What is the “Dew Point”?

Dear Tom,
You make occasional mention of the “dew point.” I have always wondered exactly what that is.
Danny Gladder, Chicago
Dear Danny,

The dew point is the saturation temperature of air, the temperature at which the air would have a relative humidity of 100 percent. Unlike relative humidity, the dew point changes only when the air’s moisture content changes. The higher the dew point, the greater is the moisture content of the air. One complication is that air’s ability to hold moisture increase greatly as it heats up. For example, when air warms from 69 to 90 degrees F., its ability to hold moisture doubles. Air that is saturated at 69 degrees means its dew point is also 69 degrees and its humidity is 100 percent. At 90 degrees, the dew point stays at 69 degrees, but the relative humidity falls to 50 percent. Dew point is a better measure of moisture content.
