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  • Welcome to Calipatria, a desert town near California's Salton Sea.

  • This is the city with the lowest elevation in the western hemisphere,

  • 55 metres below sea level.

  • And here, we've also got a visual demonstration of just what that means:

  • because that white spindly thing is a flagpole,

  • and the American flag up at the top is flying above sea level.

  • If it wasn't for all the mountains and hills between here and the Pacific Ocean,

  • then we'd be wet, but that flag would stay dry.

  • Maybe.

  • We're dry because no rivers drain into this basin.

  • The Salton Sea over there only exists because some engineers screwed up in 1905

  • and diverted a good bit of the Colorado River in there for a couple of years.

  • That water is steadily getting saltier and more polluted as it slowly evaporates,

  • and California's got to deal with that over the next couple of decades.

  • But how do we know we're 55 metres below sea level?

  • What is sea level?

  • In theory, it is the average height of the sea around the Earth.

  • But there are some problems with that.

  • Problem 1: the Earth isn't a sphere.

  • I mean, it's vaguely a sphere, don't get too excited, Flat-Earthers.

  • It's a bit squashed.

  • The force of its own spin makes it bulge by a few kilometres around the equator.

  • And when you're defining height above sea level in metres, that's a lot.

  • So geographers solved that by defining the Earth as an oblate spheroid.

  • Basically a smooth squashed sphere, just in fairly precise mathematical terms.

  • But then there's problem 2: the Earth isn't one uniform lump of rock.

  • Bits of the mantle are more dense than others,

  • so gravity isn't quite the same everywhere on Earth's surface.

  • Which means there's a bit more water sitting over those bits.

  • And by a bit more, I mean anything up to 80 metres more.

  • These aren't small amounts,

  • that could be the difference between this city being above and below sea level.

  • And to complicate things even more,

  • mountains are big enough that their gravity attracts the ocean, just a little.

  • So during the 20th century,

  • as scientists and Cold War governments started to need a global reference system

  • for spacecraft, and missiles, which were basically the same thing back then,

  • they spent a long time measuring Earth's gravity.

  • The result is the Earth Gravitational Model, megabytes in size,

  • which asks, in purely mathematical terms:

  • if this land wasn't here, but its gravity was,

  • and if there was exactly enough water to fill in the gaps...

  • where would all that water sit?

  • And how far away would it be from that theoretical smooth squashed sphere?

  • The model was finished in 1996.

  • It is available to this day on a web site that seems unchanged since 1996,

  • and it's what your phone and GPS use.

  • There are more recent, more precise models, but they are hundreds of megabytes in size,

  • and everyone's agreed that now we've got a standard,

  • let's not change it and confuse everyone.

  • Also, we'd also have to update for sea level rise,

  • whether it's best-case centimetres or worst-case metres.

  • So saying we're "55 metres below sea level" is mostly just symbolic.

  • But so are many things in the world,

  • and I'd like to think we still put some trust in them.

Welcome to Calipatria, a desert town near California's Salton Sea.

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B1 中級

とにかく海面ってなんだ? (What Is Sea Level, Anyway?)

  • 3 0
    林宜悉 に公開 2021 年 01 月 14 日
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