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Article about the phenomena of condensation in buildings.
Strictly speaking, 'condensation' describes the physical process by which substances change from a gas or a vapour to a liquid phase, usually as a result of a drop in temperature. However, the term is commonly used to describe the process when moisture in the air condenses out to form liquid water as fine droplets in the air, or on a relatively colder material. Common examples of the former in the natural environment are the formation of clouds when warmer moisture-laden air mixes with colder air above, and fog, where this occurs at ground level. Similarly mist forms when warm moisture laden air is cooled by heat loss over night. Examples of the latter include the misting up of car windows when the warm moisture-laden air within cools on the surface of the window screen, and the misting on the surface of a mirror when held in the moist air exhaled from the mouth. This occurs because reducing the temperature of the gases that make up air reduces the energy available to keep the molecules whizzing around randomly within the available space, and lets a proportion of the molecules settle down into a less mobile liquid phase, in which the motion is more limited. Conversely, molecules in the liquid phase may pick-up enough energy to leave the liquid and 'evaporate' off to join the other gas molecules randomly moving around the available space once more. In fact, at any time molecules will be 'condensing' and 'evaporating' from any liquid water. The more active and energetic the molecules are, the greater 'pressure' they exert. This is described as 'partial vapour pressure'. If the energy and hence the partial vapour pressure of the molecule in the liquid is higher than those in the air, then there will be a net movement of water into the air resulting from net evaporation or drying. Conversely, if the temperature and hence partial vapour pressure of the water molecules in the air is higher than that in the liquid or other adjacent material, there will be net condensation.