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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.
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Replies
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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.
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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.
*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.
*Jim,
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
*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.
*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
*"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.
*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
*
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
*u guys ain't suckin me into this 1..especially wit dat dense JimBlodgett...dew fallin outta da sky, the very thot..
*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
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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.
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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?
*Weaks,
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
*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
*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.......
*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.
*
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.
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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
*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
*You're right, being protected from the open sky simply slows radiation. See the ice cite below.
*Hey Andrew....Loved the science link. The one about fish farts had me rolling.ThanksJames DuHamel
*
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.