air inside "fresh". But I think that is outside of the scope of the question. – Floris Apr 17 '14 at 14:19
@Floris indeed it is just a small comment at the end ;) – DarioP Apr 17 '14 at 14:22