Relative humidity is a percentage measurement of the content of steam in the air. It is expressed by the ratio of pressure of steam contained in the air to the maximum possible steam pressure in a given temperature (vapour pressure). Instantaneous measurement of humidity is taken with the use of hygrometer while continuous registration is recorded by hygrograph. When the relative humidity totals 100%, the air is “saturated,” whereas when its percentage is lower, the air is colloquially known as “unsaturated”.
Elaborated by H. Ojrzyńska