On relative humidity

How humid the air is depends on its Relative Humidity (RH). This is the percentage of moisture in the air out of the maximum that the same volume of air can hold at that temperature. Entirely dry air has RH 0 %. As a maximum humid air has an RH of 100%. There is rarely any moisture damage if the RH is less than 70%, although there are some exceptions. As a rough rule of thumb the amount of moisture the air can hold doubles for every 10 degrees Celsius over normal temperatures. If for instance air of 20°C and RH 50% is cooled to 10°C, the RH will reach 100%. Any further cooling will cause immediate condensation. If the air is then heated, the RH will drop below 100% and any condensation that had happened would over time re-evaporate back into the air.




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