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Q & A: vapor pressure

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Most recent answer: 10/22/2007
Q:
If the vapor pressure outside is higher then the vapor pressure inside, does that mean that a force is pushing that pressure into my house? How does this work?
- Deb (age 16)
Dominican HS, New Orleans,LA
A:
If the TOTAL pressure outside were higher, then there would be a net force pushing in on the walls of your house. That's not easy to maintain, since air would leak in an make the pressures nearly equal.

It sounds as if you are asking about the partial pressure of water, whether it is different inside your house from what it is outside. The partial pressure of water is just the amount of pressure there would be if you took away all the molecules in the air except the water molecules, keeping the temperature the same. The "vapor pressure" of water is the partial pressure of water that liquid water is in equilibrium with at a particular temperature. Water molecules evaporate and recondense at the same rate when the liquid is in equilibrium with the water vapor, and the amount of water vapor in the air needed to make the recondensing rate the same as the evaporation rate depends on the temperature and on nothing else. So literally, your question is asking "if the temperature is higher outside than it is inside...". This is an interesting question, because if there is a lot of water nearby (lakes, oceans, soggy farmland after a rainstorm...) which is in equilibrium with the water vapor in the atmosphere, then the humidity outside is very high. Bringing humid, warm air in contact with colder surfaces will cause condensation ("dew"). Condensation is a special problem with air conditioned environments -- the water has to be disposed of, or it can seep around, causing mold and mildew to grow inside.

If there is no active cooling or dehumidification in your house, and the partial pressure of water vapor is higher outside than it is inside, then the water vapor will simply diffuse through the cracks in the house and the humidity (and temperature) inside the house will gradually settle into equilibrium with the air outside.

Mike W. and Tom J.

(published on 10/22/2007)

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