In the brown silence
a handful of soil
stirs into life,
germinating dust
scented by the breath
of the ripening sky

There is a delicious scent of the forest when it receives rainfall after a dry spell. The English word for it is petrichor, derived from the Greek words: petra which means stone, and ichor which is the essence of the fluid that courses through the veins of the gods. So, in Greek mythology, petrichor literally means the ‘essence of life.’

Mythology aside, there is a science to petrichor. When a forest is dry for a long period, plants secrete oils which then drips down and becomes trapped under rock and soil. When it rains, this oil is released to the air by moisture, and with it, the aromatic compounds produced by soil-dwelling bacteria known as actinomycetes or geosmin. It is this compound that gives off the distinct earthly scent of petrichor – the smell of rain after a dry spell.

And guessed what? Our noses are incredibly sensitive to this smell as any one who has walked through a forest bathed by rain can attest. In fact, we have evolved to be able to detect the smell of petrichor at levels as low five parts per trillion.

The scent is powerful but not very durable. It blooms whey dry ground is moistened by the first few drops of rain or even in response to an increase in humidity that precedes rain. But when the soil gets too wet, the smell vanishes because they get drowned out by a heavy downpour. Petrichor lovers know this, and make it a point to be out when the first flush of rain hits the dry ground, just so to take in the earthly scent.

You may ask why nature has endowed us this super-sensitivity to the smell of petrichor. The scientific theory is that it is good for our survival, or least the survival of our agrarian early ancestors because having a nose for the smell of rain after a dry spell enables them to locate water sources and areas with more fertile soil suitable for growing food, both critical factors for survival. Aside from that, geosmin is known to have a calming effect that reduces stress levels by activating the brain’s reward and emotion centers. The emotional benefits of petrichor is a reminder of nature’s goodness and why we need to reconnect with nature on a regular basis even though we live and work in cities far removed from it.

Further study

Joseph Stromberg (2013), “What Makes Rain Smell So Good?”, Smithsonian Magazine. Link:

https://www.smithsonianmag.com/science-nature/what-makes-rain-smell-so-good-13806085