Petrichor is the pleasant smell that accompanies the first rain after a dry spell. The word was coined in 1964 by Australian researchers Isabel Joy Bear and R.G. Thomas for an article in Nature (volume 993, issue 2); they derived the word from petro- (rock), a root that comes from Greek petros (stone), and ichor (the fluid that is supposed to flow in the veins of the gods in Greek mythology). A literal translation might be "stone-essence."

The originators' article explained a possible source of this scent: that vegetation gives off oily compounds which accumulate on the soil and rocks, particularly clay-based ones. (One author even uses the phrase "the smell of unbaked clay" to describe the phenomenon.) According to research released in January 2015, slow-motion videos have shown that raindrops hitting porous surfaces cause the release of aerosols, tiny bubbles of liquid suspended in gas, which allows the liquid to be carried through the air in a way that is easily smelled.

One online discussion-forum user says that 'the scent of petrichor has been extracted from rocks in Italy and marketed as "earthscent".' (I haven't found any independent confirmation of that.) In the animal world, this same scent is apparently a mating/egg-laying trigger for some wetland species. And an Edinburgh rock group has chosen to call itself Petrichor.

Sources:
http://wordsmith.org/words/petrichor.html
http://www.quinion.com/words/weirdwords/ww-pet2.htm
http://www.oswego.edu/~baloglou/misc/bombay-ice.html
http://www2b.abc.net.au/science/k2/stn/archives/archive34/newposts/250/topic250945.shtm
http://www.wisenet-australia.org/issue60/isabel_bear.htm
http://www.kzn.org.za/kzn/news/514.xml
http://homepage.ntlworld.com/rocsystems/bd/sound.html
http://newsoffice.mit.edu/2015/rainfall-can-release-aerosols-0114
http://www.washingtonpost.com/news/speaking-of-science/wp/2015/01/16/slow-mo-video-of-raindrops-reveals-how-rain-gets-its-distinctive-smell/

First, there is nothing...

We've been living in drought for ages Down Under. Water levels are running low practically everywhere. The Grampians' reservoir is apparently down to 3.5% full. Melbourne is in luxury, at levels fluctuating between about 50% and 40%. Darwin, one of the remotest cities and practically in a desert, is over 80% full. (Statistics as of today, 27 October.)

No rainfall...

The drought is so bad, that no longer is there such thing as bad weather. If it's a nice, sun-shiny day, you can sunbathe; if it's a gloomy day, we can hope for rain, and the sweet, sweet smell that comes with it. It is up there with freshly cut grass, freshly baked bread and fresh bedsheets.

One inch a day, that's all we ask...

I smelled it this morning. Here, we have a festival every October, and althought I wasn't there at the time the rain came, I could just picture everyone who was in the street either running for cover or dancing out in the miraculous water...

Sweet relief...

...but we only got it for a minute.

We have to settle for more petrichor. Not a complaint.

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