# Relative humidity (RH)

The amount of water in the air relative to the maximum amount of water that the air can hold at a given temperature. The relative humidity (RH) is the ratio of the actual water vapour pressure to the saturation water vapour pressure at the prevailing temperature. For example – if a cubic metre can hold 100ml of water at 20 degrees centigrade (273 K) and it does contain 100ml then it is said to be 100% RH. If the same cubic metre of air at the same temperature only contains 50mls of water then it is described as 50% RH.

RH = p/ps

RH is usually expressed as a percentage rather than as a fraction. The RH is a ratio. It does not define the water content of the air unless the temperature is given. The reason RH is so much used in conservation is that most organic materials have an equilibrium water content that is mainly determined by the RH and is only slightly influenced by temperature.

Notice that air is not involved in the definition of RH. Airless space can have a RH. Air is the transporter of water vapour in the atmosphere and in air conditioning systems, so the phrase ‘RH of the air’ is commonly used, and only occasionally misleading. The independence of RH from atmospheric pressure is not important on the ground, but it does have some relevance to calculations concerning air transport of works of art and conservation by freeze drying.

##### Calculator for atmospheric moisture

Air temperature°C Wet temperature°C RH% Dew point°C Lock DP

Vapour pressure Pa.    Saturation vapour pressure (svp)Pa. vapour concentration Kg/m 3. Kg water vapour/kg dry air kg/kg of dry air.

##### Instructions:

Enter your air temperature (in°C) plus the wet bulb temp, Dew point temperature or RH% All other values, except the wet bulb temp, will be recalculated when you mouse ‘click’ or ‘tab’ away from the entry.

##### Option:

Click on the ‘Lock DP’ box to enter a new temperature at the same dew point. This allows calculation of the RH that the air will acquire at a different temperature.

Note: This calculator makes approximations that limit its use to the natural climate near sea level.

Description of the calculations