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What's that smell?
Topic Started: Nov 27 2011, 07:13 PM (303 Views)
George K
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Finally
http://www.straightdope.com/columns/read/3023/what-s-that-smell-right-before-it-rains-plus

What’s that smell right before it rains?

Dear Cecil:

What’s that smell in the air (some people like it, others don't) right before it rains? I’ve lived all over the country and there’s no variation … when you smell it, you know rain is on its way.

— Nicky, Mount Prospect, Illinois


Cecil replies:

The smell is called petrichor, the scent of rain falling on dry earth. It’s caused by a couple of compounds in the soil, one of them known as geosmin, “earth-smell,” a term I found wonderfully Tolkienesque. (O geosmin! O earth-smell! A Elbereth Gilthoniel!) However, the Straight Dope copy desk recoiled, noting that any such reading would necessitate a vulgar conjunction of Elvish, pseudo-Anglo-Saxon, and Greek. We’ll therefore refrain from literary commentary and proceed in strict accord with science.

The human nose, not normally considered a particularly acute instrument, is extraordinarily sensitive to geosmin; we can detect it at a level of just ten parts in a trillion. Today this is mostly an annoyance, since in our supercilious age many prefer the fragrance of machine oil and ozone to the sweet smell of the planet. But I’ll venture to suggest it was important in an era long past.

Geosmin is produced by several types of bacteria and algae, which manufacture a volatile compound that can be kicked up when soil is disturbed, such as by gardening, plowing, or a hard rain. When a storm threatens and a few molecules of geosmin waft your way, that signifies rain is falling to windward, and in the fullness of time will fall on you.

Because we’re so attuned to it, a little geosmin goes a long way, and a lot can be decidedly unpleasant. Geosmin and another fragrant soil-borne compound, 2-methylisoborneol or 2-MIB, can make wine taste earthy, water yucky, and fish foul. (Catfish are especially susceptible.) The scent of geosmin may tell farmers their soil is healthy, but this is one area where a lot of non-farmers would be content to leave their ignorance intact.

Repellent though some find it, geosmin seems to be harmless to most animals, and in itself doesn’t signify that anything toxic is brewing. In fact, nobody really knows what it does or why we’re so sensitive to it, and most scientists, by nature practical folk, decline to speculate. But a scarcity of facts has never bothered me, and in this case we’ve got a sliver of information to go on. A couple UK scientists, wondering how Bactrian camels in the Gobi desert were supposedly able to sniff out water from 50 miles away, proposed that the animals were actually smelling geosmin carried by the wind from oases.

A survival trait so obviously useful to camels would likewise be advantageous to us. Long ago we were mainly nomads wandering in arid regions. It’s easy to imagine a parched band trudging mapless in the desert looking for the next watering hole. Then the breeze picks up, and what do they detect? Had they lacked the appropriate olfactory adaptation, nothing, with possibly disastrous consequences. As it was, if they were fortunate, they might smell the faint odor of moist earth, and with it the promise that they'd live another day.
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LWpianistin
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HOLY CARP!!!
That's one of my favorite smells.
And how are you today?
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Mikhailoh
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If you want trouble, find yourself a redhead
Fascinating. I love that smell. Always assumed it was of the atmosphere, not the earth.
Edited by Mikhailoh, Nov 27 2011, 07:27 PM.
Once in his life, every man is entitled to fall madly in love with a gorgeous redhead - Lucille Ball
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apple
one of the angels
petrichor - great word.. i always thought it was OZONE.. that's what i was told.
it behooves me to behold
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Mikhailoh
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If you want trouble, find yourself a redhead
Same here.
Once in his life, every man is entitled to fall madly in love with a gorgeous redhead - Lucille Ball
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jon-nyc
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Cheers
me too.

by the way, you don't really smell it in the city.
In my defense, I was left unsupervised.
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The 89th Key
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Mikhailoh
Nov 27 2011, 07:27 PM
Fascinating. I love that smell. Always assumed it was of the atmosphere, not the earth.
Me too
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Qaanaaq-Liaaq
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Senior Carp
From the article: “we can detect it at a level of just ten parts in a trillion”. Contrast this with “sharks can detect some chemicals at concentrations of around one part per 25 million” in his shark article from http://www.straightdope.com/columns/read/2781/shark-youre-whats-for-dinner You can see the sensitivity of our olfactory.

“The Straight Dope” column. It’s one of my favorites. I’ve been reading the column for years. No question too arcane and too esoteric for Cecil to answer.
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bachophile
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HOLY CARP!!!
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wtf?
"I don't know much about classical music. For years I thought the Goldberg Variations were something Mr. and Mrs. Goldberg did on their wedding night." Woody Allen
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Aqua Letifer
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ZOOOOOM!
The 89th Key
Nov 28 2011, 06:48 AM
Mikhailoh
Nov 27 2011, 07:27 PM
Fascinating. I love that smell. Always assumed it was of the atmosphere, not the earth.
Me too
Two different things.

Ozone is what you smell after the storm has already passed.
I cite irreconcilable differences.
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