Air is made up of water vapor combined with dry air in a way that preserves each component’s properties. Humidity is defined in a general way as the water vapor content of air. In heating, ventilation and air conditioning (HVAC), more precise terms are used; specific humidity, absolute humidity, and relative humidity.
Water vapor is a form of steam. It is steam at low temperature and pressure. Varying quantities of water vapor can be contained in air, depending on temperature. When air becomes humidified, absorbing moisture, the heat for evaporation has to come from either the air itself, or some other source. When the opposite happens, that is, when air moisture condenses, an equivalent amount of heat will be released.
Condensation is what happens when the temperature of the air is not high enough to hold the moisture it contains; at any given temperature, there is a corresponding amount of moisture that air is capable of absorbing. This is related to the air’s saturation point, defined as the point at which the air contains all the water vapor it is capable of absorbing.
For example, a body of air at 70 degrees F is saturated when it contains 8 grains of vapor per square foot, and air at a temperature of 0 degrees F is saturated when it holds one half a grain of water vapor per sq. ft. (A grain is equivalent to 0.0648 grams, or 1/7000th of a pound)
Absolute humidity is expressed either in grains or pounds per cubic foot, or grams per cubic centimeter. It is defined as the weight of the water vapor present in one cubic foot of air. It is, therefore, the same thing as the density of the air. Absolute humidity ranges from 0 grams per cubic meter in dry air to 30 grams per cubic meter (0.03 ounce per cubic foot) when the vapor is saturated at 30 °C.
Defined as the weight of water vapor per pound of dry air, specific humidity should not be confused with relative humidity, which is a measure of the percentage of water vapor, rather than weight. Specific humidity ratio is expressed as a ratio of kilograms of water vapor per kilogram of air, including water vapor.
Relative humidity is the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a prescribed temperature. Relative humidity is expressed as a percentage.
To put it another way, relative humidity is the percentage of water vapor in the mixture of dry air and water vapor at a given temperature, relative to the saturation point for that volume of air at that temperature.
Relative humidity is established by using a hygrometer or sling psychrometer to obtain the amount of water vapor in the air, then dividing that figure by the amount of moisture the air is capable of holding at that temperature, and multiplying the resulting figure by 100 to convert to a percentage.
Air Drying Effects
The drying effect of air decreases as relative humidity increases, and vice versa. The effect varies with regards to both moisture content and temperature, by definition of relative humidity. Dry air contains only a small amount of heat, since it has a low specific heat.
When the temperature is high and the relative humidity is low, evaporation of water is rapid; soil dries, wet clothes hanging outdoors dry quickly, and perspiration readily evaporates from the skin. Wooden furniture can shrink causing the paint that covers these surfaces to fracture.
When the temperature is high and the relative humidity is high, evaporation of water is slow. When relative humidity approaches 100 percent, condensation can occur on surfaces, leading to problems with mold, corrosion, decay, and other moisture-related deterioration.
The dew point is a saturation point, specifically, the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. (The condensed water being the dew.)
Any lowering of the temperature below the dew point will cause condensation of the water vapor present in the air, and the release of the vapor’s latent heat.
A relative humidity of 100% means that the dew point is equal to the current temperature. Any dew point higher than around 60 degrees F is uncomfortable for most people, as it will prevent the drying of perspiration, leading to overheating of the body. On the other end of the scale, a dew point lower than around 49 degrees F can be uncomfortable as well, causing exposed skin to dry and crack.
All of these factors relating to humidity can be controlled in an interior environment; this is referred to as air conditioning. Conditioning and controlling the ambient air is not just a matter of cooling it down, as is the commonly held notion.
The term can and does also refer to warming the air, maintaining proper circulation and redistribution of it, filtering it from contaminants like dust, chemicals and pollutants, and maintaining a comfortable humidity. Therefore, air conditioning includes heating, cooling, dehumidification, humidification, circulation and filtering/cleaning.