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 remains constant.
The total 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, a room of a given temperature of 14°C with the air containing 10 grams of water per kg will have a relative humidity of 100%. With these values it will be totally saturated. However, if this temperature was to be increased to 18°C, the relative humidity would fall to 77% which means that to reach that same total saturation point at this higher temperature a further 3 grams of water per kilogram of air must be added.
If on the other hand the air containing 13 grams of water per kilogram at 18°C was to be reduced to a temperature of 14°C, the air would then contain 3 grams of moisture in excess of what it could physically hold and this excess would be deposited. This deposit is what we know as condensation.
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