Relative Humidity

How humid the air is depends on its Relative Humidity (RH). This is the percentage of moisture held in the air out of the maximum that air of that temperature could hold. Entirely dry air has RH 0 %. Maximally humid air has an RH of 100%. There is rarely any moisture damage if RH is less than 60to 70%. As a rough rule of thumb the amount of moisture the air can hold doubles for every 10 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 would 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.