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 an aerosol. 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
organic debris or 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 from volatile oils 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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