Are You Making This Mistake with Humidity?

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water words vocabulary dew point psychrometrics hygroscopic

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.

Afterword

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.

 

Recommended Books

Understanding Psychrometrics by Donald P. Gatley

Water in Buildings by William B. Rose

 

Related Articles

Dew Point — A More Meaningful Measure of Humidity?

What Happens When You Put a Plastic Vapor Barrier in Your Wall?

Air Barriers, Vapor Barriers, and Drainage Planes Do Different Jobs

 

*I found this quote by doing a quick search on "air is like a sponge."

 

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Comments

John Nicholas

Good Points always help people keep these technical issues easy to handle and apply.  
 
Would you expand on the barometric pressure issue?

Curt Kinder

This seems to be a distinction without a difference. 
 
If anything, it obfuscates rather than clarifies. 
 
I have enough difficulty conveying the concepts of relative humidity, absolute humidity and dewpoint without getting tangled up in what you seem to be hinting at - a discussion of partial pressures and / or vapor pressures. 
 
Somewhat along the same lines, I like to ask prospective HVAC technicians a two part question:  
 
"Assuming an AC is operating and producing condensate, what is the approximate relative humidity of the supply air as it leaves the air handler coil?" 
 
Once we work our way around to the fact that supply air has very high RH...part two asks "How does that high humidity air dehumidify a room?

Sandra Wiechmann

To get a handle of Temperature/RH issues as they relate to HVAC supply air and comfort levels of occupants, many HVAC contractors and Facility Managers use our loggers to understand the situation. What is the true comfort level of a space and what temperature/RH level can be maintained to achieve this at the most efficient energy consumption level....our loggers can help you to answer this question. Have a look at the learning section of our where you will find a number of white papers and webinars that address these questions. 
www.onsetcomp.com

Allison Bailes

John N.: Thanks for asking about barometric pressure. I've removed the reference to it since it's not necessary to go down that path in this article and it sounds like I'm contradicting myself when I say dew point doesn't depend on air but it does depend on atmospheric pressure. The answer to that conundrum is altitude. The saturation curve for water vapor changes with altitude, so just as water boils at a different temperature in Aspen, Colorado, it also condenses at a different temperature. 
 
Curt K.: Whether there is a difference or not depends on your objective. If you're a Florida homeowner who just wants the air conditioner to dehumidify, it doesn't really matter how you talk about it. But if you're in the business of understanding why water vapor does what it does, it IS important to understand this distinction, especially if you really want to know the psychrometric chart. 
 
Those are great questions for your techs! Do you then go on to explain dew point? 
 

David Butler

Allison, not only does this article obfuscate (thank you Curt), but it's not entirely correct. While what you say is correct as far as it goes, the temperature of the grass has a lot more to do with dew that forms (or the temperature of a window or uninsulated metal duct that condenses). 
 
The clear night sky is sometimes referred to as a perfect black body radiator, so exposed surfaces like grass and cars will get colder than the air (or water vapor in the air), whereas the areas shaded from the sky (under a picnic table or beneath a car) will be a bit warmer. That's why my telescope corrector plate or lens used to fog up a lot when I used to live back east.

Allison Bailes

David B.: OK, when two of my smartest commenters accuse me of obfuscation, that means I'm about to learn how to explain something better. My point in the article is that the air isn't a giant sponge. I didn't want to go too far into the condensation issue, but you can't avoid talking about it because that's where the action is. I've cleaned up a couple of sentences about dew point and will add another paragraph to highlight the point I'm making.  
 
Austin: Absolutely. That's the point David Butler was making, too. I'm talking about the condition of the air and water vapor mixture here, though, and trying to make the point that the air isn't a sponge. 
 
Ted K.: Yes. I linked to it in the first paragraph and the Related Articles section.

David Butler

Allison wrote: "I didn't want to go too far into the condensation issue..." 
 
I think that's the problem. Using grass and dew as your lead-in sentence pretty much defines what the article is about. Perhaps rain (which doesn't involve anything other than dry air and water vapor) would have been a better analogy for the point you're trying to make.

Allison Bailes

David B.: You wrote: "Using grass and dew as your lead-in sentence pretty much defines what the article is about." I've reread the article a few times now, and I have to disagree. After the opening paragraph, pretty much everything else is about air, water vapor, and saturation. I mentioned dew a couple more times but only in the context of saturation.

David Butler

You missed my point. Whether you intend it to or not, the lead-in paragraph puts the reader's mind in a certain place to absorb the details that follow. In this case, your lead takes the reader in the wrong place (condensation on a surface), when the point you're trying to make is about properties of water vapor and dry air. Like I said, rain would have been a better analogy for the lead-in.

Allison Bailes

Foster L.: Yes! Unless it gets stripped down to bare nuclei and becomes a plasma. Or floats up into the attic because it's so light. ;~)

Keith Nelson

Allison, I enjoyed your "obfuscation". The distinction between the saturation of air versus the saturation of space, for me, was the missing link to clarify why "moist air" is lighter than "dry air". This concept has always irked me and now it doesn't... water vapor (H20) is lighter than air (N2/O2). I am sure this is an over simplification... 
Thanks, 
Keith

Bill Spohn

Allison,Thank you! 
You always dew point us in the right direction.