FHB Logo Facebook LinkedIn Email Pinterest Twitter X Instagram Tiktok YouTube Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe
Log In
  • How-To
  • Design
  • Tools & Materials
  • Restoration
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
Log In

Discussion Forum

Discussion Forum

Humidity v. Condensation (hey Jim Blo…

| Posted in General Discussion on April 23, 2000 06:22am

*
Andrew:

You might try looking at a fan design handbook for a detailed treatment of the subject. Years ago I used a TI59 calculator to compute air density, grains of moisture, and specific heat using barometric pressure and wet/dry bulb temperatures.

The CRC handbooks of physics and chemistry should also have the information you are looking for.

Reply
  • X
  • facebook
  • linkedin
  • pinterest
  • email
  • add to favorites Log in or Sign up to save your favorite articles

Replies

  1. RickG_ | Apr 23, 2000 06:22pm | #1

    *
    Andrew:

    You might try looking at a fan design handbook for a detailed treatment of the subject. Years ago I used a TI59 calculator to compute air density, grains of moisture, and specific heat using barometric pressure and wet/dry bulb temperatures.

    The CRC handbooks of physics and chemistry should also have the information you are looking for.

  2. Guest_ | Apr 23, 2000 09:07pm | #2

    *
    Oh, it's not for me -- I had to learn that stuff for some Chem exam "years ago" anyway -- rather I tried to explain this stuff a few months ago to Jim and others, without feeling I had really "connected." Also, I may lift the Javascript used in the cited page to learn how to do live calculations on the Web.

    Wonder where my TI got to ... I deliberately bought one of those red LED ones when they started to retire them .. have an HP 11C too.

    1. Guest_ | Apr 23, 2000 11:42pm | #3

      *Hey Andrew, I think I understood perfectly what you so patiently explained about temperature change causing water vapor to become liquid as the air cools and can no longer suspend as much vapor.What I never got a straight answer on was my question regarding "dew". I'll save a lot of Socratic exchanges and restate my belief that dew occurs when the night air cools causing water vapor to turn liquid, and the water is pulled by gravity onto whatever is below it, regardless of the temperature of that object. I believe dew is "percipitation" - it falls from the air. I know full well that this is not conventional thought and half you guys are slapping your heads right now sayin' "what the heck is he thinkin'?". But think for a second about the fact that "light" and "heat" are the same thing in a different form. It comes from the sun as light and doesn't become heat until it strikes an object it can't penetrate, like earth. Then, that heat radiates back up from the earth, into the air. The ground, and the grass on it, are warmer than the air. So how can dew be the result of "warm moist air coming into contact with a cooler surface, like a blade of grass" (Webster's Dictionary). And so we begin again. Happy Easter.

      1. Guest_ | Apr 24, 2000 12:19am | #4

        *Jim,

        First congrads. . . I look forward to the read.

        The dew point is at what temperature a "gas" in this case air, will become saturated with water vapor. It has to do with the amount of water vapor present and the present temperature of the gas.

        Even though dew is like condensation there is one slight difference, dew is more like rain that it is expelled from the gas (air) when the dew point is reached. That's why it appears on "warmer" objects in the morning.

        Condensation is where a colder object comes in contact with warmer air and causes the water vapor present in the air to condense. This is a warming process. Hope this helps.

        View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle

        1. Guest_ | Apr 24, 2000 05:35am | #5

          *Yeah, that's what I think too, Joe. But when I raised this point last Fall, several of the old gang took turns disagreeing with me, falling back on phrases like "dew IS condensation". It got pretty unwieldy, rehashing the same thoughts over and over in a 100 post thread. The one nice thing was that it stayed quite civil and I never did figure out if the folks arguing the other side figured I was too dense to "get" their point but were too polite to say that, or if they realized there was merit to what I was sayin'. And thanks for the congrads. I am still stunned by the entire thing. I get an ear to ear grin every time I think about it.

          1. Guest_ | Apr 24, 2000 06:24am | #6

            *Straight from the National Meteorologicval Society - "When the outside temperature, and the relative humidity reach the same number, dew is formed. Dew is simply water vapor that has liquified." I live in a region that has dew, each and every night of the year. Not a single day goes by that dew does not form, and cover any exposed object, being metal, plastic, wood, concrete, grass, dirt, etc... It will get anything exposed WET. Dew usually fomra just after sunset, when the temperature has started to drop. Sometimes it does not form until the wee morning hours. I have an aunt that lives in Northern Califormia, ans they leave furniture outside at night with no fear whatsoever of it getting wet. Dew just does not occur in her area. Damndest thing I ever saw (mainly because I have dew every night, and always thought it a was a normal occurance everywhere).Go figure...James DuHamel

          2. Guest_ | Apr 24, 2000 09:41am | #7

            *"Precipitate" means "to fall" -- which dew doesn't do any more than fog does. Dew forms as a direct result of condensation, which is defined as the phase change from gas to liquid. If the condensation happens in midair, we call it mist , or fog , or clouds, etc.; on the ground it's called dew. We may have lots of different unscientific words for the liquid condensate, but it's all the same process.Dew first forms on grass and cars and other things that loose heat quickly. Yes, dew can form on the underside of objects, but it's going to take longer because heat won't radiate as quickly there as it does into the open sky. By analogy, ever slept out in a tent and woke to find it raining inside? That's the "dew" from your moist breath beading up on the cold tent skin and dripping (yech). Or noticed the fogging of a surface above a boiling teakettle? Same idea.It takes lots of little droplets merging into much bigger droplets to produce rain.

          3. Guest_ | Apr 24, 2000 10:19am | #8

            *Try http://www.last-word.com for great discussions from The New Scientist about everyday phenomena, such as why does hot water freeze faster than cold? (!) So why IS the sky blue? Why do the bubbles in Guinness go the wrong way? Are giraffes more likely to be struck by lightning?*And, hey, just found something on clouds/condensation: http://www.newscientist.com/ns/981024/lastword.html

          4. Guest_ | Apr 24, 2000 12:09pm | #9

            *

            Dew point. . . is a term to describe the moment that water vapor will be "released" from a gas when that gas becomes saturated with said vapor.

            When a gas becomes saturated depends on its temperature and the amount of water vapor available. Once the Dew point is reached the now liquid water will "gather" on any surface.

            View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle

          5. Guest_ | Apr 24, 2000 01:23pm | #10

            *u guys ain't suckin me into this 1..especially wit dat dense JimBlodgett...dew fallin outta da sky, the very thot..

          6. Guest_ | Apr 24, 2000 03:22pm | #11

            *Nothing covered (with a porch, tarp, etc...) will get wet because of dew. My porch and all of its contents stay dry while EVERYTHING else gets wet. Dew does not fall from the sky like rain. You can actually feel the air start to get moist, much like being in a fog. Only difference is you cannot see the dew. If you stand outside for any amount of time after the temp drop, you will get wet yourself. Dew will form on the metal of a car, the grass, cloth, or your skin at all about the same time. I personally have not seen anything other than a car windshield that gets wet a lot quicker than another object. But what do I know...James DuHamel

  3. Kevin_Powell | Apr 24, 2000 09:01pm | #12

    *
    Instead of math, have you thought of using a psychrometric chart to graphically illustrate how air properties change with changing hemp. or NH?
    I have found these charts invaluable when instructing lumber dry kiln operators the principles of air and moisture interactions.

  4. Guest_ | Apr 24, 2000 10:49pm | #13

    *
    Nothing like Ob-Fusco-ation -- dew doesn't fall, as was well explained. The National Weather Service was also mis-quoted by James. Since the dew point is the temperature the air would have to be cooled to reach moisture saturation, it will never be the same as the relative humidity, unless you had 100% relative humidity at 100 degrees. At the dew point for a certain amount of atmospheric moisture, the relative humidity will always be 100%.

    Now, what's the "formula" (meteorological conditions) for fog to appear?

    1. Guest_ | Apr 25, 2000 12:14am | #14

      *Weaks,

      The only trait worth mentioning about you is that you're a jerk. Its quite clear you have no idea what you're talking about. Unfortunately, you fit right in here with all the other jerks.

      It support of this I present this, you state. . .

      "Since the dew point is the temperature the air would have to be cooled to reach moisture saturation"

      You being the genius that you are , I guess it never occured to you that the temperature could remain constant and the amount of moisture could increase. This would also cause saturation in the gas and cause dew/rain/condensation, whichever you prefer.

      Once again you being one of such high intellect should have been able to discern the use of the word "like" in reference to rain. You should have also realized that the comparison was made not because they "fall" like the other genius stated but, because the mechanism that produces both is the same. . .saturation.Haven't you learned yet that's its not nice to reference me. . . I can have a field day with a guy like you. . .

      View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle

      1. Guest_ | Apr 25, 2000 12:42am | #15

        *Mr. Weeks,I did not quote the National Weather Service. You did. And... you did a lousy job of it at that. I quoted the National Meteorological Society. They state in a book (that I have in my hand at this moment) that "dew" (or condensation) forms whenever the ambient temperature and the relative humidity reach the same point. Whenever the temperature drops BELOW the humidity level, the moisture saturation increases. Another point... Dew is not a scientific word. Thomas Jefferson used the term in the late 1700's, and I assure you it was not in use as a scientific term. Dew is a common term given to the wetness that forms due to moisture vapor turning to a liquid. Now I live in an area that has HIGH humidity levels. The yearly average is in the high 80's/low 90's. At night, the temp drops below this level, and voila` - you have "dew". How many times a year do you personally experience "dew"? I live with each and every day of the year. James DuHamel

        1. Guest_ | Apr 25, 2000 03:32am | #16

          *well JAMES, it aint like u got a lock on this DEW thing... we get it all the time,... and fog too.. but Joe, i think ur being harsh on steve.. haven't u recognized his pattern yet...it's dr.jeykel and mr. weeks... he's real nice and then he has trouble at home, er on the job , so rather than take it out on those he loves, he takes it out on u..cuz its ur job... here on BT..ur a big boy joe, u can do this.....i no they had the same thing in Italy...see in Ireland, every village had a sin-eater.. and every one wud give their sins to the sin-eater.. that's u , joe. its ur turn in the barrel...so pile it on guys, joe is a big enuf person to do this.. OK .....STEVE... give him ur wurst...he's ready..hah, hah, hahor , we cud all go over to the tavern and have it out over a game of darts.......

          1. Guest_ | Apr 25, 2000 04:14am | #17

            *I bet your house keeps the porch a bit warmer. Good point about the tarp -- I figure the lack of dew underneath is for lack of air circulation; plus the air itself probably settles as it cools. When air at or below the dewpoint is disturbed, condensation occurs. Without such disturbance (such as the contaminants I mentioned above that provide nuclei for rain) supercooled air won't release the moisture but will be very unstable. I don't know how far this can be pushed. Ever seen freezing rain? Wild stuff -- supercooled rain that freezes on impact.

  5. RickG_ | Apr 25, 2000 05:24am | #18

    *
    Kevin is right-a good psych chart is much easier to use for a lot of applications involving RH, grains of moisture, etc. In the real old days RH instruments used horsehair. Not sure of the current technology but the chilled mirror was the way to go about 10 years ago.

  6. Guest_ | Apr 25, 2000 07:53am | #19

    *
    Hey Andrew,

    Actually, the house keeping the porch somewhat warmer has very little bearing on the matter. What actually happens when an object is covered is that the heat escaping from the earth is trapped underneath the cover, and helps keep the ambient temp of the immediate area under the cover a little higher than the ambient temp of the air outside of the cover. This is one reason why cloud cover helps keep the temp higher than a clear night. The cloud cover helps keep the earth's heat from escaping so fast.

    But then again, what do I know...

    James DuHamel

    1. Guest_ | Apr 25, 2000 06:12pm | #20

      *Another example of phase change -- liquid to vapor -- in an unstable "super" condition, this time superheating: http://www.newscientist.com/lastword/answers/lwa456bubbles.html I think this sort of Q comes up often.Any hey, this is a direct analog of the dew forming question: ice on windshields.A psych chart -- makes me think of a diagnostic tool to evaluate psychiatric illness -- wait, psychometrics are more primitive, "the psychological theory or technique of mental measurement."OH, it's psyCHROmetric, for example http://www.linric.com/psyc_1.htm

      1. Guest_ | Apr 25, 2000 06:15pm | #21

        *You're right, being protected from the open sky simply slows radiation. See the ice cite below.

        1. Guest_ | Apr 26, 2000 06:13am | #23

          *Hey Andrew....Loved the science link. The one about fish farts had me rolling.ThanksJames DuHamel

  7. Guest_ | Apr 26, 2000 06:13am | #22

    *
    I've been looking for the math to show how temperature, dew point, and relative
    humidity relate ... the topic has come up here in the context of venting and window
    failures, because of the problem of moisture condensing within walls or roofs, or on
    windows. Sometimes just playing with some numbers makes the relationship
    easier to see.

    Anyway, try this
    NWS Javascript
    calculator
    -- there are some other interesting formulas, also.

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Video Shorts

Categories

  • Business
  • Code Questions
  • Construction Techniques
  • Energy, Heating & Insulation
  • General Discussion
  • Help/Work Wanted
  • Photo Gallery
  • Reader Classified
  • Tools for Home Building

Discussion Forum

Recent Posts and Replies

  • |
  • |
  • |
  • |
  • |
  • |
View More Create Post

Up Next

Video Shorts

Featured Story

FHB Podcast Segment: Embodied Greenhouse Gas Emissions and the Building Codes

Could a building code update make your go-to materials obsolete?

Featured Video

How to Install Exterior Window Trim

Learn how to measure, cut, and build window casing made of cellular PVC, solid wood, poly-ash boards, or any common molding material. Plus, get tips for a clean and solid installation.

Related Stories

  • Podcast Episode 692: Introduction to Trade Work, Embodied Carbon, and Envelope Improvements
  • FHB Podcast Segment: Embodied Greenhouse Gas Emissions and the Building Codes
  • Old Boots Learn New Tricks
  • Install Denim Insulation Like a Pro

Highlights

Fine Homebuilding All Access
Fine Homebuilding Podcast
Tool Tech
Plus, get an extra 20% off with code GIFT20

"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Fine Homebuilding Magazine

  • Issue 332 - July 2025
    • Custom Built-ins With Job-Site Tools
    • Fight House Fires Through Design
    • Making the Move to Multifamily
  • Issue 331 - June 2025
    • A More Resilient Roof
    • Tool Test: You Need a Drywall Sander
    • Ducted vs. Ductless Heat Pumps
  • Issue 330 - April/May 2025
    • Deck Details for Durability
    • FAQs on HPWHs
    • 10 Tips for a Long-Lasting Paint Job
  • Issue 329 - Feb/Mar 2025
    • Smart Foundation for a Small Addition
    • A Kominka Comes West
    • Making Small Kitchens Work
  • Issue 328 - Dec/Jan 2025
    • How a Pro Replaces Columns
    • Passive House 3.0
    • Tool Test: Compact Line Lasers

Fine Home Building

Newsletter Sign-up

  • Fine Homebuilding

    Home building tips, offers, and expert advice in your inbox.

  • Green Building Advisor

    Building science and energy efficiency advice, plus special offers, in your inbox.

  • Old House Journal

    Repair, renovation, and restoration tips, plus special offers, in your inbox.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters

Follow

  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X
  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences
  • About
  • Contact
  • Advertise
  • Careers
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2025 Active Interest Media. All rights reserved.

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

  • Home Group
  • Antique Trader
  • Arts & Crafts Homes
  • Bank Note Reporter
  • Cabin Life
  • Cuisine at Home
  • Fine Gardening
  • Fine Woodworking
  • Green Building Advisor
  • Garden Gate
  • Horticulture
  • Keep Craft Alive
  • Log Home Living
  • Military Trader/Vehicles
  • Numismatic News
  • Numismaster
  • Old Cars Weekly
  • Old House Journal
  • Period Homes
  • Popular Woodworking
  • Script
  • ShopNotes
  • Sports Collectors Digest
  • Threads
  • Timber Home Living
  • Traditional Building
  • Woodsmith
  • World Coin News
  • Writer's Digest
Active Interest Media logo
X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Project Guides
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Online Learning

  • Courses
  • Project Guides
  • Reader Projects
  • Podcast

More

  • FHB Ambassadors
  • FHB House
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • X
  • YouTube
  • instagram
  • facebook
  • pinterest
  • Tiktok

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 70%

Subscribe

Enjoy unlimited access to Fine Homebuilding. Join Now

Already a member? Log in

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in

Privacy Policy Update

We use cookies, pixels, script and other tracking technologies to analyze and improve our service, to improve and personalize content, and for advertising to you. We also share information about your use of our site with third-party social media, advertising and analytics partners. You can view our Privacy Policy here and our Terms of Use here.

Cookies

Analytics

These cookies help us track site metrics to improve our sites and provide a better user experience.

Advertising/Social Media

These cookies are used to serve advertisements aligned with your interests.

Essential

These cookies are required to provide basic functions like page navigation and access to secure areas of the website.

Delete My Data

Delete all cookies and associated data