This entry contributed by Dana Romero
Absolute humidity is the mass of water vapor per unit volume of air in the atmosphere. The mass of water at any given temperature is limited by the vapor pressure, which can be found in tables of water properties.
A more useful notion is that of relative humidity.
Dewpoint, Humidity, Relative Humidity
References
Young, H. D.; Freedman, R. A.; Sandin, T. R.; and Ford, A. L. Sears and Zemansky's University Physics, 10th ed. Reading, MA: Addison-Wesley, 1999.
