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Re: Dew Formation



"Perhaps someone would care to try it?"

Not really.

Essentially already done. Read below.

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

* atmosphere's window ~ 10 micron.

cf. to solar constant ~ 1.3 k W/m^2 the atmosphere (zenith) reduces this
~ by 0.86.

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!