Humidity Ratio represents the ratio of the mass of water vapor to the mass of dry air in a mixture, expressing the absolute amount of moisture present in the air. And is denoted by H. Humidity Ratio is usually measured using the Kg of water vapour per Kg of air for Specific Humidity. Note that the value of Humidity Ratio is always positive. Typically, the value of Humidity Ratio lies in the range from 0 to 1.