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Humidity and its Measurements

16th November 2018

Humidity denotes the presence of water vapour in air or other gas. Water vapour is the gaseous form of water and can be thought of much like any other kind of gas. It is normally transparent and makes up about one hundredth (or one percent) of the air around us.

Air has a given capacity to absorb water vapour and this capacity depends mainly on temperature. Generally, the hotter the air, the more water vapour it can hold.

At any particular temperature, air that contains its full capacity of water vapour is classed as saturated. The ‘relative humidity’ of the air expresses how fully saturated the air is with water vapour.

Humidity Graph

There are three main measurements of humidity: relative, absolute and and specific.

Absolute humidity (units are grams of water vapour per cubic meter volume of air) is a measure of the actual amount of water vapour in the air, regardless of the air's temperature. The higher the amount of water vapour, the higher the absolute humidity. For example, a maximum of about 30 grams of water vapour can exist in a cubic meter volume of air with a temperature in the region of 30°C.

Relative humidity expressed as a percent, is a measure of the amount of water vapour that air is holding compared to the amount it can hold at a specific temperature. Warm air can possess more water vapour (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a higher relative humidity. A relative humidity of 50% means the air on that day (specific temperature) holds 50% of water needed for the air to be saturated. Saturated air has a relative humidity of 100%.

The relative humidity of an air-water mixture is also defined as the ratio of the partial pressure of water vapour in the mixture to the saturated vapour pressure of water at a given temperature. Therefore, the relative humidity of air is a function of both water content and temperature.

Specific humidity refers to the weight of water vapour contained in a unit weight (or amount) of air (expressed as grams of water vapour per kilogram of air). Absolute and specific humidity are quite similar in concept.

For information on our humidification systems and the benefits they provide, visit our Humidification Systems page.