The point made above is important enough to restate. If you have air with a dew point of, say, 20° C (68° F), you know that if that air finds any surface with a temperature of 20° C or below, water vapor from the air will condense and form liquid water droplets, as it did on the car roof shown below. Unlike relative humidity, however, if you know the dew point by itself, you’ve got some good information. If you really want to understand the properties of a given volume of air, you have to know at least one more datum in addition to the RH: temperature (dry bulb), absolute humidity, dew point… By itself, it doesn’t give you enough information. I’m not so interested in knowing the relative humidity as I used to be. I’ve heard it said that three of the leading causes of building failures are: In fact, understanding how water interacts with buildings is one of the most important jobs of building science. Water really is an amazing substance, and if you’re wondering why I’m pontificating on it as if I were a college professor again, it’s because water and buildings have an interesting relationship. Everyone knows that, of course, but what does it mean? (That photo above is the actual dew on the actual grass on this actual morning just outside my actual house!) The appearance of liquid water occurs because of one simple fact: The temperature of the surfaces that collect this dew is below the dew point of the surrounding air. Did the grass outside have dew on it this morning where you live? It did here.
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