How temperature affects relative humidity

[anti_matter]

I have a fan in crawlspace that I can turn on to draw in fresh air. This could reduce the relative humidity by raising the air temperature, and lowering the moisture density. However, air temperature doesn't go up very much because the dirt and foundation wall tend to keep the air at around 55 degree (F).

Given the outside air temperature and RH, and the crawlspace air temperature and RH, how can I determine whether turning on the fan would reduce RH?

I could not find a formula to calculate RH at a second temperature given RH at one temperature.

I did find some formulas about vapor density and dew points. I just am not sure what remains constant when air travels to a cooler area. Does the water vapor density remains the same, or does dew point remain the same?

 

[Svein]

When the air cools, the air cannot hold as much water vapor and the relative humidity increases. If the relative humidity passes 100%, some of the vapor condenses (i. e. turns to water). See http://en.wikipedia.org/wiki/Vapour_pressure_of_water.

 

[Chestermiller]

The relative humidity is defined as the partial pressure of the water vapor in the air divided by the equilibrium vapor pressure of water vapor at the air temperature (times 100 %). The partial pressure of the water vapor is equal to the total air pressure times the mole fraction of water vapor (which doesn't change in the air stream). Using these relationships, you can calculate it for yourself. T

Chet