How People Created Ice in the Desert 2,000 Years Ago

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Today, creating ice is as easy as placing water in your freezer, but how would you accomplish the same phase-altering feat without an energy-guzzling appliance?

What may seem unfathomable at first thought was regularly accomplished more than 2,000 years ago, and in the desert of all places!

In the early evening hours, Persians and other ancient peoples of the Middle East would pour water in long, shallow stone pools no more than a foot or two deep. They would return to the pools just before first light the following morning to find the water frozen over. They'd then collect the ice and store it inside a yakhchāl, or "ice pit" (pictured above). Within these hollow, insulated domes were deep, subterranean holes where ice could be stored for months.

Okay, "What's the big deal?" you might be thinking. After all, one could easily replicate this process in frigid climes where ambient temperatures dip below freezing. But what's amazing here is that nighttime desert temperatures rarely dipped below freezing, yet ancient Middle Easterners managed to create ice nonetheless!

The secret here is a process known as "night-sky cooling." On dry, cloudless desert nights with the cold vastness of space laid bare above the surface of the Earth, heat can readily radiate from substances like water, escaping from the atmosphere to space itself, where temperatures are roughly 450 degrees below zero Fahrenheit. So much heat can radiate from water that ice can form at ambient temperatures as high as 41 degrees Fahrenheit.

Professor Aaswath Raman, an applied physicist and engineer at the University of Pennsylvania, explained the process in more detail at TED 2018.

"That pool of water, like most natural materials, sends out its heat as light. This is a concept known as thermal radiation… The atmosphere and the molecules in it absorb some of that heat and send it back… But here's the critical thing to understand. Our atmosphere doesn't absorb all of that heat… At certain wavelengths, in particular between eight and 13 microns, our atmosphere has what's known as a transmission window. This window allows some of the heat that goes up as infrared light to effectively escape, carrying away that pool's heat… So that pool of water is able to send out more heat to the sky than the sky sends back to it. And because of that, the pool will cool down below its surroundings' temperature."

Rahman now seeks to create metamaterials that play off these same principles to cover buildings, cooling systems, and solar panels. These materials would actually radiate heat to space during the day when the sun is shining, thus greatly reducing the demand for electricity to power cooling operations on earth, which already accounts for 17 percent of the electricity used worldwide.

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