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If it weren't for that pesky water vapour and other greenhouse gases, it would be nearer to -270 deg C about 3K due to the CMB radiation. However, in this case whether it's -50 or -100 deg C won't make much difference to the Arab's ice making efficacy since the warmth of the atmosphere at ground level plus the residual ground heat would predominate anyway.
About 50% more cooling at -100 than at -50. Presumably, low moisture levels in desert atmospheres help to keep sky temperature down.
It looks top-heavy. Just a puff of wind, and it will get blown over. Oh, wait a moment ....
How do you calculate that? I make it about 20%.
As the good Jonny says, I expect the real issue is thermally insulating the pond from the ground as much as possible.
That "ice in the desert" trick works mainly by evaporative cooling (taking advantage of the low humidity in the air).
Worth a quick look. They constructed walls round the pools to keep the sun off during the day. If those are steps down the side of the ice container, then it was truly massive.
I think even a 20% increase in cooling might be a little on the optimistic side. :-)
I guess a shallow pond of water will act as an insulation layer. Water isn't a good conductor of heat[1], provided there isn't a convective flow. Water has a high specific heat value so 'shallow' works in two ways in this case, shallow so it doesn't act as a heat store in its own right and shallow to minimise the thermo-siphoning effect.
[1] You might remember the classic experiment with a weighted ice cube at the bottom of a pyrex test tube full of water with the top end held in the flame of a Bunsen burner causing the water to boil with virtually no effect on the ice cube.
Interesting, thanks. That's rather more complicated --- but more effective --- than the version I'd heard of.
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