Temperature & Humidity

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Main Question or Discussion Point

The humidity of air is influenced in part by temperature, in that the higher the temperature, the higher the capacity for water vapour.

What I'm trying to figure out is why. I can come up with a couple ideas, ranging from hydrostatics to increased energy keeping liquid bonds from forming; but these are only theories, and I cannot find any actual source validating or invalidating my reasoning.

Any help is appreciated.
 

Answers and Replies

Mapes
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Your hypothesis about increased energy keeping liquid bonds from forming is probably the closest description of what happens. Imagine liquid water at two different temperatures. The partial pressure of water vapor will be higher above the sample with the higher temperature, because it is more probable that bonds will break and allow the molecule to enter the vapor phase. This translates into a higher possible humidity.
 
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The different between liquid water and water vapor is hydrogen bonding, with the water vapor having very limited H-bonds. The higher temperature allows more hydrogen bonds to break. This is true for both both the liquid and vapor. The liquid will increase vapor pressure at higher temperature as more hydrogen bonds break. The warm air will then hold more water vapor because the higher temperature prevents the hydrogen bonds from reforming.

An interesting affect are clouds. What we have is essentially a zone of water vapor that can get higher in concentration than the surrounding air without clouds. This too is due to hydrogen bonding. It causes the water vapor to concentrate as a cloud. If the cloud's water vapor was to go all the way to the liquid state, it will lower the pressure of the air near the cloud, since the water vapor is lost, lowering the impact of its partial pressure within the local air.

This next part may be unconventional, but goes along with this logic. In the lab, if we wish to condense water vapor, faster, at any given temperature, this can be done by adding pressure. But in weather, the water in rain clouds primarily condenses within low pressure systems. One way to explain this is the condensation of the clouds, due to hydrogen bonding, is at least partially helping to lower the pressure. If not, one would expect high pressure systems to be the most efficient place for rain. The latter correspond to the lab observation that more water will condense at higher than lower pressure for any given temperature.
 
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Another factor to consider is the high polarity of water molecules.
Air consists of principally nonpolar N2, O2, CO2, and noble gases.
Water vapor (polar) "dissolved" in air (nonpolar) is not very miscible.

At some given temperature, air will "saturate" with the polar water vapor
molecules, since they will be in sufficient abundance to attract to each
other and condense; but it may be more accurate to state that the nonpolar
air molecules electrostatically "reject" the polar H2O vapor and thus help
consolidate it into larger and larger clumps, as fog or clouds. This may
be a function of temperature/pressure changes at certain altitudes.

Thus air "rejecting" excess water vapor in gas phase may be the same
mechanism by which oil "rejects" excess water in liquid phase.
 

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