Answer: Moist air is a mixture of water vapor and dry air. The amount of water vapor in a certain volume of moist air (by mass) is usually much less than that of dry air, but it occupies the same volume as dry air and has the same temperature. The pressure of moist air is the sum of the partial pressures of the constituent gases (i.e. dry air and water vapor). The pressure of water vapor in moist air is called the partial pressure of water vapor, recorded as 
. Its value reflects the amount of water vapor in the moist air. The higher the water vapor content, the higher the partial pressure of water vapor. The partial pressure of water vapor in saturated air is called the saturated partial pressure of water vapor, recorded as 
.
