From the interaction of atoms and molecules in substance
Mutual transformations of liquids and gases
47. Air humidity
Water vapor in the atmosphere.
Water vapour in the air is not saturated despite the huge surface of oceans, seas, lakes and rivers. The movement of air masses, which is ultimately caused by the radiation of the Sun, leads to the fact that in some places of our planet at the moment the evaporation of water prevails over condensation, while in others, on the contrary, condensation prevails.Absolute humidity.
The air containing water vapour is called humid. The partial pressure of the water vapour contained in the air, or, as they say, the elasticity of the water vapour, can be assumed to be one of the indicators of air humidity. It is measured in pressure units (\(N/m^2\) or \(mmHg\))\(1 \frac{N}{m^2} \approx 0.0075 ~mmHg\)
The mass of water vapour contained in \(1 ~m^3\) of air may also be taken as a characteristic of air humidity. This value is called absolute humidity and is measured in density units (usually in \(g/m^3\)).
Absolute humidity and elasticity of water vapors are related by the equation of state \( p = \frac{1}{\mu} \frac{m}{V} R \,T = \frac{\rho}{\mu} R \,T\).
Relative humidity.
Knowledge of absolute humidity (or water vapor elasticity) does not indicate how far water vapor is from saturation under these conditions. Namely, the intensity of water evaporation (or its condensation) and, consequently, the loss of moisture by living organisms depends on it. That is why relative humidity is introduced, which shows how far water vapor at a given temperature is from saturation.The relative humidity \(r\) is the ratio of the elasticity \(p\) of the water vapour contained in the air at a given temperature to the pressure \(p_0\) of the saturated vapour at the same temperature, expressed as a percentage
\(r = \frac{p}{p_0} \times 100\%\)
Dew point.
When cooling humid air at constant pressure, its relative humidity increases, because the lower the temperature, the closer the elasticity of vapour in the air to the elasticity of saturated vapour. Eventually, the vapour becomes saturated. The temperature at which the water vapour becomes saturated is called the dew point.When the air cools down to the dew point, the condensation of vapors begins, the fog appears and the dew falls out.
The dew point also characterizes the humidity of the air, because it allows you to determine the partial pressure of the water vapor with the help of the table of the dependence of the saturated vapor pressure on temperature.
Hygrometers.
Humidity is measured with special instruments, hygrometers and psychrometers.The condensation hygrometer allows to detect the dew point. The simplest device of this type is made of a metal box, the front wall of which is well polished. Inside the box, an easily evaporating liquid ether is poured and a thermometer is inserted. Running air through the box with a pump causes strong evaporation of ether and quick cooling of the box. The thermometer notices the temperature at which dew droplets appear on the polished wall surface. The appearance of dew indicates that the water vapor has become saturated.