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Dew Point vs Humidity

Here at ATMOX, we talk about crawl spaces and attics and moisture all the time. The goal is to keep products easy-to-understand and easy-to-install. At the same time, behind the scenes, the product design incorporates a lot of cool technology and science. We are always looking for innovation and new features, but the basic operating principles of an ATMOX controller haven’t changed. Unlike most other moisture control products in the crawl space and attic world, ATMOX has always focused on a dew point comparison – not relative humidity. So how do you explain dew point vs humidity?

This blog post is for all the fellow science people out there who like digging in. If you fell asleep in your high school environmental science class, then you won’t want to continue reading – just know the ATMOX products work based on scientific principles. For all the science enthusiasts who need a refresher – this one’s for you. . .

So, let’s go over some basic science definitions and principles to better understand humidity vs dew point.

Relative Humidity Definition:

The ratio of water vapor in the air to the maximum amount of water vapor in the air that could be present if the air were completely saturated at a given temperature. Expressed as percentage, 100% rH would represent the maximum amount of water able to be held at a given temperature.

The part that is often forgotten or misunderstood about relative humidity (rH) is that the “relative” is referring to that “given temperature” from the definition. So humidity is relative to temperature. The warmer the air mass, the more water vapor the air can hold.

Dew Point Definition:

The temperature where any air mass when cooled to that temperature reaches 100% humidity. It is expressed as measure of degrees.

Dew point is a better measure of absolute measure of the water vapor in the air within a given air mass. When thinking about being outside or in weather, it is also a better measure of how humid it feels.

Air with a lower dew point is drier than air with a higher dew point.

Video Explanation

Here’s a simple video by the National Weather Service in Chicago explaining Relative Humidity vs Dew Point.

Air Mass Examples

Relative Humidity Comparison

Think of the air mass as a sponge or cup that can hold moisture. As the temperature increases, the sponge or cup gets bigger and bigger and can soak up more moisture.

Although each cup in the video may represent 50% rH – they all have different amounts of water vapor. The small cup or cold air isn’t holding as much water as the large cup even though they are all at 50% rH.

Let’s look at example. If all other things are equal, which air mass do you think has more water vapor in the air?

Air Mass A: Relative Humidity = 45% and Temperature = 95 degrees F
Air Mass B: Relative Humidity = 90% and Temperature = 70 degrees F

Click for answer.

Air Mass A has more water vapor. Even though relative humidity is only 45%, the air is so much warmer that it can hold more water vapor. Air Mass B appears to be nearing saturation since it is at 90% rH, but it is actually holding less water vapor and is a drier air.

Dew Point Comparison

Now look at the dew points of each of those air masses to see the difference. Which air mass is wetter?

Air Mass A: Dew Point = 70 degrees F
Air Mass B: Dew Point = 67 degrees F

Click for answer.

As stated earlier, air with higher dew points are holding more moisture. So just as before, Air Mass A is wetter.

Higher dew points are holding more moisture, so Air Mass A with a 70 degree dew point is wetter than Air Mass B. Therefore, if Air Mass A represented outside summertime air and Air Mass B represented the air in the crawl space, the ATMOX controller would look at its dew point comparison and determine that the outside air was not beneficial and would not operate exhaust and intake fans in standard operation.

Let’s look at how the relative humidity alone can be so misleading. Air Mass A has a relative humidity of only 45%, so it must be good, right? Not true once you know that the temperature is at 95 degrees. With a dew point of 70 degrees, this would be a fairly hot and sticky summer day.

Temperature Comparison

Now look at another example with a different Air Mass A in a cooler temperature and higher humidity. Air Mass B stays the same as before. Now which air mass has more water vapor in the air?

Air Mass A: Relative Humidity = 90% and Temperature = 60 degrees F
Air Mass B: Relative Humidity = 90% and Temperature = 70 degrees F

Click for answer.

Air Mass A now has a dew point of 57 degrees F. Air Mass B remains at dew point of 67 degrees F. Although the relative humidity is the same between the two, Air Mass A is now a drier air mass than B.

Proper Ventilation with Proper Information on Dew Point vs Humidity

For proper ventilation with outside air, the air masses have to be compared to determine whether or not the outside air will be beneficial to the space. Learn more about the operating principle of the ATMOXsphere, and why it’s so important to understand Humidity vs Dew Point.

Post Author: ATMOX TOM