Dewpoint - The temperature at which air is cooled enough to reach its saturation point (100% humidity) for a given volume of water vapor in the air. If the air cooling occurs because of adiabatic expansion, clouds are formed; if it is due to contact of the air with a colder object, condensation (known as dew) is formed on the object; and if it is due to mixing of a warmer, moist air parcel with a cooler, dry parcel, fogs and condensation trails are created. Unlike humidity, the dewpoint is not based upon the temperature or pressure of the air, but solely upon the water vapor content. It can be measured using a dew point hygrometer, from the known temperature and relative humidity and by measuring the difference in wet and dry bulb temperatures. It is of importance in predicting fogs, frost, dew and overnight minimum temperatures.
[Bulletin of the American Meteorological Society; v74; 1323-1325; 1993.]
[Mechanical Engineering; v116; 30; 1994.] 
Atmospheric Chemistry Glossary