The dry and wet thermometer is composed of two common thermometers, one is used to measure the air temperature, which is called the dry bulb thermometer; the other is wrapped in the ball with gauze soaked in distilled water, and the lower end of the gauze is immersed in distilled water, which is called the wet bulb thermometer. Because the gauze wrapped in the wet bulb thermometer evaporates and absorbs heat after absorbing water, the indication is smaller than that of the dry bulb thermometer.
When the air is dry, the gauze of the wet bulb thermometer evaporates quickly and absorbs more heat, so the difference between the readings of the two thermometers is large. The greater the difference between the two thermometers, the drier the air.
When there is a lot of water in the air, the gauze of the wet bulb thermometer evaporates slowly, absorbs less heat, and the difference between the readings of the two thermometers is small. The smaller the difference between the two thermometers, the more humid the air is.
By comparing the reading difference of two thermometers with the data of temperature and humidity conversion table, we can know the humidity.