Comments on: Are You Making This Mistake with Humidity?

By: Bill Spohn

Bill Spohn — Fri, 15 Aug 2014 10:00:35 +0000

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

By: Keith Nelson

Keith Nelson — Fri, 15 Aug 2014 01:32:55 +0000

Allison, I enjoyed your "
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

By: Allison Bailes

Allison Bailes — Thu, 14 Aug 2014 22:42:00 +0000

Foster L.:
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. ;~)

By: David Butler

David Butler — Thu, 14 Aug 2014 20:49:43 +0000

You missed my point. Whether
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.

By: Allison Bailes

Allison Bailes — Thu, 14 Aug 2014 20:41:46 +0000

David B.:
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.

By: David Butler

David Butler — Thu, 14 Aug 2014 20:00:05 +0000

Allison wrote: "I didn't
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.

By: Allison Bailes

Allison Bailes — Thu, 14 Aug 2014 19:49:30 +0000

David B.:
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.

By: David Butler

David Butler — Thu, 14 Aug 2014 18:48:34 +0000

Allison, not only does this
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.

By: Allison Bailes

Allison Bailes — Thu, 14 Aug 2014 18:40:37 +0000

Sandra W.:
Sandra W.: And we're among the companies that use Hobo data loggers. Did you see the article I wrote about them?

Hobos in the House - Let the Data Logging Games Begin!

For those who want to check them out, here's a direct link to the Onset website.

By: Allison Bailes

Allison Bailes — Thu, 14 Aug 2014 18:36:16 +0000

John N.:
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?