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When air holds the maximum possible amount of vapour, the vapour exerts what is called the saturation vapour pressure. If the water vapour present is less than this maximum then the air is capable of taking up more moisture. The ratio of actual pressure to the saturation vapour pressure at any given temperature, expressed as a percentage, is called the relative humidity (rh). When apiece of wet wood is exposed to air which is not already saturated (ie., its relative humidity is less than 100 %), evaporation takes place from its surfaces. the rate of evaporation is dependant on the difference between the vapour pressure exerted by the wet wood surfaces and that of the air/water vapour mixture in immediate contact with them (Pratt 1974).
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