Relative humidity if is often talked about in flood restoration and dealing with wet building assessment and repair.
Air always contains some moisture in the form of water vapour, however most of the time this is not noticeable to our senses. The amount of vapour that can be held in air depends on temperature, with a greater holding capacity in warmer air.
When air is holding the maximum amount of vapour possible at a given temperature it is considered saturated. This may feel stuffy and noticeably humid to the senses. Dry air, which contains a small amount of moisture, will feel crisp and refreshing when cold but when warm will dry the throat and can lead to a hard cough.
Relative Humidity is the degree of saturation, as it expresses as a percentage the amount of water vapour that air is holding relative to its saturated level. Saturated air is therefore at 100% RH. Air at 50% RH is holding only half of the vapour it could do at that temperature.
If that same sample of air is cooled, the RH will increase because the maximum weight of water vapour that air can hold is reduced in the cooler air. Conversely, if the air is warmed, RH reduces as the air at is capable of holding higher levels of moisture.
When measuring water vapour, it is expressed in terms of weight of water in a given weight of dry air. Usually this is in grams of water per kilogram of dry air.