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The amount of water(g) that air can accommodate depends on temperature. The absolute humidity then is defined as the percentage of water to air and decreases when the temperature decreases. The relative humidity is the ratio between the current and the maximum humidity, which increases with decreasing temperature. The temperature at which the relative humidity reaches 100% is called the dew point.

Anyway, why can air keep more water when it is warmer?

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For a substance to be a gas, the kinetic energy of the molecules that make it up must be higher than the electrostatic potential energies that hold it together. When we talk about air "holding" water we are really talking about the amount of water that would be in the vapor phase relative to the amount that would be liquid at equilibrium.

Since temperature is a measure of the average kinetic energy of molecules, as the temperature increases, the fraction of water that can exist as vapor increases. Since air is already a gas under any conditions where water vapor can exist, it looks like the air "holds" more water as temperature increases.

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    $\begingroup$ Ah, so it actually has nothing to do with any air-property. $\endgroup$ – TMOTTM Jun 12 '14 at 14:13
  • $\begingroup$ Yes, the vapor-to-liquid ratio for water at equilibrium would increase with temperature for air or any other gas (even vacuum). The exact value would depend on the total pressure of the gas phase, and that would be a function of the composition of the gas. However, the general trend would be the same. $\endgroup$ – thomij Jun 13 '14 at 0:52
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The liquid-vapor coexistence line of the phase diagram of water has a positive slope $\mathrm{d}P/\mathrm{d}T$. Humidity in air is a partial pressure of water vapor. If partial pressure of water is increased starting from $<100\%$ humidity, it will reach the vapor-liquid transition and any additional water vapor will simply condense into droplets ($=100\%$ humidity).

The first-order phase transition of condensation will force any water molecules added to a saturated system to condense into liquid, and so there is a high limit to the humidity at a given temperature.

An increase in temperature corresponds to an increase in the saturation vapor pressure, thus at warmer temperatures there will be a higher partial pressure of water vapor in the air at a given $\%$ humidity.

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