The wet and dry bulb thermometer is an instrument for determining air temperature and air humidity.

Structure: consists of two of the same ordinary thermometer, one for the determination of air temperature, called the dry bulb thermometer; the other in the ball part of the distilled water soaked gauze wrapped in distilled water, the lower end of the gauze immersed in distilled water, called the wet bulb thermometer. As the gauze wrapped wet bulb thermometer after absorbing water evaporation and heat absorption, so the number than the dry bulb thermometer shows the number of small. Wet bulb gauze often need to change the new.

When the air is dry, the wet bulb thermometer gauze evaporates quickly, absorbing more heat, the difference between the two thermometers is relatively large. The larger the difference between the two thermometers, the drier the air.

When there is a lot of water vapor in the air, the gauze of the wet bulb thermometer evaporates slowly, absorbs less heat, and the difference between the two thermometers is small. The smaller the difference between the two thermometers, the more humid the air.