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#1 Otto Piechowski

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Posted 25 July 2013 - 10:25 PM

I have some questions about dew, which I would like to ask and to which I’d like some scientific response.

Specifically, dew doesn’t fall; does it?

And since we are talking about dew, let’s just lay out all the questions I have about dew, scientifically speaking:

Question 1: Dew doesn’t fall; right?
Question 2: What is the cause of dew?
Question 3: What is this radiative cooling thing and how does it work?

#2 deSitter

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Posted 25 July 2013 - 11:01 PM

There are three ingredients for dew - exposure to the sky, calm winds, and high humidity.

Any object will attempt to come into thermodynamic equilibrium with its environment. If something is left under the sky, then part of that environment is deep space, which is very cold. An object will radiate away its heat energy into the infinite reservoir of space. Counteracting this is heating by convection of the surrounding air. If he air is very still, a layer sets up (boundary layer) that is very close to the object, and stagnant, that is, hard to disperse. This layer cannot be easily warmed by convection, and loses more heat energy to space than the ambient air, and so is colder than the air just above it. If the boundary layer gets cold enough, the water vapor in this cooler air will condense out like sweat on a beer bottle. That's dew.

Dew can be prevented by minimizing exposure to the sky. That's what a dew cap on a refracting telescope does.

So dew forms, not falls.

-drl

#3 Otto Piechowski

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Posted 25 July 2013 - 11:11 PM

Danny, thank you. That's clear...except for one thing.

You wrote, "An object will radiate away its heat energy into the infinite reservoir of space."

I have a very hard time getting my mind around that concept your words clearly describe. It doesn't make sense to me that the infinite reservoir of perfectly cold space which is so far away, would have an impact on my little telescope, because my little telescope is a mile or more from that reservoir. I don't see how it (space) impacts it (the scope).

Is it that that infinite reservoir of space and its coldness is impacting the body/column of air; the heat in that body/column of air, which is in contact with the scope? Is that it?

I know its hard to believe, but I have a hard time getting my mind around this radiative cooling thing. If the air is cooled by that infinite reservoir of space (I like the poetic image your words convey by the way), what can't that awesome reservoir just equally impact my scope when it happens to be just slightly, but directly, underneath the overhanging canopy of a tree?

Otto

#4 deSitter

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Posted 25 July 2013 - 11:18 PM

A warm object gets colder by emitting infrared radiation. Space will just suck all this up and not return anything above a few degrees K. In contrast the telescope will also attempt to come into radiative equilibrium with the house, trees, etc. which are at more or less the same temperature, so more or less as much will be received as given. One good thing to do when the telescope is not in use, is to point it at something warm, like the ground, instead of space.

-drl

#5 Otto Piechowski

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Posted 25 July 2013 - 11:26 PM

One of the reasons I like use a small refractor is because I can also protect it from dew forming on the objective by that highly scientific and technical act, described in liturgical words as, "laying on of the hands".

With say, a three inch refractor this works well. You wrap your warm palms around the dew shield and around the OTA just behind the objective and you can feel, sometimes, the OTA go from damp to dry. Kind of cool. As to the wording, it was my friend Jeff Barbour who came up with that. First time he used it I laughed at the alternate use of a liturgical phrase.

Otto

#6 GlennLeDrew

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Posted 25 July 2013 - 11:45 PM

At a given air temperature, stand outside at night under a cloudy sky and a clear sky. You will feel colder under the clear sky.

A clear sky of typical moisture content will present an equivalent black body temperature of roughly -30C, winter or summer, day (yes, day) or night. And so while not at a few degrees Kelvin (!), the sky does present as a pretty cold heat sink.

During the day, the Sun's energy more than compensates for the heat radiating into the sky.

Cloud cover typically has a base temperature warmer than -30C (unless very high cirrus). And so cloud usually acts somewhat like a blanket.

Under a clear sky exposed objects cool radiatively. But to some extent this is ameliorated by the (usually, away from cold northern winter locales) warmer ground, vegetation, etc. and so the full radiative balance determines the rate of cooling.

On top of this, conduction via the air in immediate contact plays a part in the cooling rate. If the air is still, radiative cooling chills the air in contact with the ground, thus bringing the air temperature down at a higher rate. But a breeze stirs up the air, mixing warmer air from higher above the ground downward through turbulent interchange, this retarding the rate if air cooling,

This is a *very* brief introduction to the subject!

Oh, dew most certainly does not fall, as it is not precipitation. It condenses spontaneously and directly on a suitably chilled surface.

#7 Jarad

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Posted 26 July 2013 - 06:49 AM

Glenn's answer was pretty thorough. I will just add that the reason it usually looks like dew "falls" is because tends to form on the surface exposed to the sky, which is usually the top. The bottom surface of most object is radiating toward the ground, and receiving radiation back from the ground, so usually stays warm enough not to get condensation on it.

But you can easily get dew on the bottom of a cold object. On my porch, we have glass-top table. It's quite humid where I live (annoyingly so), and if I put a cold drink on top of the table, it cools the glass below it, and condensation forms on the bottom of the table under the drink (not to mention the sides of the drink itself). So dew can condense from any direction.

Jarad

#8 deSitter

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Posted 26 July 2013 - 08:47 AM

A clear sky of typical moisture content will present an equivalent black body temperature of roughly -30C, winter or summer, day (yes, day) or night. And so while not at a few degrees Kelvin (!), the sky does present as a pretty cold heat sink.


DUH! (red faced) of course you are right

-drl

#9 Otto Piechowski

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Posted 26 July 2013 - 09:36 AM

Danny, Glenn, Jarad, thank you.

I agree, dew forms and does not fall. And I understand why it forms; the cold causing humidity in the air to condense on a cooler surface.

It's that cooler surface and the radiative thing I'm just not getting; though you have explained it well.

I suspect you guys know some things about heat which I do not.

I think of my metallic OTA. It contains heat. I think of heat as a substance, a quality, a stuff; not stuff like metal to be sure, but something with substance to it. Stuff, quality, substance can be taken away from the OTA....but it seems to me it has to flow away, transmit away by contact with something else like a cloth wiping paint off a surface.

this idea of radiative cooling, infrared rays transmitting heat away from the OTA at, I guess its "c", through an atmosphere that has substance of its own and its own heat...I'm just having a problem.

I know how this works, this understanding thing; I need to reread your words a few more times, let them percolate in the presence of actual experience of watching the OTA or SCT corrector dew up on a pristine night, or not dew up under some conditions and then it will just....bang....I got it.

Otto

#10 Jarad

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Posted 26 July 2013 - 11:10 AM

think of my metallic OTA. It contains heat. I think of heat as a substance, a quality, a stuff;


It is energy. It can leave in a couple of ways: blackbody radiation, conduction, or convection.

Conduction and convection are through contact with objects or air. We'll ignore those for the moment.

The one most relevant to dew is radiation. Every object is constantly emitting blackbody radiation. The intensity and spectrum depends on the object's temperature and composition.

Every object is also absorbing radiation from its surroundings. How much depends on it's absorbance (color), and on how much radiation is hitting it.

So for your scope, during the day the sun is in the sky. The sun is very hot, and gives off lots of radiation in the infra-red to ultra-violet range. This hits the top side of your scope, adding energy. Your scope is radiating energy in all directions, but at much lower intensity and longer wavelengths (mostly infra-red). So during the day it absorbs more energy than it gives off, and it gets hot. The top side gets hotter than the bottom side, since the bottom side isn't getting radiation from the sun, only from the ground (which is much lower intensity, since the ground is about the same temperature as the scope).

At night, the top of the scope is exposed to the night sky. So it radiates infra-red to the sky, but receives much less back. So now the top cools down. The bottom radiates infra-red to the ground, but since the ground is about the same temperature as the scope it receives back about the same amount of energy as it radiates, so it doesn't cool much.

So the top cools faster than the bottom, and gets below the dew point first. So dew forms on top. If your tube is metal, it conducts fairly well, so the bottom will start to cool by conduction and eventually dew up as well. Aiming a hair-dryer at it will blow warm air over it, adding heat by conduction and convection, warming it up and evaporating the dew.

The actual temperature change depends on the total transfer of energy by radiation, absorption, conduction and convection. When something receives more energy than it gives off, it gets hotter. When it gives off more than it receives, it gets colder.

Make more sense?

Jarad

#11 GlennLeDrew

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Posted 26 July 2013 - 11:26 AM

The 'something' of heat 'contained' by a substance is the vibration of the atoms/molecules, or if free to move (as in a gas) their speed of motion. At absolute zero all such movement ceases.

#12 Otto Piechowski

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Posted 26 July 2013 - 11:46 AM

Yes, it does make sense.

I have, until now, never begun to appreciate this thing of infra-red radiation and its role in transmitting heat.

Thank you. Otto

#13 ColoHank

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Posted 26 July 2013 - 11:51 AM

I have a very hard time getting my mind around that concept your words clearly describe. It doesn't make sense to me that the infinite reservoir of perfectly cold space which is so far away, would have an impact on my little telescope, because my little telescope is a mile or more from that reservoir. I don't see how it (space) impacts it (the scope).



"Thou canst not stir a flower without troubling of a star."
Sir Francis Thompson

#14 Otto Piechowski

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Posted 26 July 2013 - 12:52 PM

I read it a second time, Jarad. It really does make sense to me. I was getting caught up on the concept of infinite cold space sucking heat out of the OTA. But what is going on is the OTA is always emitting heat in the form of infrared radiation and, at night, is getting very little heat back from the direction of outer space.

Glenn, at absolute zero the vibration of atoms/molecules etc. ceases.

First of all, can things get to absolute zero? I think the answer will be "no", but I want to ask.

Second, if a thing could get to absolute zero, would it then cease to exist?

Otto

#15 Otto Piechowski

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Posted 26 July 2013 - 12:54 PM

Hank, for philosophical reasons I believe the statement "Thou canst not stir a flower without troubling of a star." is correct.

From the perspective of physics, is this statement correct? to what degree correct? in what sense(s) correct?

Otto

#16 llanitedave

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Posted 26 July 2013 - 08:36 PM

First of all, can things get to absolute zero? I think the answer will be "no", but I want to ask.

Wikipedia is your friend here. The article, rightly or wrongly, states pretty firmly that "absolute zero cannot be achieved". We can get within billionths of a degree, though.

Second, if a thing could get to absolute zero, would it then cease to exist?


No, but I don't see how we could detect it, since we detect objects by the radiation they emit. It it's at absolute zero, it's emitting no energy. Anything we do to it to make it detectable would have the effect of imparting energy to it, and then it would no longer be at absolute zero.

#17 Otto Piechowski

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Posted 27 July 2013 - 09:56 AM

"No, but I don't see how we could detect it, since we detect objects by the radiation they emit. It it's at absolute zero, it's emitting no energy. Anything we do to it to make it detectable would have the effect of imparting energy to it, and then it would no longer be at absolute zero. "

Well, that certainly makes sense, Dave.


OK, you've gotten me to feel I understand how radiative cooling contributes to dew formation.

Now, I would like your instruction about the atomic/subatomic end of radiative cooling.

Does heat cause the atoms/molecules to vibrate (dance, shimmy and shake, whatever it is they do) or is this vibration what heat is?

What is the process/mechanism that cause the vibration to become infrared radiation?

Infra-red is just photons of a particular energy in the electro-magnetic spectrum, just to the side of visible light photons. Right?

#18 Mister T

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Posted 27 July 2013 - 10:32 AM

Wouldn't it still have gravity/mass?

would it condense upon itself into a black hole? :question:

#19 llanitedave

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Posted 27 July 2013 - 09:31 PM

"No, but I don't see how we could detect it, since we detect objects by the radiation they emit. It it's at absolute zero, it's emitting no energy. Anything we do to it to make it detectable would have the effect of imparting energy to it, and then it would no longer be at absolute zero. "

Well, that certainly makes sense, Dave.


OK, you've gotten me to feel I understand how radiative cooling contributes to dew formation.

Now, I would like your instruction about the atomic/subatomic end of radiative cooling.

Does heat cause the atoms/molecules to vibrate (dance, shimmy and shake, whatever it is they do) or is this vibration what heat is?

What is the process/mechanism that cause the vibration to become infrared radiation?

Infra-red is just photons of a particular energy in the electro-magnetic spectrum, just to the side of visible light photons. Right?


Remember that the outer boundary of every atom is a negatively charged electron shell. The electrons are spaced at well-defined distances from the nuclei depending on their individual energies. When you impart energy to an atom, the part you are energizing is the electrons, making them jump to a higher orbits.

Expanding the outer shell means the atom is occupying a greater volume, and is more likely to interact with other atoms. In a dense substance, of course, atoms are already interacting, and since it's the negatively charged electrons that are encountering one another, there's going to be a repulsive effect (it's a lot more complicated than that, of course, since the positively charged nucleus can't be ignored), and the electrons are going to be undergoing changes of momentum and exchanges of energy as a result. If a photon loses energy in the exchange, it falls into a lower orbital.

Any time an electron moves from a more energetic orbit to a less energetic one, it emits a photon -- the energy of the photon corresponds to the quantum energy level of the electron shells that were jumped between. Opposite when an electron absorbs a photon. If the photon is of the right energy, the electron will absorb it and jump to the appropriate orbital shell. Otherwise, the photon will be ignored.

Since atoms are always moving, this little approach-and-retreat interaction between the outer shells not only transfers the atomic momentum to nearby atoms, but it squeezes some electrons into lower orbitals, causing them to give up energy as photons. These photons might be absorbed by other electrons in other atoms, but they might also escape from the substance. The net effect is that there's less energy, and the substance cools down.

If you bombard it with photons from outside, some of these photons will be absorbed by atomic electrons, and they'll react by expanding their shells, jostling the atoms alongside them and continuing the dance.

Another thing to remember is that temperature is not the same thing as heat. Heat is the total thermal energy contained by a substance, temperature is the intensity of the motion of the atoms within it. A very dense substance may have a lower temperature than a less dense one, while still containing more heat -- either because there are more atoms to move, or the individual atoms contain more momentum. Heat radiation is not merely infra-red -- a very hot substance can give off visible light as well, as the burners on our stove remind us. The frequency of the photons emitted is a function of the temperature of the substance, the total energy contained within ALL the photons emitted is a function of the heat being transferred.

This was all a pretty clumsy explanation, but I hope it triggered in someone else the desire to relate it more clearly.

#20 Otto Piechowski

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Posted 27 July 2013 - 10:13 PM

Thanks, Dave. I could understand that explanation easily. It was helpful.

Otto

#21 Otto Piechowski

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Posted 27 July 2013 - 10:16 PM

That was an easy to understand and helpful explanation, Jarad.

I am curious about the word blackbody. I have heard that word use in association with heat and in association with quantum physics before, but I have never thought to ask; why do we refer to blackbodies when we are talking about heat? I suspect the word black doesn't (so much) refer to color as to a certain energy state, but I am only guessing.

Otto

#22 gavinm

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Posted 27 July 2013 - 11:41 PM

Blackbody refers to an object where the colour of the light depends only on its temperature (as opposed to burning something like copper that is green because of electron transitions) - think of iron at a blacksmith - red hot, white hot etc

Stars are kind of blackbody (ie hot-blue, cool-red) but with lots of other things going on as well to complicate things..

#23 StarWars

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Posted 28 July 2013 - 03:39 PM




Dew Point... :grin:

http://en.wikipedia.org/wiki/Dew_point

The dewpoint temperature is the temperature at which the air can no longer "hold" all of the water vapor which is mixed with it, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature.

If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%.

#24 Carl Coker

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Posted 05 August 2013 - 04:51 PM

That was an easy to understand and helpful explanation, Jarad.

I am curious about the word blackbody. I have heard that word use in association with heat and in association with quantum physics before, but I have never thought to ask; why do we refer to blackbodies when we are talking about heat? I suspect the word black doesn't (so much) refer to color as to a certain energy state, but I am only guessing.

Otto


We call them blackbodies because a perfect blackbody absorbs all radiation that hits it, so it is black by definition. Similarly, Black paint is black because it absorbs all visible light that hits it.






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