Reference point
Stand in a concrete yard as the first rain of the season comes down.
Do this before you read on. An entry only works when you have just smelled the thing.
Earthy, damp, slightly musty, with a cold mineral note. It arrives very quickly and fills the whole space rather than coming from one direction.
People detect the main compound of this smell at concentrations many orders of magnitude lower than almost any other smell. That is why nobody has to learn to recognise it, and why people describe it in much the same way regardless of culture.
The biggest surprise: this smell does not come from the rain. It comes from bacteria living in the soil, and the rain only throws them into the air. What you smell when the rain starts is the product of an organism, not of the weather.
How to smell it
Go out into a concrete or brick yard just as the first rain of the season begins, within the first two or three minutes. After that the smell thins quickly. If you miss it, you can stand in by watering a pot of soil that has been dry for a long time — the effect is weaker but of the same nature.
Why this one is hard
Because the time window is very narrow and cannot be repeated at will. It is one of the few entries where you have to wait. Rain on foliage also needs rain, coffee blossom opens only a few days a year, and termite mushrooms appear only after the first rains.
Don’t confuse with
This entry
First rain
Earthy family
First rain has a cold mineral note; straw mushroom is fishier, with a metallic note.
This entry
First rain
Earthy family
Both contain geosmin. First rain has a cold mineral note and comes from the air; stone moss is darker green, lower, and has to be scraped off to be smelled.
This entry
First rain
Earthy family
Clean clay gives a pure mineral smell, without the mustiness. First rain has geosmin from bacteria, so it has an extra biological layer.
This entry
First rain
Earthy family
The same shower, two places. Under the leaves it is green and wet; in an open yard it is earthy and mineral.
Scientific note
Geosmin is secreted by Streptomyces bacteria in the soil. The human nose detects it at a few parts per trillion; published figures range from four to one hundred depending on the study. The widely accepted hypothesis is that this ability had survival value: geosmin points to water, and for ancestors living on dry savannah that information was worth a life.
A comparison is often made with this compound: that the human nose is two hundred thousand times more sensitive to geosmin than a shark is to blood. The figure has spread very widely but does not stand up.
The reason is simple: humans smell geosmin in air, while sharks smell blood in water. Comparing parts per trillion in air with parts per million in water is not a valid comparison — the two media differ completely in molecular density and in how substances diffuse to the sensing organ.
Moreover, both figures vary widely. Published geosmin thresholds range from four to one hundred parts per trillion; shark thresholds range from one part per million to one part per billion depending on the species and the substance. The ratio of two hundred thousand is the result of picking the most favourable number on both sides.
The truth is remarkable enough without the shark comparison: this is one of the lowest thresholds ever measured for human smell, and it explains why the smell of first rain arrives instantly even though the molecule is fairly heavy.
In Vietnamese life
For a long time this smell could only be described with a whole clause. The name petrichor, built from Greek roots, was coined only in 1964. Lacking a word does not mean lacking the experience — it only means nobody had felt the need to name it.