Dear Tom,
What is “dew point”?
— Ray Cook, Wheaton
Dear Ray,
It’s a question we hear frequently. Here’s the definition: The dew point is the temperature to which air must be cooled in order for saturation to occur. And here are some related facts: The dew point temperature is an absolute measure of how much water vapor is in the air. For example, air at 60 degrees with a dew point of 50 degrees contains exactly the same amount of moisture as air at 90 with a dew point of 50. As air is cooled, its ability to contain water vapor decreases, and air chilled down to its dew point temperature will have a relative humidity of 100 percent. Relative humidity (RH) is an expression of how saturated the air is. An RH of 75 percent means air contains three-quarters of the moisture it could hold.
