 What causes the smell after rain?
[ 2006-07-10 11:27 ]
下過雨后，空氣里常常有股強烈的味道。有人說這是因為雨水把世界都沖洗了一遍，各種東西都開始散發他們本來的味道。實際的情形又如何呢？看看下面這篇文章，你就會知道這些怪味道都是從哪里來的了。
 As it turns out, the smells people associate with rainstorms can be caused by a number of things. One of the more pleasant rain smells, the one we often notice in the woods, is actually caused by bacteria!Actinomycetes, a type of bacteria, grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produce spores in the soil. The wetness and force of rainfall kick these tiny spores up into the air where the moisture after a rain acts as anaerosol. The moist air easily carries the spores to us so we breathe them in. These spores have a distinctive, earthy smell we often associate with rainfall. Since the bacteria thrives in moist soil but releases the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell.
Another sort of smell is caused by the acidity of rain. Because of chemicals in the atmosphere, rainwater tends to be somewhat acidic, especially in urban environments. When it comes in contact with organicdebrisor chemicals on the ground, it can cause some particularly aromatic reactions. It breaks apart soil and releases minerals trapped inside, and it reacts with chemicals giving them a stronger smell. These reactions generally produce more unpleasant smells than bacteria spores, which is why the after-the-rain smell isn't always a good one.
Another after-the-rain smell comes fromvolatileoils that plants and trees release. The oil then collects on surfaces such as rocks. The rain reacts with the oil on the rocks and carries it as a gas through the air. This scent is like the bacteria spores in that most people consider it a pleasant, fresh smell. It has even been bottled and sold for its aromatic qualities!
These are a few common rain smells, but there are also all sorts of other scents after it rains. There is lots of aromatic material that the moisture and impact of rain can stir up, and the moist atmosphere following a downpour is particularly good at carrying these particles through the air. So, when you talk about the after-the-rain smell with a friend, you may mean one thing while your friend is thinking of something else. You'll both agree, however, that the air has a much stronger aroma to it after a good rain.
actinomycete：放射菌類
aerosol：浮質，氣體中的懸浮微粒,如煙,霧等
debris：碎片, 殘骸
volatile：揮發性的
(英語點津 Annabel 編輯)
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