Answer:
Relative humidity depends on the amount moisture actually in the air at some temperature to the amount of moisture that the air can hold.
For instance, at 25 deg C the air has a saturation pressure of 23.76 mm of Hg.
That means that if at that temperature the pressure of the water vapor in the air reaches 23.76 mm Hg then the air is 100% saturated.
I believe the standard air pressure at 25 deg C is 760 mm Hg.
Now if the pressure of the water vapor in the air is 1/2 * 23.76 = 11.88 mm then the air is 50% saturated and the relative hunidity is 50 %.
At 100% relative humidity rain occurs (the air can't hold any more water).
