What is dewpoint?

In technical terms, the dew point is the temperature to which the air would have to be cooled in order to make the relative humidity reach 100%. In other words, it is the temperature at which the moisture in the air would begin to separate and turn to liquid form, as fog, precipitation, etc.

There is a very important scientific truth hidden in the previous statement: air's capacity to hold moisture varies with temperature. Warmer air can hold more water vapor. Cooler air cannot hold as much.

Relative humidity is "relative" because it adjusts for this varying capacity. It is an incredibly useful metric for a wide range of tasks. However, it does mask this underlying reality, that 50% humidity at 65°F is a different amount of moisture than 50% humidity at 90°F. In fact, the warmer air has just over twice as much of moisture, even though the relative humidity is the same.

Dewpoint helps reveal this discrepancy. 65°F at 50%RH corresponds to a dew point of 46°F. 90°F at 50%RH yields a dew point of 69°F. Higher dewpoint, more moisture.

Ventilation Decisions and Dewpoint

Using relative humidity alone can lead to mistakes when trying to cure moisture problems with ventilation. Imagine you're faced with a cool, damp basement sitting at 60°F and 70% relative humidity. Gross, right?

Outside, it's hot, but seemingly relatively dry 90°F and 50% relative humidity. Opening a window will help dry out the basement, right?

Not so fast!

Let's look at the dewpoints.

60°F @ 70% RH => 50°F dewpoint

90°F @ 50% RH => 69°F dewpoint

So exchanging the basement air for outside air is actually bringing MORE moisture in! When that outside air is cooled surfaces like basement walls and floors, the relative humidity goes up. If the surfaces are below the dewpoint outside air, liquid condensation will form. Which is the reason it's called dewpoint!

A similar effect, but operating in reverse is why leaky houses get dry in the winter.

Outdoors, it might be 25°F at 50% RH. This is a dewpoint of 9°F. Warm that same air, with the same moisture content, up to 72°F and you get a desert-like relative humidity of under 9%!

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Dewpoint and Comfort

Many people find the dew point to be a more direct measure of moisture-related comfort than relative humidity. Specific guidelines vary, but here's a fairly typical breakdown: