Dew point
Also known as: dew point temperature
Short definitionThe temperature at which the water vapour in air starts to turn into droplets as the air is cooled. Any surface colder than this sweats, and ices up if it is below zero.
Air cooled without any change in its water content saturates at some temperature and releases water; that temperature is the dew point. For air at 30 °C and 50% humidity it is 18.4 °C. The evaporator surface runs colder than the dew point of the room air, so it collects moisture. If the surface is below 0 °C the moisture turns to frost and defrost is needed.
In a project the issue shows up on the outer surfaces of panels, doors and pipes. Insulation thickness is chosen so that the outer surface stays warmer than the dew point of the ambient air. Humid coastal towns need thicker panels and a sound door gasket. On freezer room doors a frame heater keeps the frame above this limit; otherwise the door freezes shut.