Absolute Humidity, Relative Humidity, Specific Humdity
Humidity is the amount of moisture (water vapor) in the air. It can be expressed as absolute humidity or relative humidity. Absolute humidity is the mass of water vapor divided by a unit volume of air (grams of water/cm3 of air). Relative humidity is the amount of water vapor present in the air divided by the maximum amount that the air could contain at that temperature. Relative humidity is expressed as a percentage. The relative humidity is 100% if the air is saturated with water vapor and 0% if no water vapor is present in the air at all.
Absolute humidity is the total amount of water vapor present in a given volume of air. It does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to roughly 30 grams per cubic meter when the air is saturated at 30 °C
The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapour (H2O) in the mixture to the saturated vapour pressure of water at a given temperature. Thus the relative humidity of air is a function of both water content and temperature.
Specific humidity (or moisture content) is the ratio of water vapor mass to the air parcel's total (i.e., including dry) mass and is sometimes referred to as the humidity ratio. Specific humidity is approximately equal to the "mixing ratio", which is defined as the ratio of the mass of water vapor in an air parcel to the mass of dry air for the same parcel.