On Sat, Jul 19, 2003 at 05:03:07PM -0500, Dan Minette wrote:
> Ronn's closer to right on this than you are.
Dan, you and Ronn are both straying from the topic that we were discussing, and neither of you have addressed the question that was asked. Additionally, Ronn made an incorrect statement about conductivity, which I corrected.
> There is no arguing that heat capacity has something to do with this > phenomenon, but I see Ronn's statement relating to the long term lack > of thermal equilibrium. For example, we can have the air temperature > falling below 0F, while the frost line in the ground is very shallow > > So, since the essence of the condition he was describing is the long > time it takes to reach thermal equilibrium; the best single thing to > cite is the poor conductivity.
This is a really silly argument, Dan. The BEST single thing to cite is something directly relevant to the question which was being discussed, which neither you nor Ronn did. Please try to focus on the topic. As far as the tangent that Ronn went down, the best single thing to cite is Laplace's (or Poisson's) equation, which I did even before he went down the tangent.
I was responding to the following post:
At 11:52 AM 7/19/03 -0400, Erik Reuter wrote:
On Sat, Jul 19, 2003 at 10:55:49AM -0400, Robert J. Chassell wrote:
> Presuming the end caps are mostly low-conductive stone or regolith, > the major temperature determiner for the end caps should be, I think, > the air, although indirect light will have an effect.
I think that would be difficult. Heat conduction between air and a surface is usually very small compared to heat conduction by a solid, even a "low conductivity" solid. For example, just a few feet (say, 2m) below ground on earth, the temperature can be quite different than the air temperature. Ground temperature is often quoted as being about 10-15C year round, even during hot summers or cold winters. Presumably this is because the ground conducts heat much better than the air and spreads heat out evenly throughout the ground. Any heat absorbed by the ground from the air is conducted away and spread around resulting in a uniform temperature mostly independent of the air temperature. Also, the mass of earth is hugely greater than the mass of air, so even if the air transferred all of its heat to the ground, the ground temperature would not change a great deal. I am having a hard time describing what is really a 3 dimensional heat flow problem that could be simulated by assigning boundary conditions, heat sources, and solving Laplace's differential equation over the volume in question.
Comments follow:
At 11:52 AM 7/19/03 -0400, Erik Reuter wrote:
On Sat, Jul 19, 2003 at 10:55:49AM -0400, Robert J. Chassell wrote:
> Presuming the end caps are mostly low-conductive stone or regolith, > the major temperature determiner for the end caps should be, I think, > the air, although indirect light will have an effect.
I think that would be difficult. Heat conduction between air and a surface is usually very small compared to heat conduction by a solid, even a "low conductivity" solid. For example, just a few feet (say, 2m) below ground on earth, the temperature can be quite different than the air temperature. Ground temperature is often quoted as being about 10-15C year round, even during hot summers or cold winters.
So Erik was the one who introduced the topic he is now calling a "tangent": the question of why the temperature a few feet below the ground on Earth is nearly constant year-round.
Presumably this is because the ground conducts heat much better than the air and spreads heat out evenly throughout the ground.
As I said, this is _not_ true: the upper few feet of the ground does not conduct heat very well but acts as an insulator, keeping the temperature a few feet below the surface relatively constant regardless of seasonal variations in air temperature. FWIW, the same effect occurs on the Moon, where the lunar regolith acts as an insulator to keep the variations in temperature a few feet down small in spite of diurnal surface temperature variations on the order of 500°F.
Any heat absorbed by the ground from the air is conducted away and spread around resulting in a uniform temperature mostly independent of the air temperature.
No, that is not the primary reason that the subsurface temperature remains relatively constant. The primary reason is that any heat does not penetrate very far. This is of course why root cellars and caves remain at nearly constant temperatures of about 288K = 15°C = 59°F year-round.
Also, the mass of earth is hugely greater than the mass of air, so even if the air transferred all of its heat to the ground, the ground temperature would not change a great deal.
This is correct. And admittedly if the atmosphere contained many times more heat than it does — assuming we were even here to debate about it in that case ;-) — more total heat could be transferred to the ground. There would still be a substantial temperature difference between the surface and a point a few feet underground, though, because of the insulative properties of the soil.
I am having a hard time describing what is really a 3 dimensional heat flow problem that could be simulated by assigning boundary conditions, heat sources, and solving Laplace's differential equation over the volume in question.
You would still need to take into account the thermal conductivity of the soil. And observations still show that the temperature a few feet below the surface on Earth and the Moon stays nearly constant despite large diurnal or annual variations in surface temperature, showing that the material which composes these layers is a good thermal insulator, _not_ a good thermal conductor, which is what the statement in the previous post which begins "Presumably this is because the ground conducts heat much better than the air and . . ." seems to suggest.
Applying this to the subject line of the thread: some of those who have written about building orbital habitats have pointed out that a few feet of lunar regolith would serve to insulate the habitat against heat loss.
-- Ronn! :)
Ronn Blankenship Instructor of Astronomy/Planetary Science University of Montevallo Montevallo, AL
Disclaimer: Unless specifically stated otherwise, any opinions contained herein are the personal opinions of the author and do not represent the official position of the University of Montevallo.
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