It is a simple device that allows you to determine the humidity by measuring the difference in how much water evaporates. Compared to a simple structure, humidity can be measured relatively accurately. Before electronic hygrometers became popular, psychrometers were mainly used.

The wet-bulb hygrometer consists of two thermometers, a dry-bulb, and a wet-bulb. Humidity is measured using the temperature difference between the two thermometers.

The dry-bulb is like a normal thermometer. Shows the current temperature.
The wet-bulb is a thermometer wrapped in a thin, damp cloth. Cooling by evaporation takes place and falls below the temperature of the dry bulb thermometer. The dryer the air, the lower the temperature is due to the heat of vaporization.

If the air is full of water vapor, evaporation does not occur. In this case, the dry and wet bulbs’ temperature is the same, and the humidity is 100%.