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Our house has a glass sliding door to the shower. The shower has the dimensions of about 2 feet wide, 5 feet long, and 6 feet high. Above the door (and shower head) there is about 1 foot of open space before the ceiling to let the heat escape.

Sometimes I forget to slide the door all the way to the end of the other side, leaving about a 1 inch space in between the door and the wall. Cold air comes through so much that I always known when there is a crack, the amount of cold air I feel does not seem proportional to the opening at all. I would think that the amount of heat being produced by the shower would be enough to where I don't feel the cold air, but that does not seem to be the case.

What I'm thinking is that it is not that the heat is leaving through there, heat is escaping through the top since heat rises, but rather the cold air is coming through the opening due to a difference in pressure. Is this the case? How come it does not seem proportional to the opening?

How come I can feel a fair amount of cold air coming from the small opening at the opposite end of the shower?

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Is the narrow opening at the end of the shower where the showerhead is? If so, then Anthony X's answer can be expanded slightly to include entrainment of air by the moving column of water. This should be a small effect. –  Eric Towers Jul 7 at 15:42
    
@EricTowers It can be at either end, both seem to have the same effect –  Zach Saucier Jul 7 at 15:43
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up vote 22 down vote accepted

The answer is a combination of physics and physiology.

The warm water in the shower very quickly heats up the air in the shower, and warms up your skin. It also drives up the humidity of the air in the shower. You acclimate very quickly to the temperature/humidity conditions in the shower as being "normal".

With the door left open a crack, you allow convection to occur... the warmer, more humid air in the shower escapes out the space at the top, replaced with cooler, drier air from the bathroom. Convection is driven by the difference in density between the warm humid air and the cooler drier air; it's not just the difference in temperature which alters density... a molecule of water is lighter than a molecule of air, so humidity lowers density for equal temperature and pressure.

The draft feels unusually cool because it is somewhat cooler than what you've acclimated to (stronger subjective experience), and because the drier air in the draft is able to draw much more heat from your skin through evaporation than the warmer, more humid air.

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tl;dr windchill –  ratchet freak Jul 7 at 8:21
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Just want to point out that there are no "molecules of air". Air is a mixture of gaseous substances. Thus, each compound of air has a different molecule. –  golem Jul 7 at 9:45
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