"Nemo" wrote in message
...
I've heard the Romans and Arabs used to make ice by exposing water to
the desert sky at night, covering it in the day. After a few days'
cycling it had radiated enough heat into the clear desert sky to freeze.
Doesn't work in areas where there are clouds at night.

Earth gives off heat at night. Clouds reflect the heat back so it will not
cool down as fast. Clear skies allow the earth to radiate its heat more
readily.

Desert nights can get exceedingly cold with their cloudless skies. (this
according to an Egyptian friend and neighbor).