Air humidity that is the extent of wet and dry air. According to different needs, usually respectively in absolute humidity, vapor pressure, relative humidity and dew point temperature said. Its size and change will directly or indirectly caused the cloud, fog, precipitation phenomena such as the evolution and extinction.

Meteorological departments in the air humidity has several, including relative humidity, absolute humidity, dew point and vapor pressure, etc.. Relative humidity is the most commonly used. Relative humidity is the percentage of units (%), and the air is not water vapor relative humidity zero, air capacity has reached maximum water vapor (referred to the air is already saturated), the relative humidity is 100%.

Air humidity measurement commonly used dry ball thermometer. It is the same two thermometer, the dry bulb thermometer is used to measure temperature; wet bulb thermometer with the mercury ball moist gauze wrapped with gauze and immerse in the bottom Shuiyu Lane. Wet gauze so that the ball always kept moist conditions (and hence known as the wet bulb thermometer). Wet gauze on the ball in the air without water saturated when evaporation. The decision to speed evaporation in the air relative humidity : humidity at the slow evaporation, moisture evaporation hour fast. Humidity is 100%, the moisture content of the air is saturated, water stop evaporation. Is the evaporation of water consumed calories, so wet bulb thermometer readings will be reduced. Therefore, in addition to saturated air, that the relative humidity of 100% (this time the wet bulb thermometer readings and the same dry bulb thermometer), dry bulb thermometer readings better than the wet-bulb thermometer readings higher. The greater the margin both said more air drying, the lower the relative humidity. Therefore the use of wet and dry bulb temperature can send to know the relative air humidity levels. Meteorological departments have been using the control book publishing, it is easy to check with the required information.