Re: Greenhouse

[William Beaty <billb@eskimo.com>]

On Sun, 29 Sep 1996, Rauber, Joel              Phys wrote:

> > salt. Today we use polyethylene sheet for some greenhouses, the
> > same material that is used for infrared windows in vacuum apparatus.
>
> I have the following question:  Are not both affects happening 
> simultaneously and contributing to the warmth inside the greenhouse (or 
> car)? (It doesn't get to hot inside my car on a cloudy day, or at night, up 
> here in the upper midwest).  So it strikes me that one should be asking 
> which is the predominate affect?

Hi Joel!

I recall seeing an in-depth analysis of this problem years ago (I don't
have the ref.) The conclusion:  the "greenhouse effect" only becomes
significant for a real greenhouse in dead still air conditions.  If there
is any convective mixing proceeding vertically, then the "greenhouse
effect" makes only a trivial addition compared to the temperature rise
caused by inhibiting of convection.

I think of it like this: imagine an isolated square meter of earth's
surface with a 50km column of air sitting upon it.  

The sun heats the meter-square surface but not the atmosphere (at least
not much).  The square of surface then conductively heats the adjacent
air.  Winds then mix this hot air throughout the entire column, so the
square meter of earth is responsible for keeping the 50km deep atmosphere
warmed.

Now what if we placed a transparent plate slightly above our meter of
earth?  It would stop the convective mixing.  To a first approximation,
the air temperature under the plate would rise continuously as long as the
sunlight was hitting it, and the atmosphere above it would cool to
absolute zero!  If we add other effects back in, then we'll find that the
air under the transparent plate rises to a certain high temperature and
the atmosphere ends up cooler.  Now pack millions of these 1-meter columns
together, and you have the real situation.

Think of the high temperature within a car on a sunny day.  The car is hot
because it has trapped the hot which would otherwise get spread throughout
the entire atmosphere by convective mixing.  So, it's not hard to explain
why the inside of your car gets hot during the day.  It's harder to
explain why the temperature inside doesn't rise to infinity! 

Oooo, interesting tack:  the temperature inside a greenhouse would rise to
infinity if not for conductive and RADIATIVE cooling processes.  SO, for a
real greenhouse, the "greenhouse effect" doesn't explain the high
temperature inside, instead it explains the COOLING process which keeps
the temperature from rising towards infinity!  Twisted enough for ya? 

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