Re: atmospheric blanket / greenhouse effect

[Bernard Cleyet <anngeorg@PACBELL.NET>]

Pse. read "in the light of" JD's recent post.

bc

"Come to think of it, two lines of evidence suggest
that the Earth's atmosphere is not very optically
thick.
 -- The night sky in the high desert is perceptibly
colder than the night sky in a humid locale, due
to radiative cooling.  So certainly it is *sometimes*
thin."


Overnight our bedroom (2nd floor) dropped only two deg. F.   A few days
ago it dropped more than ten.  I ascribe the difference to thick fog and
cloud cover.

For clean dry air one atmosphere thick (zenith) the solar constant is ~
0.12 W/cm^2;   above the atmosphere ~ 0.14, therefore, ~ 14% loss.   For
wet & dusty easily 40% loss.   (p. 102 V. II, Lei)     Nearly all the
absorption is due to water (fig. 12.10 - p. 99 and fig. 12.12 *, p. 103)
.    For earth radiation, it's absorption) mostly due to CO2.    The
main window (~ 8 => 14 micron T ~ 0.8 => 0.4)) transmits ~ 35%. above 14
micron CO2 absorbs.  (fig. 12.10 *, p. 99 and GE Radiation Calculator).

Sky spectral radiance is ~ 7 W / m^2 - sr - micron;  peak at ~ 9 micron,
from the horizon.   From the zenith, the window (8 => 14 micron) causes
a dip by a factor of ~ 1 / 10.  (figs. 12.16 a & b, pp. 109 & 110).
This curve  is a little flatter than one would expect from a single
black body at ~ 300 K (night).  It is above 6 W ... from 7 => 14.5
micron.    The day's dip is considerably less and the horizon curve is
closer to a single temp. black body.  The max. radiance is greater.
(9.7 W / cm^2-sr-micron at ~ 9.3 micron)

* These figs. are similar to those LW referred.  12.12 is a family for
various thicknesses of atmosphere.

Levi discusses Rayleigh and Mie scattering including data on rain drops
and aerosols; tables include back scattering.   He also has an appendix
on Random walk in two dimensions.

bc




John S. Denker wrote:

> On 07/28/2003 03:10 PM, Robert Cohen wrote:
> >> For a thick atmosphere, if you want a self-consistent theory, you
> >> need to divide the atmosphere into layers and iterate your
> >> argument.  The Nth layer shields the N-1 lower layers as surely as
> >> the first layer shields the surface.  The effect is cumulative.
> >> The radiation is _diffusing_ outwards.
> >
> > Yes, I agree with you,
>
> :-)
>
> > yet somehow I don't think I disagree with myself.
>
> And maybe I don't disagree with you, either.
> See below.
>
> > Yes, it is better to divide the atmosphere into layers.
>
> Agreed.
>
> > My argument
> > assumed a single layer which, as you point out, may not be a good
> > approximation to the earth's atmosphere although I'd argue it is
> > roughly correct.  I'd also argue it is not even close to being
> > correct for Venus.  For Venus, one definitely needs to subdivide into
> > layers in order to get such a hot surface temperature.
> >
>
> It depends on what "roughly correct" means.
>
> The single-layer model is correct at the level of
> dimensional analysis.  But there's more to physics
> than dimensional analysis.
>
> Perhaps it's worth noting that diffusion is nonlinear.
>  -- For a process such as a wheel rolling uniformly
> in the +x direction, things are linear.  A million
> microradians is the same as one radian.
>  -- For a random walk, things are *not* linear.  A
> million random one-micron steps are not equivalent
> to a single one-meter step.
>
> Or maybe you're implicitly claiming that the earth's
> atmosphere is optically thin.  That may be.  That's
> where this discussion began:  I said I don't know
> how high we have to go before the atmosphere becomes
> optically thin.  (We agree Venus must be very thick.)
>
> Come to think of it, two lines of evidence suggest
> that the Earth's atmosphere is not very optically
> thick.
>  -- The night sky in the high desert is perceptibly
> colder than the night sky in a humid locale, due
> to radiative cooling.  So certainly it is *sometimes*
> thin.
>  -- The magnitude of the heat-trap effect (I can't
> bring myself to call it the greenhouse effect) is
> only about 35 degrees;  if there were a huge
> number of layers the effect would be bigger.  This
> is blatantly working backwards from the temperature
> to get the "explanation" of the temperature, but
> it tells us what ballpark we should be looking in.
>
> I stand by my assertion that knowing the thickness
> is important.