Comments on: Dew Point — A More Meaningful Measure of Humidity?

By: Cameron Taylor

Cameron Taylor — Thu, 13 Sep 2012 07:50:13 +0000

Commercial HVAC, where
Commercial HVAC, where consistent humidity control is important (such as in museums, libraries, clean rooms, dry rooms, etc.) are transitioning from relative humidity control to dew point control. RH sensors in the return air stream (a common location for measuring RH, along with in the conditioned spaces) can be skewed by elevated or depressed dry bulb temperatures, causing the system to overly humidify (for winter heating/humidification) or dehumidify (for summer cooling/dehumidification).

That said, each way we measure humidity has importance. Wet bulb is important when dealing with cooling towers and other forms of evaporative cooling. It also comes into play when evaluating a/c real time performance. Relative humidity is important when dealing with preservation of items, such as books, musical instruments, museum collections, etc.

That said, I personally find dew point useful to track when a building may require humidity to be added or removed. That is, an outdoor to indoor air dew point comparison can be useful in many ways when evaluating building air leakage.

By: David Butler

David Butler — Tue, 30 Aug 2011 07:05:21 +0000

Ted, hope those light bulbs
Ted, hope those light bulbs were LED's :->

When I lived back east, my brother and I used to combine camping trips with observing. When dew points were high, we would erect one of those wall'less utility tents over our observation planning table, where we kept the eyepieces and sky maps. Otherwise, everything would get soaking wet.

By: Ted Kidd

Ted Kidd — Tue, 30 Aug 2011 05:35:13 +0000

"This is because the
"This is because the night sky (absent clouds) acts as a black body radiator, absorbing heat from anything that has direct line-of-sight of the sky. You can observe this effect by noticing the absence of dew under a picnic table."

David, that is awesome! LIGHTBULBS FLASHING!! Thanks!!

Another nice article Allison!

By: Allison Bailes

Allison Bailes — Wed, 24 Aug 2011 05:53:50 +0000

Elaine:
Elaine: Thanks! Don't I know you from somewhere? ;~)

By: Elaine

Elaine — Wed, 24 Aug 2011 05:07:23 +0000

Well said, as usual. Plus,
Well said, as usual. Plus, it's always satisfying to use the word "datum" correctly. :-)

By: Allison Bailes

Allison Bailes — Mon, 22 Aug 2011 22:41:35 +0000

David:
David: Understanding temperature and moisture is endlessly fascinating! Thanks for mentioning this issue that puzzles so many HERS rater test takers.

By: David Butler

David Butler — Mon, 22 Aug 2011 22:29:13 +0000

One thing I learned as an
One thing I learned as an amateur astronomer is how outdoor surfaces can drop below the dew point even though the air remains above the dew point. This is because the night sky (absent clouds) acts as a black body radiator, absorbing heat from anything that has direct line-of-sight of the sky. You can observe this effect by noticing the absence of dew under a picnic table.

How does this apply to astronomy? The large front lens on a Schmidt-Cassegrain telescope easily fogs up, ruining your observing session (well, not so much here in AZ). Through my years of observing in the Southeast, I learned that dew point forecasts provided by the NWS are pretty much useless.

Fortunately, low-power heaters and dew shields are available to mitigate this problem. Dew shields work well because they dramatically reduce the amount of sky the lens is exposed to. I made my own very effective dew shield from Reflectix lined with black felt. So don't tell me that stuff is good for nothin ;-)

By: Allison Bailes

Allison Bailes — Mon, 22 Aug 2011 21:32:01 +0000

John: Yes,
John: Yes, of course, you're right, and let it be noted here in the comments that the appearance of liquid water on a surface depends on the nature of the surface. I probably should have mentioned it above but was planning to save that for a future article. Thanks for keeping me honest!

By: John Brooks

John Brooks — Mon, 22 Aug 2011 19:50:17 +0000

"If you have air with a
"If you have air with a dew point of, say, 20° C (68° F), you know that if that air finds any surface with a temperature of 20° C or below, water vapor from the air will condense and form liquid water droplets..."

Allison, Not Just "Any Surface"...It depends on the material(sorptive vs nonsorptive)

see Rose page 81&82

By: Allison Bailes

Allison Bailes — Mon, 22 Aug 2011 19:23:17 +0000

Matthew:
Matthew: Yep, we give our students a healthy dose of psychrometrics in our classes. In addition to the Trane psychrometric chart, which I've linked to directly here, I really like Bill Rose's book, Water in Buildings. I don't know if there's a better presentation of the theory and application of this crucial subject.