ABSTRACT
This chapter considers how water vapour behaves – particularly when subject to temperature changes. It sets out the moisture levels necessary for human comfort, before examining the problems caused by excessive levels of moisture, and how they may be prevented. Water vapour is produced by the evaporation of liquid water as part of the continuous cycles of evaporation and condensation which occur within the earth’s atmosphere. The amount of water vapour in a volume of air can be expressed as by its moisture content. The choice of measurement – moisture content or vapour pressure – depends on the purpose of analysis. Saturation is expressed in terms of the saturation vapour pressure (SVP) which is the vapour pressure exerted by saturated air. The dew point is a way of considering saturation in terms of temperature. When a body of air is cooled its SVP will be lower and, at certain temperature, will be the same as the actual vapour pressure.
