# [ExI] puzzle

spike at rainier66.com spike at rainier66.com
Thu Jul 2 15:06:04 UTC 2020

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> On Behalf Of William Flynn Wallace via extropy-chat

Subject: Re: [ExI] puzzle

Here is what the books says:

Around sundown, Egyptian women placed water in shallow clay trays on a bed of straw.  Rapid evaporation from the water surface and from the damp sides of the tray combined with the nocturnal drop in temperature to freeze the water - even though the temperature of the environment never fell near the freezing point.  Sometimes only a thin film of ice formed on the surface, but under more favorable conditions of dryness and night cooling, the water froze into a solid slab of ice.   bill w

BillW, this was a matter of debate when I lived in the Southern California desert.  We set up recording high-accuracy temperature measuring equipment, demonstrated that under certain conditions frost would form on the cars when the temperature never dropped below about 6C, which is a long ways from freezing, warmer than a typical refrigerator by a lot.

If you think about it, that is amazing.  It requires an air mass with sufficient moisture to hit the dew point, followed by a dryer air mass blowing in, freezing the dew.  Our big debate is how the radiation played into it.  I did the models and showed it has a big affect: when those conditions are met, it is always really clear skies, so the moisture radiates heat out into cold space in accordance with Planck’s law.  You hafta take into account Bolzmann’s constant, do all the sophisticated stuff to predict the temperature of a surface particularly at night.

In retrospect it shouldn’t be all that anti-intuitive.  Many of us have stood near a good hot campfire on a cool evening, or an even better example is stood 50 meters out from a housefire: you feel the heat when a big flame gets going.  Clearly that isn’t convection: the heated air goes up.  You can feel a lot of heat when you can’t even smell smoke.  Radiation is important.  Air temperature is only good for a first order approximation.

Thermodynamic equations are cool.

spike

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