On Sat, Jul 19, 2003 at 06:02:21PM -0500, Ronn!Blankenship wrote:
> At 06:20 PM 7/19/03 -0400, Erik Reuter wrote: > > >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.
Did you read what I wrote? Do you claim air conducts heat better than dirt? Do you claim that the temperature of the air is more homogenous than the temperature of the dirt?
By your statement, the atmosphere should all be at the same temperature because air is a good insulator.
> 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.
No, that is not the primary reason. The primary reason is that temperatures follow Laplace's (or Poisson's) equation which depends on the conductivity and heat capacity and geometry of the system. AS I SAID BEFORE.
> You would still need to take into account the thermal conductivity of > the soil.
Which I ALREADY WROTE in my original post: that is part of solving Laplace's equation.
> 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.
No, I did not write that the earth is a good thermal conductor. Since you quoted it, maybe you can read it again. If I am wrong that dirt conducts heat better than air at 1atm, then please cite some numbers. If I am wrong that solving Laplace's equation is the most accurate way to model the system, then what is your model and how is it better?
Perhaps I have misunderstood the question.
What I have been saying is that if I were to perform the experiment by obtaining a 2-meter or so probe with a sharp end, mount a thermocouple at that end, and drive it into the ground, then place an identical thermocouple at on the surface, the one on the surface would record substantial variations in temperature both diurnally and annually, while the one a couple of meters below the surface would report a nearly constant temperature in the neighborhood of 288K, and that the primary reason for this is that the soil does not conduct heat well. If this is not what you were referring to, I'm sorry if I misunderstood you. While indeed I can make a mathematical model of the heat flow, if the results of that model do not match the observations, it is the model which is incorrect and must be changed. I don't know of any way to derive an accurate value for the thermal conductivity of the soil from theory: this is something which must be measured empirically and plugged into the equations in order to get the correct results from the model.
I hope we agree on the above.
If so, all I have been saying is that the primary reason that the temperature a few feet below the surface of the Earth or the Moon � where it has been actually measured � remains nearly constant while the temperature at the surface varies by tens or hundreds of degrees is that the material which forms those few feet is a poor conductor of heat.
> 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.
You STILL haven't addressed the question of modeling the air temperature inside the habitat.
That's true. The only issue I have been attempting to address this afternoon is the one of why the temperature a few feet below the surface of the Earth remains nearly constant.
-- 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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