ABC6 WeatherLab - Dew point
The official definition of dew point temperature is: "the temperature to which the air must be cooled to reach complete saturation". This is also known as 100% relative humidity.
It's called the "dew" point temperature because that is the temperature to which the air must be cooled to produce "dew" (on grass,for example). This often happens overnight on late Spring nights.
But that still doesn't really explain the true meaning of dew point.
The way I look at it is this: In a given amount of air, if you tell me the dew point temperature, I can use a graph to determine how much moisture there is in the air. For example, if you use a dew point measuring device to determine that the dew point temperature of the air is 52°F, then I can use the chart to tell you that the air sample contains roughly 10 grams of water vapor per kilogram of air.
Therefore, just by knowing the dew point temperature, you automatically know the actual amount of moisture in the air. That's it. That's the whole thing. The higher(lower) the dew point, the more(less) moisture there is in the air. You don't need to know the actual air temperature for this exercise.
Given a Spring day with highs in the 70s, dew point temperatures in the 30s/40s means the air is dry and comfortable, whereas dew point temperatures in the 60s/70s means the air is sticky/humid.
-Kevin Coskren, ABC6 StormTracker WeatherTeam