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

或幾百顆熱氣球，把幾百噸的水運上天空。

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

然後我們得把這麼多水噴成小到足以漂浮的水滴。

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

簡單來講，這真不簡單。但同時，

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

我們的大氣層在世界各地升起一朵又一朵的雲，

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

高度可達海拔20公里，完全只用來自地表的水和動力。

surface. Cumulus clouds, for example get their start

例如積雲，開始時陽光的能量切斷水分子間的連結，

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

例如積雲，開始時陽光的能量切斷水分子間的連結，

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

讓海、植物和土壤中的水蒸發。 上方的空氣有了濕度和熱能，

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

較冷、較重的空氣會向下包夾它， 並把它向上推，如同隱形的熱氣球。

balloon. Surprisingly, this balloon’s cargo doesn’t weigh it down – in fact, the more

驚人的是，這顆氣球不會因重量而下沉─事實上，

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

飄上去的水量越多，氣球還會越輕。

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

就是這麼奇怪，因為水蒸氣和氮氣、氧氣(大氣主要成分)

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

同為氣體，而基本物理指出，相同體積的氣體

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

會有相同數量的分子，無論種類為何。而水是由

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

兩個氫加一個氧構成，比兩個氮或兩個氧還輕。因此

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

暖濕空氣甚至比暖乾空氣還輕。當隱形氣球上升，

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

外界維持氣球邊界的氣壓變小，分散其內部熱能並

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

使其降溫。最後上端的空氣冷到足以讓水蒸氣凝結，

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

成為小水滴，遠看就像一縷輕雲。當氣球其他部分

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

繼續上升，水蒸氣繼續冷卻，並在同樣高度凝結，

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

形成一片平底雲朵，彷彿無中生有般往上長。

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

另外，當水分子彼此結合，從蒸氣凝結成水滴，

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

它們會釋放從地表蒸發時所吸收的能量。

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

這加熱了周遭空氣，讓它上升並吸起更多下方濕空氣，

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

濕空氣凝結又釋放熱量，持續驅動並強化上升氣流。

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

即使是一小團積雲，凝結釋放的總能量也很巨大─

is huge – equivalent to about 270 tons of TNT. And if the supply of water vapor is much

相當於270噸的TNT。如果水蒸氣的供給更多，

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

釋放的能量能製造高達平流層的雲柱， 以及猛烈的上升氣流，

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

產生強烈的風暴以及葡萄柚大的冰雹。 我看今日這天不像個玩熱氣球的。