Most of us have noticed the unusual yet strangely pleasurable smell that permeates the air just after rain begins to fall. But have you ever wondered what causes that unique scent? Happy Ali sniffs out the answer and discovers an intoxicating synthesis of chemistry, bacteria, and the blood of the Gods.
Take a walk in your backyard the next time that rain clouds gather for an impending downpour. You will notice a couple of unique changes going on around you.
First, there will be an increase in humidity, the signal that moisture content in the atmosphere is rising. Then, as the very first raindrops begin to fall from the sky, an unusual and yet quite pleasant earthy smell rises all around you.
I grew up in a small country community in a farming region of rural Australia. As a seven-year-old child, I recall quite distinctly standing under the branches of a giant almond tree in the front yard of our home on a summer’s evening watching dark, mountainous storm clouds roll in from the west.
A unique smell would suddenly saturate the already damp air, right about the same time that a big fat dollop of warm rain hit the top of my head. Ever since, I have associated that unusual, heady fragrance with feelings that can be best described as a mixed sense of relief, happiness and even comfort.
That distinctive aroma – a fusion of clean air and damp soil – has a name. It’s called Petrichor. OK, it’s not a name that instantly conjures up that alluring, some might say captivating scent, but that is probably because it was first coined by two Australian scientists, Isabel (Joy) Bear and Richard Thomas, back in 1964 in an article entitled Nature of Argillaceous Odour that they co-wrote for the distinguished British academic journal, Nature.
Now, Nature of Argillaceous Odour is a big mouthful by any definition, even an academic one. They were investigating a phenomenon that they described, somewhat drily, in this way:
“That many natural dry clays and soils evolve a peculiar and characteristic odour when breathed on, or moistened with water, is recognized by all the earlier textbooks of mineralogy.” They went on to say that the smell is widely recognized and associated with the first rains after a dry spell or drought and also noted that “There is some evidence that drought-stricken cattle respond in a restless manner to this ‘smell of rain’.
Joy and Richard set out to discover what – exactly – made the smell. They steam-distilled rocks and stones that had been exposed to hot dry conditions out in the open. What resulted was a yellowish coloured oil that had been chemically bonded to the rocks and soil but then released by moisture.
They called it Petrichor. And the origin of the name is much more poetic than the name suggests. It comes from the Greek word petra, meaning rock and another Greek word, ichor, which is the liquid that flowed through the veins of the Gods who lived on Mount Olympus. Petrichor thus means, quite literally, the blood of the stone and it is the result of a complex set of chemical reactions.
The smell begins as the humidity starts to rise before a rainstorm. Tiny particles of moisture begin to form in the microscopic crevices and holes on the surface of rocks and in the soil. It’s just enough moisture to activate the oil and set the aroma free to be spread by the wind. When rain starts to fall the smell intensifies as bacteria – the same bacteria used to produce commercial antibiotics – in the soil release a molecule known as geosmin that adds yet another layer to the smell. (Geosmin is so strong a smell that humans can detect it at just one part per billion)
Finally, if lightening occurs at the same time, then electrified ozone in the air creates yet another layer of smell.
And here’s another fun fact. The “rain smell” was isolated and reproduced by perfume makers in the Indian region of Uttar Pradesh during the 1960s. They called the scent Mutti ka attar, which translates to English as “earth smell”.