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Re: Dew Formation



Good question. I must think about.

Reminds me of Arabs who made ice in the desert using blankets (ca. 500
CE ?) reported by G. Gamow.

I've thought about it. "It" refers to the earth. The sky is
reflecting a part of the earth's radiation, except for the window, with
flat response. The graph has ~ the typical black body shape with the
peak @ ~ 10 microns.

I guestimate the reflectivity is ~ 10% Totaling the radiance from the
graph: v. ~ 50 W / m^2 *.

Assumptions: earth's emissivity 1, flat earth. BB @ ~ 300 K radiates
~ 500 W / m^2 -- 50 back.

* the text's discussion gave only W / m^2 - micron, the graph W / m^2
-micron - sr. ( A factor of 6.) More to think about. Cosines, etc.

bc

Brian Whatcott wrote:

If Levi gives an equivalent night sky temperature of 27 degC
how do you suppose ice forms by radiative cooling?

Brian W.

At 10:13 PM 3/13/2004, you wrote:


"Perhaps someone would care to try it?"

Not really.
...
Levi reports the atmosphere's spectral radiance as ~ > 0.8 W / m^2 - sr
- micron from 5 micron => > 20. (any angle)

From ~ 0 to 2 deg. it rises to ~ 10 X ( 8 W ...) between 7 and 14
micron *. by 20 micron it is ~ 3 W / m^2 etc. measured at ~ 3 km, 8
deg. C, and clear night. I assume no moon
...
I suppose this radiance is due to absorption from the earth's
radiation. It approximates a black body at 300 deg. K Would be
interesting to see how it changes with time.

bc



Brian Whatcott wrote:



At 01:04 PM 3/13/2004, John M., you wrote:




When dew forms on clear nights, it will heavily wet lawns but will
hardly ever leave visible moisture on concrete walkways or asphalt
roads. There are many factors that regulate condensation:
temperature, available condensation nucleii, curvature, hydrophilic
surfaces, etc. - but I am having difficulty identifying the primary
one in this case. Any ideas out there?




The primary mechanism is radiation exchange with 2 pi steradians of a
3 kelvin blackbody plus high resistance thermal conduction pathways
to the ground and the (still) air.

--
John Mallinckrodt mailto:ajm@csupomona.edu




If we are to believe John, a vacuum enclosure with a suitably
transparent lid would allow a black chromed copper square (say),
if well insulated at sea-level, to cool to 5 or 6 K overnight?
I don't think so.
I think you would find it rather difficult to radiatively
chill any object at sea level to below -100 degC

Perhaps someone would care to try it?

Brian Whatcott Altus OK Eureka!







Brian Whatcott Altus OK Eureka!