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Doug Linzey wrote:
> Here is an excellent Web site designed to counter any argument put
> forth by global warming sceptics:
> <http://gristmill.grist.org/skeptics>
>
> Doug Linzey
>
Hi Doug & All, Dec 16, 2006
I visited this site some time ago, branched to
A Few Things Ill Considered: Water Vapor is Almost All of the Greenhouse
Effect
<http://illconsidered.blogspot.com/2006/01/water-vapor-is-almost-all-of.html>
and then downloaded a copy of
http://www.atmo.arizona.edu/students/courselinks/spring04/atmo451b/pdf/RadiationBudget.pdf
and entered stall mode. Those who eat, drink and sleep climate models,
and know all the current benchmarks and conventions, could no doubt
read this with ease but ideally prose should inform those who don't
know. To the uninitiated, it is stiff going.
One sentence (lower right, p. 199) will illustrate what I mean.
"Here we assume a "solar constant" of 1367 W m^-2 (Hartmann 1994),
and because the incoming solar radiation is one-quarter of this, that
is, 342 W m^-2., a planetary albedo of 31% is implied."
Stripping this sentence to bare bones we are told that-- 'Because
the incoming solar radiation is one-quarter of the "solar constant" a
planetary albedo of 31% is implied', which is of course nonsense. So one
has to ask what the authors might have had in mind when they wrote this.
Taking this one component at a time, the solar constant is defined
as the energy received per unit area by a plane surface placed outside
our atmosphere and perpendicular to the sun's rays (variously given in
older sources as 1.94 to 2.0 gram calories/minute/cm^2) when the earth
is at its mean distance from the sun. Using the conversion Watt =
0.01433 kg cal/minute and 1.94 g/cal/min/cm^2 one gets 1354 Watts/ m^2;
very similar to the 1368 used by the authors.
From Fig. 7 and associated text it is evident that the various
fluxes per square metre are considered to be additive i.e. no allowance
is made for the greater surface area at the top of the atmosphere than
at the surface of the earth. And from Fig. 7 one can also see where the
31% albedo came from; outgoing longwave radiation of 235 W/m^2 (assumed
equal to absorbed solar flux) and an incoming flux of 342 W/m^2.
i.e. (342-235)*100/342= 31%
And taking this assumed atmosphere depth of zero into account, along
with the above, one can now deduce what they assumed everyone would
accept as given. Because the surface area of a sphere is 4pi*r^2, the
area of a disk is pi*r^2, and because the effective area that intercepts
the sun's rays at any instant is a disk of radius r, the average
incoming solar flux (day & night; summer & winter) will be 1/4 of the
solar constant, i.e.
Average flux to earth = (Flux/m^2 to disk)*(area of disk)/area of earth
= (Solar constant)* pi*r^2/ 4pi*r^2
= 1367/4
=~342
So if worrying about global warming keeps you awake at night you
should read some of this paper. You may worry as much as previously but
weariness will put you to sleep.
Their Fig. 7 is interesting. They have omitted any consideration of
latent heat of fusion, perhaps because average temperatures near the
surface are above freezing and this is a one-dimensional static balance
sheet.
QUESTION: Should not the solar constant be referenced to mean d^2 where
d is the distance from earth to the sun ?
Yours truly, Dave Webster, Kentville
>
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