From: throopw@sheol.UUCP (Wayne Throop)
Newsgroups: talk.origins
Subject: Holden and the greenhouse effect
Summary: review of a simplified greenhouse
Date: 05 Nov 92 01:29:12 GMT

:: From: news@fedfil.UUCP (news) (Ted Holden)
:: It is the Sagan version of SGH which has
:: the heat CAUSED by the 2% or less of solar energy, without bothering to 
:: explain why the middle cloud layer which absorbs the vast bulk of the 
:: remaining 98% of solar energy is COOLER which violates the 2'nd law.

No, SGH has the TEMPERATURE of the surface caused by an input of 2% of
the input heat.  Ted still confuses rate with quantity, temperature
with heat, and still hasn't analyzed how the heat can escape the
surface.

: From: mrjg8679@uxa.cso.uiuc.edu (Michael R James)
: Message-ID: <Bwz703.9yn@news.cso.uiuc.edu>
: You will have to do better.  Picture a house with one window
: heavily insulated from the outside.  The walls of the house can
: absorb 98% of the incoming radiation and still remain cooler
: than the interior (assuming we are sufficiently insulated
: of course).

The "we are sufficiently insulated" applies mainly to the window.  The
walls can be insulated any way at all (as long as they are opaque and
windproof), and the interior would HAVE to be hotter, since to drive
heat out through the walls, the "little 2% that could" will have to
boost the internal temperature above that of the walls, since no heat
can be transfered from the interior *through* the wall until it has
been transfered from the interior *to* the wall.

This little greenhouse with a small window is a very good model, by the
way.  It leads well to the realization that Ted's model of the
situation without internal residual heat, namely a cool surface and a
hot atmosphere, simply cannot happen, no matter HOW little energy gets
to the surface.  The surface must be *at* *least* as hot as the
atmosphere once a stable state is reached, and that's even if zero
energy reaches the surface.

To see why, consider the ways heat can escape from the surface of
venus, assuming the surface is cooler than the atmosphere.  There are
essentially three cases.  First: it could radiate out.  But if it is at
a lower temperature than the overlying atmosphere (true by hypothesis),
and if the atmosphere is effectively opaque in the thermal spectrum the
surface emits (true of venus' CO2-rich atmosphere), the surface will
gain more heat from the blackbody radiation from the atmosphere than it
will lose.  It will get hotter, until it equalizes with the
atmosphere.  To do otherwise would mean that heat was transfered from a
cool surface to a hot atmosphere.

Second: it could convect out, because "heat rises".  But the atmosphere
above is hotter still (by hypothesis), so the atmosphere next to the
surface stays put (because it's cooler than that just above) until it
equalizes with the higher atmosphere.  To do otherwise would mean
cooler gas was rising above warmer gas.

Finally, it could conduct out.  But the only direction heat would flow
is from the hotter atmosphere to the cooler surface, again until they
equalize.  So we can see quite clearly that there's just no way for the
heat to escape the surface, until the surface is at least as hot as the
atmosphere.  And any solar energy input *at* *all* at the surface, even
as little as 2%, or 1%, or 0.0001%, will make the surface *hotter* than
the upper atmosphere.  Only the 2% makes it in, but *nothing* makes it
back out until the surface is hotter than the atmosphere.

The consequence is, *Ted*'s picture of what would happen if there were
no internal heat on venus violates thermodynamics, since it requires
heat to be transfered from a low-temperature surface into and through a
high-temperature atmosphere.

The small-window greenhouse model Michael brings out above makes this
clear (if heat can't go back out the window), since it focuses us on
how to drive heat out through the wall, and this can only happen by
conduction (since the wall is solid and opaque).  Given this simple
case, we can then see that the other two possibilities for venus
(convection driven upwards in a gravity well, and radiation into an
opaque band of the atmosphere) must behave in exactly the same way.
The case analysis above is oversimplified, ignoring for example the
role of pressure differences in the extreme temperature difference
between the top and bottom of the atmosphere, but it captures the
essence of the situation.

It's very clear that the thermodynamic egg is on Ted's face, not Sagan's.
--
Wayne Throop  ...!mcnc!dg-rtp!sheol!throopw