Are You Making This Mistake with Humidity?
I want you to think about humidity today. Look at the photo below. What do you see? Dew drops on blades of grass. Where did that water come from? Out of the air. What was it about the air that caused the water to go from the vapor state to the liquid state? Wait! Don't answer! If you go any further, you're likely to make the mistake that so many people do.
What is air?
Before I tell you what the mistake is, let's talk about air. What's it made of? Most people answer that question by listing the components of air and their percentages: 78% nitrogen, 21% oxygen, 0.9% argon... You can look up lists of all the common components, but there's another way to look at air that's more relevant to building science.
Air is made of only two components: dry air and water vapor. Dry air has all those things in the list minus water vapor. The reason we separate air this way in building science (and meteorology, too) is that dry air is made of things that remain in the gas phase throughout the temperature and pressure ranges we normally experience. Water vapor has the distinction of being the only component of air that regularly changes between gas, liquid, and solid phases.
Psychrometrics is the science of humid air, and that's where this way of looking at air comes from. The process of water vapor condensing on cool blades of grass at night is because the water vapor in the air has found a surface with a temperature below the dew point. It's reached the condition we call saturation. And that's where the mistake happens!
The humidity mistake
What is it that gets saturated when water vapor condenses and forms dew? Think carefully. Did you say it's the air? If so, you've just made the humidity mistake.
Recall that we just said air is made of two components: dry air and water vapor. Compared to solids and liquids, air has a really low density. Yeah, there are a lot of molecules, but they see lots of empty space as they zip around. We can assume (to a satisfactory level of accuracy) that the dry air and water vapor molecules do not interact with each other, each behaving like an ideal gas and obeying the ideal gas law.
What that means is that the dry air component has no effect on whether or not the water vapor molecules reach their saturation condition—dew point. The water vapor could be in the space all by itself and you'd still get dew at the same temperature.
So if you say things like the following, you're making the humidity mistake:
Dew forms when the air is saturated with water vapor.
100% relative humidity means the air contains the most water vapor it can hold.
"Dry air is like a sponge, it soaks up moisture from wherever it can find it."*
Technically, the first two statements could be seen as correct, but it's better to substitute the word "space" for air. If you want to be correct in what you say about humidity—and who doesn't—then you'll understand that dew point or saturation has nothing to do with the dry air component. It's all about the water vapor and the temperature of the air, which is a mixture of dry air and water vapor.
This isn't an easy topic to wrap your head around. Although I used condensation of water vapor on blades of grass to talk about the condition of saturation, my concern here is with the state of the mixture of dry air and water vapor and how we talk about it, not the phase change.
I decided to write on this topic last night when I was reading Don Gatley's book, Understanding Psychrometrics. On page 23, he wrote, "Saturation water vapour pressure depends on the temperature only." And later on the same page, "...saturation has nothing to do with the air. A saturated water vapour condition exists when the volume contains the maximum possible number of water vapour molecules." Then he quoted C.F. Marvin from a 1900 publication on psychrometrics:
Faulty Conceptions. A false notion that the air has a certain capacity for moisture is wideley prevalent, and is perpetuated by all such expressions as "The air is partly saturated with moisture," "Weight of aqueous vapour in a cubic foot of saturated air," etc. It should always be clearly observed that the presence of the moisture in any given space is independent of the presence or absence of air in the same space (except that the air may slightly retard the diffusion of the vapour molecules). It is more correct to say, in the above cases, that "the space is partly saturated with moisture," or that "the moisture is in a partly saturated condition or is superheated."
The takeaways here are:
- Air does not get saturated or partly saturated with water vapor.
- A volume of space can be saturated or partly saturated with water vapor.
- The degree of saturation depends on how many water molecules are in a volume of air and the temperature of the air.
The psychrometric chart puts all the variables together so you can see what happens under changing conditions of temperature and moisture content. If you want to understand air and humidity, that's where it all comes together.
Understanding Psychrometrics by Donald P. Gatley
Water in Buildings by William B. Rose
*I found this quote by doing a quick search on "air is like a sponge."
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