Re: Dew Formation

[John Denker <jsd@AV8N.COM>]

Quoting Bernard Cleyet <anngeorg@PACBELL.NET>:

> I suppose this radiance is due to absorption from the earth's
> radiation.  It approximates a black body at 300 deg. K

This supposition bespeaks a physical model that I don't
understand.  At the very least, there is an alternative model
that ought to be considered, to wit:

The atmosphere radiates 'because' it has a temperature.  It has
a temperature 'because' of a number of inputs, including
 -- direct heating by solar radiation
 -- latent heat liberated by moisture when it condenses in the
    atmosphere
 -- heat transferred by contact with the surface
 -- etc. etc. etc.

> Levi reports the atmosphere's spectral radiance as ~ > 0.8 W / m^2 - sr
> - micron from 5 micron => > 20.  (any angle)

I really doubt Levi is telling the whole story.  My friends who own
big IR telescopes tell me the contribution they receive from the
atmosphere depends on the _water column_ they have to look througth,
and not much else.  If the air is dry, they can more-or-less see
right through it.  So it's good to have a site with high altitude
and/or dry weather.

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

I've repeated the experiment myself, making ice in the desert
using styrofoam coolers et cetera.  Works fine.  Impresses the
tourists.