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Re: Dew Formation



Hi!

interleaved:

John Denker wrote:

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

this implies thermal inertia. theoretically, check from the SH of dry
and wet air column equivalent to ~ V.~ 5 miles. Experimentally by
checking its cooling more rapidly than the earth cools. (I suggested
this as an expt. previously.) there is data on altitudinal variation of
temp. and water content. High altitude wind, I suspect, is important in
this.

-- latent heat liberated by moisture when it condenses in the
atmosphere

The amount of moisture can be determined (remote sensing) and therefore,
the amount of internal energy "released" determinable. This would tend
to make the atmosphere temporally isothermal. this may be circular as
the data may be interpreted by assuming atmospheric radiation. Note
data I collected later.

-- heat transferred by contact with the surface

if the air is still, I don't think significant. The data's graph I
described was probably taken at an observatory as given at 3.4 km
altitude. Data off Zenith is far from the earth's surface, i.e. > 3 km.

-- 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.


These windows are labeled: * 1.25µm (J), 1.6µm (H), 2.2µm (K) and 3.5
µm (L). The additional 4.8µm (M) and 10.2µm (N) are difficult to use
because of the atmospheric radiation. They are necessary for cool stars
e.g. M's. All except two [M and N are divided by H2O] in the region of
interest (~ 5 => 30 micron) are surrounded by CO2 and O2 absorption, not
H2O. The last H2O absorption is the one between M and N (5.5 => 8
micron. There is a big one beyond the region of interest.

* used as similar to vis color difference to determine temp. of stars.


I have an IR thermometer (Radio Shack, $30). I suspect it's one of
these semiconductor thermopiles, argon filled with an alumina window and
vis. filter. [Pasco sells one W/O the filter for much more. It outputs
total energy from ~ 0.3 => 4 micron.] I checked its calibration with
various BB approximations: open mouth, ear canal, fireplace soon after
fire, paper tubes in refrigerator and freezer, etc. Results rather
consistent and close to expected temperature.

Last night, after an almost rainy day, I aimed it at the very foggy sky
(~ 11 PM) 11 deg. C aimed at the ground 12 deg. The fog tho v. heavy
was not near the ground. Tonite I'll try again. It's a v. clear hot
day, so I think it may be a clear evening. Walking home after buying
one IR T.; pointing at the Zenith, it was off scale. (claimed: -20 =>
200 deg. C). It was a clear night. I'll not be able to measure the
temp. in the Zenith direction, but slightly above the horizon it may read.


It was a somewhat clear evening 'till ~ 2200. then the sky got warmer!

Here's the data:

Zenith Ground Time & comment

11 deg. C 12 2300 (2004 03 13) v. foggy not ground
level

-15 +15 ~ 1800 sunset v. clear (2004 03014
-18 +12 1825
-18 +11 1850
-20 +12 1935
-16 * +11 2020
-18 +11 2115 still rather
clear usual stars visible
-21 +08 2230 noticeably
overcast most stars still vis.
-9 * +10 2400 rather uniformly
overcast one star (planet?) vis.


-18 +10 0645 sun rise v.
clear (2004 03 15; full moon 44 bce,
Caesar,
the only popular Roman dictator murdered.)

* Warming by condensation?

Because of variation in direction, etc., I guestimate +/- 1 deg. c



bc, late.



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.