Relative Humidity is the measure of how much moisture is contained in the air at a given temperature, and expressed as a ratio between the amount of moisture contained in the air and the maximum amount of moisture the air could hold if that temperature remained unchanged.
The amount of moisture that air is capable of holding will alter with the temperature, the warmer the air the more moisture it can retain. Therefore, the Relative Humidity changes not only by the amount of moisture, but also the temperature.
In practice, if moisture had been generated in a cold room such as the bathroom, and the air had reached the maximum amount of moisture that it could hold at this given temperature, the relative humidity would be 100 percent. By increasing the room temperature (without generating more moisture) the humidity would decrease (to 90% for example). This change in relative humidity would make it possible for 10% more moisture to be added, which would take the level back up to the 100%. If on the other-hand the temperature was to drop to its original temperature, the moisture added would then be in excess of the 100% and would be shed as condensation.
