What is relative humidity?  Relative basically means relating to something. It is the amount of water vapor in the atmosphere and how much water vapor the air can hold at that particular temperature. And this is the percentage of moisture in the air. But this relationship can be observed only in a certain range of temperature.

What is absolute humidity?  Basically absolute means real value. And it is the amount of water present in a given mass of air at a given temperature. Absolute humidity (AH) is obtained from the following simple formula.  AH = water vapor/air ratio.