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  • If we wanted to make a cloud entirely from scratch, we’d first need a fleet of jumbo

  • jets or several hundred hot-air balloons to haul hundreds of tons of water up to the sky.

  • And then, somehow, we’d need to disperse all that liquid into a mist of droplets small

  • enough to float. In short, it wouldn’t be easy. And yet,

  • our atmosphere manages to pump out one cloud after another all over the world at altitudes

  • of up to 20 kilometers above sea level, using water and fuel carried all the way from Earth’s

  • surface. Cumulus clouds, for example get their start

  • when solar energy evaporates water from oceans, plants, and soil by breaking the bonds that

  • hold water molecules together. As the patch of air above collects moisture and heat cooler,

  • heavier air sinks around it pinching it off and pushing it aloft like an invisible hot-air

  • balloon. Surprisingly, this balloon’s cargo doesn’t weigh it downin fact, the more

  • water vapor it collects before lift-off, the lighter it gets.

  • As weird as that sounds, it’s because water vapor is a gas just like the nitrogen and

  • oxygen that make up most of the atmosphere. Basic physics dictates that a given volume

  • of gas has the same number of molecules regardless of what those molecules are. And water is

  • made of H plus H plus O, which is lighter than both two Ns and two Os. So warm, humid

  • air is even more buoyant than warm, dry air. As the invisible balloon goes up, the falling

  • pressure outside allows it to keep ballooning, which spreads out its internal heat and lowers

  • its temperature. Eventually, the air at the top cools enough for the water vapor there

  • to condense into droplets, which look from afar like a thin wisp of cloud. And as the

  • rest of the balloon rises, water vapor continues to cool and condense at the same altitude,

  • creating a flat-bottomed cloud that appears to grow upward out of nothing.

  • What’s more, as the condensing water vapor molecules bond together into liquid droplets,

  • they release the energy they absorbed from Earth’s surface when they evaporated. This

  • heats the surrounding pocket of air, giving it lift and sucking more moist air up behind

  • it [updraft], which cools and condenses and releases heat, which fuels lift and strengthens

  • the updraft. Even in a small cumulus cloud , the total energy released from condensation

  • is hugeequivalent to about 270 tons of TNT. And if the supply of water vapor is much

  • larger, the energy released can produce stratosphere-high pillars of cloud with violent updrafts, fierce

  • electrical storms, and grapefruit-sized hailstones. Not good weather for hot air ballooning.

If we wanted to make a cloud entirely from scratch, we’d first need a fleet of jumbo

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Why Are There Clouds?

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    Yrchinese posted on 2017/02/11
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