Blog Post 2: Help! What’s the Difference Between Relative Humidity and Dew Point?

Many people confuse relative humidity with dew point.  They often hear about dew point on the weather channel, but aren’t really sure what it means. Let’s take a closer look at these two concepts.

To put it simply, relative humidity is the amount of moisture in the air compared to what the air can “hold” at that temperature.  So if I was to say that the relative humidity, which is always measured in a percentage, was 100% that means that the air is holding its maximum water vapor capacity at that temperature.

It’s important to understand that relative humidity is relative to the temperature.  In order to understand this concept, let’s take a look at the diagram below.  The glasses represent the temperature, so the smaller the glass the colder the temperature and vice versa.  When looking at the first glass you notice that colder air has a smaller capacity for holding moisture and has a high relative humidity.  As the temperature increases (glass gets bigger), the air is able to hold more moisture and the relative humidity decreases.   In fact, for every degree that the temperature increases, the relative humidity decreases about 2%.

RH Chart

We don’t always use relative humidity to describe the amount of moisture in the air.  Sometimes we refer to dew point.  The dew point is the temperature at which the relative humidity equals 100%.  Unlike RH, the dew point does not change with air temperature.  In that sense it is an “absolute” measurement of the amount of water vapor in the air.

Dew Point is also a great measure for comfort.  If the dew point is below 60 degrees it is comfortable, but if the dew point gets above 70 degrees then it gets very sticky and uncomfortable.  If you know temperature and relative humidity, you can calculate the dew point using a psychometric calculator or an online calculator, like this one www.dpcalc.org

Come back next week for part three in our series discussing the sources of moisture in your home!

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