humidity

Air humidity is the water content in the air (atmosphere). The amount of moisture the air can absorb depends on its temperature.

Air humidity in physics and technology
Air that does not contain any water vapor is called dry air, air containing water vapor is called moist air. The following ways of expressing air humidity are often used for physical calculations and technical applications.

Absolute humidity
Absolute humidity is the weight of water vapor in a volume of moist air. It is denoted {\displaystyle \rho _{p}} and has the unit kg.m-3.[1]

{\displaystyle \rho _{p}={\frac {p_{p}}{r_{p}.T}}}

where:

• {\displaystyle p_{p}} - partial pressure of water vapor in the air at a given temperature (Pa)
• {\displaystyle r_{p}} - specific gas constant of water vapor (J.K-1.kg-1)
• {\displaystyle T} - temperature (K).

Relative humidity
Relative humidity is the ratio of the amount of moisture to the maximum possible amount at the same temperature. It is denoted {\displaystyle \varphi } and given as a percentage.[1]{\displaystyle \varphi ={\frac {m_{p}}{m''_{p}}}{\dot {=}}{\frac {p_{p}}{p_{s}}}}

where:

• {\displaystyle m_{p}} - mass of water vapor in air at a given temperature (kg)
• {\displaystyle m''_{p}} - maximum weight of water vapor in saturated air at a given temperature (kg)
• {\displaystyle p_{p}} - partial pressure of water vapor in the air at a given temperature (Pa)
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• {\displaystyle p_{s}} -partial pressure of water vapor in saturated air at a given temperature (Pa).

{\displaystyle x={\frac {m_{p}}{m_{v}}}=0,622.{\frac {p_{p}}{p-p_{p}}}}

where:

• {\displaystyle m_{p}} - mass of water vapor in air (kg)
• {\displaystyle m_{v}} -mass of dry air (kg)
• {\displaystyle p_{p}} - partial pressure of water vapor in the air at a given temperature (Pa)
• {\displaystyle p} - wet air pressure (total) (Pa)

Air humidity in meteorology

In meteorology, it is defined by the following quantities (the first three are listed in the previous chapter):

absolute air humidity - amount of water vapor in grams in 1 m3 of air
relative (=relative) air humidity - absolute air humidity divided by the humidity of saturated water vapor (that is, the maximum amount of water vapor) at the same temperature in the same volume
specific (=measured) air humidity - amount of water vapor in grams in 1 kg of dry air
dew point deficit - air temperature minus dew point temperature
water vapor pressure - partial pressure of water vapor in hPa
saturation supplement - vapor pressure at saturation above water at the daily temperature minus the actual vapor pressure
In the atmosphere, water is found in all three states. There is a constant exchange of moisture between the active surface and the atmosphere, which takes place through turbulent flow and molecular diffusion. The content of water vapor in the air affects the course of various processes in the atmosphere, e.g. for radiation balance, air purity and the like.

Fluctuations in air humidity characteristics are generally related to periodic changes in air temperature. Dry air has a relative humidity of 0%, air saturated with water vapor has a relative humidity of 100%.

Rozdelenie vzduchu podľa jeho vlhkosti

 

The area of pleasant perception of air humidity depending on the temperature of the dry bulb
According to the moisture content, the air can be divided into:[1]

unsaturated air - the partial pressure of water vapor in the air is lower than the pressure of saturated vapor at the same temperature,
saturated air - the partial pressure of water vapor is the same as the pressure of saturated vapor at the same temperature,
supersaturated air - saturated air containing additional moisture in a different state: in the form of mist or innovat.
Moisture and man
If we heat the cold air to a comfortable temperature in winter, even with a constant amount of water vapor in the air, its relative humidity will drop. It is caused by the fact that at a higher temperature, the air can absorb a larger amount of water vapor. The most pleasant air humidity for humans is at a relative humidity value of 50-60%. Therefore, it is also necessary to deal with air humidification or drying with air conditioning and heating devices in buildings.
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