雨后的气味从哪里来？
Why rain has a smell
Name: ________________ Date: ________
雨刚落下时，你有没有闻到过一股特别的气味？它有点像泥土，又和平时的泥土不太一样。人们常把它叫作“雨的味道”，但线索并不只在天上，也在脚下。
2015年，麻省理工学院的研究人员用高速相机观察水滴落地。他们发现，水滴碰到带有小孔的表面时，会把一些空气困在里面，形成很小的气泡。
气泡向上移动，破裂时，把更小的水滴送进空气里。这些小水滴可能把地面上的气味物质一起带走。研究人员提出，这或许能解释雨后泥土气味传播的一种方式。
不过，并不是雨越大，这种过程就越明显。在他们的实验中，类似小雨和中雨的条件产生了更多微小水滴；猛烈的撞击反而没有那么多。地面的性质也会影响结果。
这项实验给出的是一种可能的传播机制，不是说每一次雨后的气味都只有一个来源。下次闻到那股味道时，你可以想一想：刚才的雨落在了什么地方？
Key words
| Word | Pinyin | Meaning |
|---|---|---|
| 天上 | tiānshàng | in the sky; in heaven |
| 观察 | guānchá | to observe; to watch; observation |
| 表面 | biǎomiàn | surface; face |
| 形成 | xíngchéng | to form; to take shape |
| 向上 | xiàngshàng | upward |
| 移动 | yídòng | to move; to shift |
| 地面 | dìmiàn | ground; floor |
| 物质 | wùzhì | matter; substance |
| 或许 | huòxǔ | perhaps; maybe |
| 传播 | chuánbō | to spread; to disseminate; to propagate |
| 明显 | míngxiǎn | obvious; clear; evident |
| 实验 | shíyàn | experiment; test; to experiment |
English (for the teacher)
Just as rain starts to fall, have you ever caught a particular smell? It’s a bit like soil, yet not quite the same as soil on an ordinary day. People often call it “the smell of rain”, but the clues aren’t only up in the sky; they’re also under our feet.
In 2015, researchers at MIT used high-speed cameras to watch drops of water hit the ground. They found that when a drop lands on a surface with tiny pores, it traps some air inside and forms very small bubbles.
The bubbles rise, and when they burst they send even smaller droplets into the air. These tiny droplets may carry scent compounds from the ground away with them. The researchers suggested this might explain one way the smell of soil spreads after rain.
But it isn’t the case that the harder it rains, the more noticeable this process becomes. In their experiments, conditions like light and moderate rain produced more tiny droplets; hard impacts actually produced fewer. What the ground is like also affects the result.
The experiment offers one possible way the smell spreads; it doesn’t mean the smell after every rain has only one source. Next time you catch that smell, you might ask yourself: where did that rain just land?