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Psychrometric Chart and Humidity

A chart consisting of lines and curves that shows the relationship between temperature, humidity and air volume. From the psychometric chart reading, a variety of information can be obtained, including the specific humidity, dew point, vapor pressure, and wet-bulb temperature (all on Wikipedia).

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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 theair 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 (for sea level). Such a chart is shown above in simple form.
CBD-1 Humidity in Canadian Buildings
by N. B. Hutcheon, Canadian Building Digests, National Research Council Canada