To quantify the humidity of air used absolute and relative humidity.

Absolute humidity measures the water vapor density p_And, in the air, or its pressure p_And.

A clearer idea of the degree of humidity of the air gives the relative humidity In. Relative humidity is measured by the number, showing, how many percent is the absolute humidity R_And from the density of water vapor p_N, need to saturate the air at its existing temperature:

Relative humidity can be determined by vapor pressure, as virtually the vapor pressure proportional to its density. So you can define and so: relative humidity is measured by the number, showing, how many percent is the absolute humidity R_And the pressure of water vapor p_N saturating the air at its existing temperature:

Thus, relative humidity is determined not only by the absolute humidity, but also the air temperature. When calculating relative humidity values of p_N or R_N we must take from the tables (cm. table. 9.1).

Find out, as the air temperature changes can affect humidity. If absolute humidity R_And equal 0,0094 kg/m3 at 22°C. Since the density of saturating vapour p_N at 22°C is 0,0194 kg/m3 (table. 9.1), the relative humidity is about 50%.

Suppose now, the temperature drops to 10°C, and the density R_And will remain the same. Then the relative humidity will be 100%, t. e. the air will be saturated with water vapor. If the temperature drops to°C b (for example, night), that from each cubic meter of air will condense onto 0,0094—0,0073=0,0021 kg of water vapor (falls the dew).

Temperature, at which the air is in the process of cooling becomes saturated with water vapor, is called the dew point. In the above example, the dew point is 10°C. Note, that at a certain dew point the absolute humidity R_And can be found in table. 9.1, since it is equal to the density of saturation vapor p_N the dew point of. Finding then on the same table R_N for a given air temperature, can by the formula (9.1) to calculate the relative humidity In.

Recall, in the absence of centers of condensation of steam can be supersaturated. Therefore, clean air can be cooled below the dew point, and vapor contained in it to condense will not be.