The hygrometer

Relative humidity is measured with instruments called hygrometers. The most widespread   are the hair type hygrometer, that exploit the unique property of human hair to stretch proportionally with the relative humidity (as those who have curly hair well know, when the air is humid, hair becomes more curly and knotted).
In meteorology more precise instruments called psychrometers (from the Greek word psycros, cold) are used. These consist of a couple of thermometers positioned side by side, one of which has a bulb covered with a cloth soaked with water. So long as the relative humidity is less than 100%, the thermometer with the wet bulb shows a lower temperature than the dry one, and the difference increases  when the relative humidity decreases. By means of special tables it is therefore possible to calculate the relative humidity from the difference in the temperatures measured by the two thermometers. In fact, in order to make the water of the wet bulb evaporate, energy is necessary, which is subtracted from the bulb, which cools. The speed of evaporation, and therefore of the bulb cooling, is greater when the relative humidity is lower (i.e. when the air is dryer). When relative humidity is equal to 100% and the air is saturated with water vapour, instead, evaporation on the wet bulb stops and the two thermometers show the same temperature.

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