Portugal - Atlas do Ambiente I.6 - Humidade do Ar

Summary

There is ever more or less water in the atmosphere but this water is normally as vapour and so is invisible. The hygrometric state of atmospheric air is expressed through various physical parameters among which is the relative humidity of the air. The relative humidity of a sample of moist air at temperature Tis the ratio in per cem of the mass of vapour m in the sample to the mass m' which the air would have if it were saturated at the same temperature. A sample of atmospheric air taken near the surface of the Earth contains ever a certain amount of water vapour but for several regions this quantity is so small that is not easy to measure it with simple processes. The most frequent instruments for the determination of the humidity of the air are the hygrometers and the psychrometers. The hygrometers are based on a method using the change in dimensions of hygroscopic substances such as the human hair if it is not greasy, and the psychrometers are based on a thermodynamic method. The humidity of the air variations are particularly conditioned by temperature variations and the nature of local air masses. The spacial distribution of the atmospheric humidity as well as its diurnal and annual variation may be affected by local factors. The values of the atmospheric humidity ar 9 a.m. local time are very close to the mean values for the day. Relative humidity of the air is a very important climatological element because when associated with other climatic elements it influences the