Dew Point

A saturated volume of air contains all the water vapor that it can hold. Saturation is usually reached by the air being cooled until its saturation vapor pressure equals the actual vapor pressure. The temperature of the air at that point is called the dew-point temperature, or simply, the dew point. If the air is cooled below its dew point, condensation occurs because the amount of water vapor in the air exceeds the maximum amount that can be contained at the lower temperature. Further cooling therefore causes clouds, fog, or dew to form. Under ordinary circumstances the actual vapor pressure cannot exceed the saturation vapor pressure by more than a very small amount.

Cooling near the surface normally results from contact with cool ground or water. Cooling to the dew point may also occur by lifting moist air to higher altitudes; it is thus cooled adiabatically. For example, consider air with a temperature of 80°F. and a vapor pressure of 0.362 inches of mercury. Referring to Table: Saturation Vapor Pressure, we find that if the air is cooled to 50°, the actual vapor pressure will equal the saturation vapor pressure. Therefore, 50° is the dew point.