Water is known by different names in different states. It can be measured in many ways and described with various terms. The following article explains the behavior of water vapor in air and clarifies the terminology used to describe it.

It is said that a beloved child has many names. This also applies to water, including water in gaseous form, which is the source of all life on our planet. Most of us have heard of relative humidity and dewpoint temperature, but there are many other ways to measure the presence of water. Partial water vapor pressure, absolute humidity, frostpoint, mixing ratio, wet bulb temperature, and even enthalpy all describe the humidity of a gas.

When the term humidity is used, we usually mean water vapor in a gas, typically air. Moisture, on the other hand, is used for liquids and solid materials. The term moisture also applies to extremely dry gases, when water vapor is considered an impurity.

PROPERTIES OF GAS MIXTURES
A full understanding of the various terms for humidity and moisture require some basic knowledge about the properties of gas mixtures. In a gas mixture such as air, the total pressure (same as atmospheric or barometric pressure) of the gas is the sum of all the individual pressures of its gas components. The atmospheric pressure, usually around 1000 hPa, is the total of the partial gas pressure of nitrogen (~775 hPa), oxygen (~10 hPa), argon (~10 hPa), carbon dioxide (~0.4 hPa), and a number of other gases with lower partial pressures. All gases produce the same pressure and volume with the same number of molecules, so the partial pressures also represent the proportion by volume of the various gases. On this basis, 21% of the total volume of dry air is oxygen and around 1% is typically argon.

WATER VAPOR PRESSURE—Pw (hPa, PSI, Pa, mbar, mmHg, inHg, mmH₂0, or inH₂0)
The air temperature dictates the maximum partial water vapor pressure in air, in other words, the water vapor saturation pressure. The ability of water to be gaseous form is strongly dependent on its temperature (Figure 1). The higher the temperature, the higher the partial pressure of the water vapor. The partial water vapor pressure in the immediate presence of liquid water equals the saturation pressure at that specific temperature.