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The measure of the amount of the air's water vapor present at a particular time is called absolute humidity. It is expressed as a specific, absolute, or relative value, value, and should not be confused with relative humidity, which is expressed as a percentage. Within the field of air psychometrics, measures of humidity are important elements.
Absolute humidity is determined using the mass of the air's water vapor. It can be expressed in either kilograms of water vapor or pounds of water vapor. The mass of water vapor is then considered in terms of volume. This measurement of density is usually expressed using grams per cubic meter or pounds per cubic foot. In other words, absolute humidity measures the density of water vapor.
Humidity is an important aspect of weather and provides an additional aspect to measuring the environment in terms of temperature. More specifically, absolute humidity provides a means for weather services to express how the temperature of the environment feels. A higher absolute humidity, and thus relative humidity, means that the weather feels much hotter. The air is moister and generally feels heavy or thick.
The weather during a high level of humidity is sometimes referred to as humid or muggy. If there is a very low level of humidity, the air can feel very dry or even stale. In locations where the humidity is very low, people may experience dry skin more often than usual. A higher level of humidity causes people to perspire or sweat more.
Relative humidity utilizes the ratio of vapor pressure to vapor pressure saturation. If a household or weather service refers to humidity or uses a humidistat, they are more often than not referring to relative humidity. Relative humidity uses the absolute humidity, or current level of humidity, in relation to the maximum humidity to offer a percentage.
Also related to humidity measures are air psychometrics, steam and humidification, dehumidification, and recommended relative humidity. Humidity is a central aspect within the study of air and the cycle of moisture and water. Respiratory conditions or illnesses also relate closely to humidity. Moist air, when the air is above the recommended humidity level (RH), can allow dangerous bacteria, fungi, viruses, or mites to thrive. For this reason, public buildings must measure their humidity levels and pay particularly close attention to humidity levels at all times.