An unexpected light shower, after a long dry period, which in some parts of the world, has also been hot and dusty. As the first drops hit the ground, an evocative, intoxicating smell feels your entire olfactory system, Petrichor.
It is a smell that has occupied the attention of scientists and perfumiers for almost two hundred years. You may catch hints of it in your favourite perfume or aftershave.
The name Petrichor is derived from the Greek words for rock, Petros, and ichor, in Greek mythology, a substance that flowed through the veins of the gods. It was coined in the 1960s, by Australian researchers, Isabel Bear and Dick Thomas. Up until then, the smell had the not so poetic appellation of argillaceous odour.
If like most people you find Petrichor appealing, you have chemistry to thank. Like all smells, this too is a chemical reaction.
Oils from certain plants are absorbed in between rocks and in soil, during dry seasons. When it rains, a molecule, geosmin, which is made by a particular bacterium, streptomyces, are released in the air.
All that remains for you to be transported back to your childhood, or your favourite place, is your sense of smell, and the magic is complete.
Bear and Thomas’s work was preceded by among others, TL Phipson, who noted his interest in this chemical reaction within “American Scientist” magazine, as long ago as 1891.
As with every attraction in nature, beauty, or in this case a luscious smell, Petrichor has a hidden purpose.
Like plants attracting birds, so their seeds can be eaten and spread farther afield, it seems the Petrichor producing bacteria attract other creatures which voraciously consume it, helping it to spread its spores far and wide. Being eaten, as part of the reproductive process.
Humans are particularly sensitive to Petrichor, which may also be tied up with their survival. Some researchers suggest this sensitivity is a primordial inheritance from ancestors who depended on rains for survival.
The next time you smell Petrichor, and you think of your perfume, or aftershave, you may actually be remembering your human ancestors’ relief that the rains are coming.