Petrichor – How We Long For A Whiff of It

One of the great pleasures of rain falling on dry ground is the marvellous smell that results.

For those of us who have been deprived of this pleasure for so long but who remember it with great fondness this mention in Wikipedia, the free encyclopedia is enough to bring tears to our (dry) eyes.

Petrichor is the earthy scent produced when rain falls on dry soil. The word is constructed from Greek, petra, meaning "stone", + ichor, the fluid that flows in the veins of the gods in Greek mythology.

The term was coined in 1964 by two CSIRO researchers, Isabel Joy Bear (Australian) and Roderick G. Thomas (British), for an article in the journal Nature. In the article, the authors describe how the smell derives from an oil exuded by certain plants during dry periods, whereupon it  is absorbed along with another compound, geosmin, a metabolic by-product of certain Actinobacteria, which is emitted by wet soil, producing the distinctive scent; ozone may also be present if there is lightning. In a follow-up paper, Bear and Thomas showed that the oil retards seed germination and early plant growth. This would indicate that the plants exude the oil in order to safeguard the seeds from germination under duress.

In 2015, MIT scientists used high-speed cameras to record how the scent moves into the air. The tests involved approximately 600 experiments on 28 different surfaces, including engineered materials and soil samples. When a raindrop lands on a porous surface, air from the pores forms small bubbles which float to the surface and release aerosols. Such aerosols carry the scent as well as bacteria and viruses from the soil. Raindrops that move at a slower rate tend to produce more aerosols; this serves as an explanation for why the petrichor is more common after light rains.

Some scientists believe that humans appreciate the rain scent because ancestors may have relied on rainy weather for survival.

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