Karachi Photo Blog

Monday, December 17, 2007



Making ice through radiation heat loss

We have all heard of bygone desert people making ice under nighttime clear skies. The physics of that phenomenon is not too hard to understand. When you radiate to outer space, you are radiating to almost a 0 K object. [Remember that the space does not have a temperature—you can have temperature of an object, but you cannot have temperature of nothingness, which is what space is.]
Why nighttime? Because during the day even when the sky is clear there is too much scattered sunlight the water would be bombarded with.
Why do it in desert? Because a desert is normally very dry, its air is low in humidity and that means minimum obstruction between a body losing heat through radiation and outer space. So, if you can have water facing nighttime clear sky and the receptacle holding this water is so well insulated that heat from sides and bottom has a lot of difficulty being transferred to water then you can gradually cool water and eventually freeze it.

Fortunately I did not have to go to a desert to witness this phenomenon. I saw it in Rancho San Antonio Park where I often go for a walk. Lately the temperatures in Silicon Valley have been low--but still above freezing. Rancho San Anotonio Park has puddles from the last rain. I found puddles with vegetation around them to be still liquid, but a few which were very shallow and had maximum advantage to lose heat to nighttime clear sky were frozen. Ice was made through radiation heat loss.

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