Date: Sun, 31 Mar 2002 22:23:11 -0500 (EST)
From: "Keith F. Lynch" <kfl at keithlynch.net>
To: WSFAlist at keithlynch.net
Subject: [WSFA] Re: Interesting Inventions
Reply-To: WSFA members <WSFAlist at keithlynch.net>

Steve Smith <sgs at aginc.net> wrote:
> Take a large shallow pond.  Water will evaporate, cooling it.  Cover
> it with straw during the day to keep it from warming up.  Remove the
> straw at night to expose the pond to the sky (radiation temperature
> about -268 degrees C (5K)).  The water will freeze.

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.
--
Keith F. Lynch - kfl at keithlynch.net - http://keithlynch.net/
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