Volcanoes 101 . Site map . There is always water vapor in the air. The amount of this moisture is called humidity. Relative humidity is the amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated.
Relative humidity is expressed as a percentage.
Temperature effects how much moisture air can hold, and as the temperature decreases, relative humidity increases. This is called the dew point. Explore the States of Matter . The Exploratorium has a cool experiment on this topic. E-mail us with general site comments or suggestions.