To: WSFAlist at keithlynch.net
Date: Sun, 31 Mar 2002 23:44:51 -0500
Subject: [WSFA] Re: Interesting Inventions
From: ronkean at juno.com
Reply-To: WSFA members <WSFAlist at keithlynch.net>

On Sun, 31 Mar 2002 23:38:35 -0500 Steve Smith <sgs at aginc.net> writes:

> "Keith F. Lynch" wrote:
> > 3K, not 5K.  But the freezing has little to do with the radiation
> > temperature of the sky, and plenty to do with the relative
> humidity.
> > Water will always seek the dew point.  That's how a wet-bulb
> > thermometer, for measuring relative humidity, works.  If the dew
> point is above the freezing point of water, water will not freeze.  If
> the dew point is below the freezing point of water, water will
> eventually freeze unless there's a source of heat, such as direct
sunlight.
>

> 5 degrees.  The extra two degrees comes from water vapor in the
> atmosphere.  (Not much water vapor in the desert).
>

Whether it's 3 degrees K or 5 degrees K probably does not make much
practical difference in this case.  Either way, the sky is very, very
cold relative to the water.

> The data came from a Scientific American article a few years back
> that talked about various "traditional" means of keeping cool in the
> desert. The article  stated that radiation cooling was the main reason
that
> it actually froze.  Evaporation (dew point) helps, of course.
>
> I can personally vouch for the fact that the Arizona desert cools
> off *very* quickly after dark.  Very little water vapor in the air.
>

The Arabs say 'night is the desert's winter'.  The original message
concerned the ancient Roman method of obtaining ice in Syria, by
uncovering a shallow pond at night.  Syria is mostly dry, with a high
average elevation.  There is a 9,000 foot peak 40 miles southwest of
Damascus, and Damascus is at least 2,000 feet above sea level, I think.
So the high elevation suggests that average temperatures are fairly cool.
 The dry air implies a low dew point, which supports Keith's view, but
the dry air also implies a lack of cloud cover, which supports Steve's
contention that radiation is the main cause of ice formation.  It's the
lack of clouds, I think, which makes night the desert's 'winter'.

Question:  If the dew point was a little above freezing, and a very
shallow _insulated_ pond were exposed to the cloudless night sky, could
the water at least skin over with ice in a few hours, due to heat loss by
radiation?  Presumably, under those conditions, evaporation alone could
not result in ice formation, though if there was some wind, evaporation
could quickly pull the water temperature down towards the dew point.

Ron Kean

.

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