As the monsoon advanced across South India, Wikipedia shared an apt factoid on twitter last week, ‘The smell of rain is “petrichor.” ’ The oxford dictionary defines petrichor as ‘a pleasant smell that frequently accompanies the first rain after a long period of warm, dry weather.’ No wonder most of us love that ‘fresh wet mud’ smell – it heralds an end to the sultry summer and the onset of cloudy breezy weather, but more about that in a bit.
The word petrichor is derived from a combination of the Greek words ‘petra’ meaning stone, and ‘ichor’, the fluid that flows in the veins of the Gods in Greek mythology. Petrichor was first described in a paper published in Nature journal in 1964 by Australian CSIRO scientists Isabel Joy Bear and Richard G. Thomas, and the process that gives rise to the fragrance we perceive is fascinating indeed.
During dry periods, plants exude an oil that retards seed germination and early plant growth, this oil is absorbed by clay – based soil and rocks (hence the words ‘petra’ and ‘ichor’). The smell itself comes about when increased humidity fills the pores of rocks, soil, etc. with tiny amounts of water. While it’s only a minuscule amount, it is enough to flush the oil from the stone and release petrichor into the air; this must be why we get that slight whiff of the rain smell before it actually starts raining. This process is further accelerated when actual rain arrives and makes contact with a porous surface; air from the pores forms small bubbles which float to the surface and release aerosols, such aerosols carry the scent and spread it. The article that I referred to for much of this information also has a super slow motion video released by scientists at MIT last year that helps explain this process.
So now you know it, Petrichor – that fresh fragrance that makes everyone’s spirits rise, is not the fragrance of the rain, neither is it that of wet mud, it’s actually due to an oil that’s released from the rocks or soil into the air just before the rain begins to fall. Did the description of the process in the last paragraph kill the romance of that fragrance for you, or make it even more interesting ? I hope it’s the latter, because more deconstruction follows – this time of why we find the smell of the rain so pleasant.
The reason petrichor makes our spirits rise may actually be hardwired into our memories, part of our collective consciousness – some scientists think it’s due to our species’ reliance on rain for a plentiful supply of plants and game animals throughout history. This article describes some evidence that may support this hypothesis. Anthropologist Diana Young of the University of Queensland in Australia, who studied the culture of Western Australia’s Pitjantjatjara people, has observed that they associate the smell of rain with the color green, hinting at the deep-seated link between a season’s first rain and the expectation of growth and associated game animals, both crucial for their diet.
So next time you draw in a deep breath of that fresh wet mud smell and feel invigorated, think of how that reaction connects you to your ancestors from centuries ago.
Next post : Now that we’ve serendipitously stumbled upon the topic of fragrance and the collective consciousness, we’re going to take this thread further – look out for more examples in our next post.
- Zenobia Driver