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  • [♩INTRO]

  • In July 1983, a weather station in Antarctica recorded the lowest air temperature

  • ever measured on Earth.

  • It was about -89 degrees Celsius

  • which is colder than the average temperature on Mars.

  • But 25 years later, scientists analyzing satellite data found small pockets

  • high up on the East Antarctic Plateau where temperatures routinely drop

  • about five degrees lower, making them the coldest spots we know of on Earth.

  • But these pockets aren't just for record books:

  • They're also helping us understand how cold it's possible to get

  • on the surface of our planetand what conditions it takes to get that cold.

  • These record-breaking temperatures happen during long polar nights

  • within little hollows in the ice sheet,

  • more than three-and-a-half kilometers above sea level.

  • On still nights when the sky is clear, the air near the surface of the snow cools

  • down as heat radiates into space,

  • and it becomes denser than the slightly warmer air surrounding it.

  • As a result, it starts sliding downhill along the ice sheet

  • and sinks into these hollows.

  • If the wind is light enough, the super-cold air gets trapped down there,

  • and it acts like a kind of ice pack,

  • which brings down the temperature of the surface even more.

  • When that happens, the air temperature drops down to those record low

  • until weather stirs things up again.

  • But the lack of wind isn't the only thing you need to get those temperatures.

  • It also has to be extremely dry,

  • because water vapor can keep heat from escaping.

  • See, heat might feel like this abstract thing,

  • but it's just waves of infrared radiation.

  • So when heat escapes our planet,

  • infrared waves are radiating from the ground and into space.

  • When temperatures drop, there's less heat coming from the surface,

  • so the waves carrying that heat away lose energy and stretch out.

  • And those longer waves are more easily absorbed by water vapor in the air.

  • So if there's any moisture in the air,

  • a lot of that radiation won't escape into space;

  • it will be absorbed by the water and sent back to the ground to warm things up.

  • In fact, to get below about -90 degrees, you need less than a millimeter's worth

  • of water in the entire atmosphere stretching from the ground all the way into space.

  • In other words, if you wrung out all the water from that air,

  • it wouldn't even reach the one-millimeter line on a rain gauge.

  • Those are some strict requirements.

  • But for the researchers studying this, the really surprising thing wasn't just that

  • the temperature got so low in these pockets:

  • It was that, over time, it dropped to nearly the same low temperature

  • over and over again.

  • Like, across 14 years and hundreds of kilometers of the Plateau,

  • the lows at the coldest sites hovered right around -94 degrees,

  • just above the all-time low of -98.

  • So scientists wondered if the temperature was hitting some kind of threshold

  • if it was reaching the coldest temperature possible on the surface of our planet.

  • And that seems to be what's happening!

  • See, even on still, dry nights, the heat doesn't radiate away forever.

  • Instead, a lot of that infrared radiation gets trapped by carbon dioxide

  • because while CO2 always absorbs some heat,

  • it's especially good at absorbing these longer wavelengths.

  • So at that point, the air itself is trapping heat

  • and sending it back toward the surface.

  • The atmosphere essentially acts like a blanket,

  • and heat can only escape from the surface really slowly.

  • Eventually, around -98 degrees Celsius, the temperature is dropping

  • less than half a degree a day

  • too slowly to really change before the weather shifts and winds stir things up.

  • In theory, if air could sit still for days or weeks on end,

  • the temperature could drop lower, but on a planet with so much weather,

  • -98 degrees is essentially as cold as it gets.

  • Which, hey, is fine by me.

  • It takes some creative thinking to find and demystify something

  • like the coldest place on Earth.

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  • [♩OUTRO]

Thanks to Brilliant for supporting this episode of SciShow.

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