The water vapour contained in the atmosphere is called air humidity. It plays an important part in the physical and chemical conversion of air pollutants. High humidity, for instance, favours the formation and growth of aerosols which cause a more frequent occurrence of haze and fog in polluted air, and, therefore, an alteration of the meteorological radiation budget. Due to precipitation, evaporation, and long-range transport of air masses, the water vapour content of the air varies considerably in different localities.This guideline describes hygrometric and psychrometric procedures by which the content of air humidity can be determined. Besides the measu