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Science facts you never learned in school: can you smell rain coming?

3 min read

Have you ever wondered why we can smell the rain?
Most of us have probably smelt that lovely fresh, earthy scent produced when rain falls on dry soil.
It’s not just gratitude that makes rain smell so appealing after a long period of dry weather, as there’s actually some chemistry involved too.
Long been chased by scientists and even perfumers, now scientists have discovered why people can smell the rain, and it has even a name, petrichor.
The word come from Ancient Greek πέτρα (pétra), meaning “rock” or πέτρος (pétros), literally “stone”, and ἰχώρ (ikhṓr), the ethereal fluid that is the blood of the gods in Greek mythology.
The term is coined by scientists Isabel Joy Bear and Richard Thomas in their 1964 article “Nature of Argillaceous Odour”, published in the journal Nature.

In any case, a sensitive snout is smelling ozone, petrichor and geosmin.
In other words, the nose smells oxygen, the debris that raindrops kick up and wet soil-dwelling bacteria, called Streptomyces.
First comes the ozone, the oxygen fried by lightening that changes its chemistry for O2 to O3. This has a sweet, pungent zing and winds carry it down from the upper atmosphere to your waiting nose.
Ozone is a naturally occurring gas in the atmosphere.
Its name is derived from the Greek word, ozein, which means smell. Electrical charges, such as those from lightning or man-made sources, split the atmospheric oxygen molecules into separate atoms. These free oxygen atoms combine quickly with other oxygen molecules in the air to form ozone. Ozone molecules are carried down from higher altitudes to nose level by a storm’s downdraft.
If you smell a lot of that—look out!

Then comes the raindrops.
Of course rain itself has no scent.
Scientists discovered that water drops hitting surfaces like soil or leaves knock particles up in the air. A raindrop hitting an uneven surface traps bubbles of air that shoot upwards and burst from the top of the water droplet like fizz in a champagne glass.
These bubbles can float long distances before they pop and you can smell the pollens, dirt, oils or city scum.
Petrichor comes in fact from plant oils that have accumulated over dry periods—primarily in plant leaves. Secreted oils settle into the pavement or soil and are released when “disturbed” by the rain.
The smell of petrichor is stronger after long periods of drought as the amount of oils have built up over long periods of time.

Finally, the wet soil triggers the bacteria or blue-green algae to release geosmin, that great earthy smell so loved by gardeners, that can be detected by animals as well as humans.
Interestingly, some scientists believe that humans appreciate the rain scent because ancestors may have relied on rainy weather for survival, while camels in the desert also rely on petrichor to locate sources of water such as oases.

So while the science behind petrichor isn’t all that romantic….(who’d have guessed we’re actually smelling bacteria secretions and lightning?) the end result certainly is.
And you? Do you like smell of the rain?

Images from web – Google Research

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