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BMS Controls Glossary



Humidity is the presence of water vapor in the air. Water vapor is the gaseous state of water and is invisible. The amount of water vapor in the air determines the humidity. The higher the humidity, the more water vapor present, and vice versa.

The saturated vapor pressure (SVP) is the pressure exerted by water vapor when it is in equilibrium with liquid water. The higher the SVP, the higher the humidity. Relative humidity (RH) is a measure of how much water vapor is present in air relative to how much could be present at saturation. RH is typically expressed as a percentage, and values less than 100% indicate that air can hold more moisture.

For example, an RH of 50% means that the air can hold half as much moisture as it could if it were saturated. Dew point is the temperature to which air must be cooled at constant pressure to become saturated with respect to a particular substance. The dew point for water is reached whenRH = 100%. At this point, further cooling will cause condensation on surfaces exposed to the air (e.g., windows, outdoor furniture). The term “humidity” often refers to RH, but can also refer to other measures such as absolute humidity and specific humidity. Absolute humidity is the mass of water vapor per unit volume of air and is commonly expressed as grams per cubic meter (g/m3).