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greenhouse misconception



On Sat, 7 Feb 1998, LUDWIK KOWALSKI wrote:

I remember reading somewhere that a small greenhouse box in which glass was
replaced by an IR-transparent crystal had essentially the same temperature
for two substances. It is the absence of convection, they said, that is the
main cause of high temperatures in green houses.

But what if the air above this device was not allowed to convect? Bury
the NaCl-windowed greenhouse under a thick layer of aerogel, and compare
it to an IR-opaque greenhouse. When air is not convecting, would there
not be a difference?

Here's a thought-experiment I made as an undergraduate when I first
encountered the idea that "greenhouse effect" did not apply to
greenhouses. I'd heard that convection dominates the process, but these
were just words, and I sought gut-level understanding.

Rather than imagining the open atmosphere above the earth, I ignore
horizontal air movements in this way: imagine instead a small square
surface 1 meter on a side. Put it at the bottom of a 20KM square column
of air. Put walls around this column if you like. (In fact this is not
too far from the truth, since any horizontal wind just moves air between
neighboring columns. With a large grid of columns, horizontal wind does
little.)

__
|\_\|
| || / \
| || |
| || |
| || |
| || | 20 KM column of air
| || |
| || |
| || |
| || |
| || |
| || |
| || |
| || |
| || \ /
|_ ||
\_\
1 sq. meter of earth at
the bottom

Since the atmosphere is mostly transparent to sunlight, the sun does not
heat this air column much. Instead the 1 m^2 patch of land absorbs the
major portion. Sunlight heats the earth tremendously in comparison to the
air.

The earth then heats the bottom of the air column by conduction (and
radiation?) If I stick a 20KM stirring rod into my column and thrust it up
and down, this is analogous to convection, and the entire column of air is
therefor heated by the square of earth at the bottom. (or I could place
two of my walled columns together, poke holes between them at top and
bottom, and let a true convection loop operate.)

Now, what if we add a square membrane of transparent plastic right above
our patch of earth? It would stop the vertical mixing, but would still
allow sunlight to hit the earth. The air below the membrane would heat up
tremendously, but would not be allowed to mix with the air above the
membrane. We'd have a bubble of hot air trapped against the earth.

Chris Bohren in "Clouds in a glass of beer" points out that the thickness
of a glass greenhouse is also an issue. For dead-still nonconvecting air
with thick-glass greenhouses, the "greenhouse effect" could very well
dominate. Authors who use the traditional explanation of greenhouse
operation aren't entirely wrong, they just have ignored the more common
process.

I don't know if my thought experiment works for everyone, since I was an
EE student at the time, and a long column of various materials with
source/sink on either end resembles a transmission line or a resistor.

Also, now that I look at the details again after decades, I have
questions. The plastic membrane of the greenhouse still heats the
atmosphere column like the bare earth did. And the air BELOW the membrane
convects and allows that air to take on the same temperature as the earth.
Does the plastic membrane REALLY do nothing else than limit convection?
If the membrane was one molecule thick, so it did not limit conduction
between the air inside the greenhouse and the air outside, wouldn't the
greenhouse cool down as if the membrane was missing? Maybe "limit
convection" is a bad description, if the membrane is actually insulating
the air within the greenhouse from the air outside. Ah, I see it: even if
the membrane is infinitely thin, it still presents a 1m^2 area of contact
between the two bodies of air. Without the membrane, air within the
greenhouse streams upwards and takes on an immense contact area with the
outside air.

How can we write equations about this setup without involking the verboten
concepts "heat" and "heat flow"?

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William J. Beaty SCIENCE HOBBYIST website
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