Why does rain smell good? The science of petrichor

The smell of rain hitting dry earth was named in a paper published in the journal Nature in March 1964 by two Australian researchers, Isabel Bear and R. G. Thomas. They proposed the term petrichor, from the Greek words for stone and the fluid that flows in the veins of the gods.

The paper was a careful piece of empirical work. They isolated, from rocks and soil that had been exposed to long dry weather, an oily substance that produced the characteristic smell when wet. They called it the petrichor essence. Until that moment, the smell was something everyone recognized and no one had described chemically.

The defining molecule in petrichor is called geosmin, from the Greek for earth-smell. It is produced primarily by streptomyces bacteria, which live in soil almost everywhere on the planet, and by some cyanobacteria in water. Geosmin is what makes a beet taste like beet, what makes some lake water taste muddy, and what makes wet soil smell the way wet soil smells.

The human nose is unusually sensitive to it. Reported detection thresholds vary by source, but humans can register geosmin at concentrations on the order of parts per trillion. There are very few molecules we are wired to smell at lower concentrations. We are, at the chemical level, a species that reads the soil.

Geosmin is one part of the smell. Bear and Thomas's other contribution was identifying the role of plant oils. During long dry periods, certain plants secrete oils that accumulate on the surface of soil and porous rocks. When rain hits, those oils are mobilized and aerosolized along with the geosmin and other compounds.

The full smell of rain after a long dry spell is therefore a layered one. Earth bacteria. Plant secretions. Sometimes ozone, depending on the storm. The fact that we tend to read the entire layered scent as a single pleasing experience is a reminder of how the brain composes smells out of components most of which it does not separately register.

Before some storms, particularly thunderstorms, the air carries the cleaner, sharper smell of ozone. Ozone is produced when lightning splits oxygen molecules, which then recombine into the three-atom variant. It is the same molecule that protects the upper atmosphere, encountered in much smaller concentrations near the ground.

Ozone is not technically part of petrichor. It often arrives just before. The combination of the two, the sharp ozone in the moments before rain and the soft earth-and-plant smell after the rain hits, is part of why a thunderstorm smells the way it does.

There is no settled scientific answer to why humans find petrichor pleasing. There are good hypotheses. The most discussed is that ancestors who tracked rainfall well had access to more reliable water and food, and a sensitivity to geosmin would have been a quiet survival advantage. The smell, in this read, is one of the oldest weather forecasts available to the body.

Whether or not the evolutionary story is exactly right, the result is consistent. A long dry spell ending in rain is one of the most reliable pleasant smells available to the human nose, across cultures and across centuries. The chemistry takes nothing away from the feeling. It just lets us name what is happening.

There is something quietly settling about the fact that the smell most universally read as pleasant is the smell of soil in contact with water. We are, structurally, animals that find ground and weather agreeable. A small reminder of this on the first afternoon of a long-awaited rain is one of the more reliably restorative experiences a person can have without trying.

The smell of rain on dry earth is not a single global smell. Different soils, plants, and microbial communities produce different versions of petrichor. The first rain on the high desert smells different from the first rain on coastal scrub. The first rain in a temperate forest smells different from the first rain on a cleared field.

The differences are part of why a particular place can hold a particular memory of rain. The smell is a chemical signature of the soil, the plants, and the bacteria native to it. To return to a place after years away and catch its specific petrichor in the first afternoon shower is to be reminded that the ground has its own face.