Re: Global dimming masking greenhouse effect

From: Edward Green (spamspamspam3_at_netzero.com)
Date: 01/19/05 

Date: 18 Jan 2005 19:35:41 -0800

jimp@specsol-spam-sux.com wrote:

> Waffle?

Sorry. Overly harsh. Perhaps "light crispy wafer". :-)

> OK, how's this?

<see previous post for details>

Experiment? Whoa. Heavy!

I have a few problems with your proposal: for one thing, I don't see
how we are to control for variations in relative humidity and air speed
in the oven vs. outside; for another, it will be difficult to regulate
the temperature of a household oven that finely; and for yet another,
I'm not sure I see the point of including the semi-closed bottle: yes,
trapping of air and increased humidity over the water will slow
evaporation, and I'd then expect net evaporation to be rate-limited by
diffusion or mixing at the bottle mouth. In other words, I'm fairly
sure you are right a priori about the insigificance of illumination in
this case (and while removing the over light was a nice touch, I
wouldn't expect that little bulb to do jack).

Ok.

Let me make a counter-proposal which seems to me to answer my own
objections: I take two 2L soda bottles, cut the bottoms off to make
two identical saucers, and set them outside in the noonday sun. One,
however, I cover with a sun screen at a height of about 1 foot, on four
slender sticks.

Now, it seems to me, we've automatically controlled for equality of
temperature, humidity and wind conditions: well, ideally the object
casting the shadow on saucer two might be even farther away in case it
modifies wind patterns. Or we could have two identical canopies, one
clear, the other reflective... but now we've failed to control for
heating of the blocked sunlight! We want to isolate the
photoevaporative effect alleged.

You know, this is actually somewhat ticklish...

Also, I've thought of a simple consistency check: If N moles of water
are directly "photoevaporated", then this would still require
absorption of N times the molar heat of evaporation directly by light
absorption at the surface. I don't know ... water looks kind of
transparent to me. And the adsorption had better be right at the
surface, not a um down, or that kicked molecule is going to thermally
equilibrate before it escapes so we're back to simple heating. Is
water more strongly absorptive in wavelengths other than the visual and
the interface? UV?

You know what? I think this rules the proposed effect out. To the
extent classical electrodynamics is adequate to describe the passage of
light between two different media, there is, AFAIK, surface reflectance
and transmittance, and bulk absorbance, but no specific "surface
absorbance": we don't deposite energy at the interface. Since
classical electrodyamics is known to be adequate to treatement of
interfaces (?), I deduce that any special photoevaporative effect must
be insignicant.

Satisfied?

There are however intermediate kinds of effects possible: say some
wavelength range is absorbed strongly in the first few mm or even cm of
water, not right at the interface. Now that looks like simple heating,
but it might be again that the surface temperature of the water is
higher than ambient under these conditions: e.g., if a thermometer in
air reads 30C 1 m about the surface in the dark, and also in the
sunlight, it might be that the surface temperature of water is really
> 30C in the latter case. That looks like "photoevaporation", but is
really a more macroscopic non-equilibration. There are also the
possibility of confounding variables galor: the air may simply be more
active, on average, at given local average temperature and humidity, in
brighter light.

Hmm... I beginning to tend to the idea that the effect Franz mentions
is real, but maybe mainly of academic significance, whereas there may
be many other effects related to the level of irradiation which are
really mediated by the local water temperature and air temperature and
relative humidity at the surface layer, but which may be confounded
with a true photoevaporative effect, since we can't measure these
mesoscopic variables.

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