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Psychrometric chart of heating and cooling of air-vapor mixtures

It is important to realize that what is normally referred to as "humidity" is actually relative humidity. It is a measure of the amount of water vapour present in the air expressed as a percentage of the maximum amount that the air can hold at that particular temperature. When the temperature is changed, the relative humidity changes, since the capacity of the air for holding moisture increases with increasing temperature.
These relationships are most conveniently represented graphically in psychrometric charts.

When an air-vapor mixture is heated or cooled, without the addition or removal of moisture, the resulting "process" may be plotted as a horizontal line on the chart: AB.
The process AB represents room air at 75°F and 40% relative humidity being cooled in contact with a cool window surface. When cooled to 49°F the mixture becomes saturated...

...if cooled further, water will be removed by condensation on the window surface: BC.

This situation is typical of many that occur in buildings and when fully understood gives much insight into many humidity problems.

As the air continuse to cool it will lose moisture until, here, it reaches D — at which point it is at 20°F and 20% relative humidity.
Outside air that enters a building in winter has a low moisture content, and when heated provides low relative humidity unless substantial quantities of water are evaporated within the building.
This may be verified by reference to the process shown by the line D E, which shows that air from outside at 20°F and 100 per cent relative humidity will, when heated to 75°F, have only 12 per cent relative humidity. The moisture content will have to be increased from 0.2 to 0.74 per cent to provide 40 per cent relative humidity.
The complete cycle.

Source: Hutcheon, N. B. Humidity in Canadian Buildings, CBD-1, Canadian Building Digest, National Research Council Cnada