Why Can You Smell Rain Before It Falls?

The sky changes color before the rain starts. The wind shifts. And then there is the smell — something earthy and clean and difficult to name, arriving a few minutes before the first drop hits.

You’ve always known that smell. You probably like it. You may not have thought much about where it comes from.

It comes from bacteria, ozone, and sixty-million-year-old plant chemistry. And the reason you find it so pleasant might be traced back to when your ancestors needed rain to survive.

The Word and What It Names

In 1964, two Australian researchers — Isabel Bear and R. G. Thomas — published a paper in Nature describing a distinctive scent associated with rain falling on dry earth. They named it petrichor, from the Greek petra (stone) and ichor (the fluid that runs through the veins of the gods in Greek mythology).

But petrichor is not one molecule. It is a sensory experience produced by at least three overlapping sources that arrive at different times, in different conditions, and from different origins.

Source One: Geosmin

The most important molecule in the petrichor story is geosmin — a bicyclic alcohol produced by soil bacteria, primarily Streptomyces species. Streptomyces are gram-positive bacteria that form thread-like filaments in soil and are responsible for producing a substantial fraction of the world’s antibiotics, including streptomycin.

As part of their life cycle, Streptomyces produce spores. Geosmin is a byproduct of that sporulation. Rain disturbs the soil, aerosolizes the spores, and in doing so releases the geosmin they carry.

What makes geosmin remarkable is the sensitivity of human olfaction to it. The human nose detects geosmin at concentrations as low as 5 parts per trillion — roughly equivalent to one drop dissolved in the water of 200 Olympic swimming pools. For comparison, the detection threshold for mercaptan — the odorant added to natural gas specifically because it’s supposed to be detectable at extremely low concentrations — is orders of magnitude higher.

This extreme sensitivity is not an accident. When geosmin gets into drinking water — which it occasionally does, producing the characteristic earthy taste of tap water in certain regions — humans find it intensely unpleasant, even at concentrations far below any health threshold. We are wired to detect trace amounts of geosmin and, in water, to reject it.

In rain-soaked soil, however, the same molecule registers as pleasant. The context matters, but the sensitivity appears to be real and ancient.

Source Two: Plant Chemistry

The second component was identified in Bear and Thomas’s original 1964 paper.

During drought conditions, many plants produce oils that appear to function as growth inhibitors — potentially preventing competing seeds from germinating in dry conditions when resources are scarce. These oils accumulate in the soil and on rock surfaces, adsorbed onto clay minerals and buried in the substrate.

When rain arrives, the water displaces these oils, releasing them into the air. Bear and Thomas extracted the accumulated oils from rocks and soil and found they contributed significantly to the characteristic scent of rain on dry earth. The aroma was stronger after prolonged drought — more oil had accumulated.

This component is the original petrichor. It is the earth releasing what it has been holding.

Source Three: Ozone

The smell before a storm — distinct from the smell of rain on soil — is ozone.

Lightning causes nitrogen and oxygen in the atmosphere to combine into nitric oxide, which reacts further to produce nitrogen dioxide, which reacts with moisture and UV light to produce ozone (O₃). Ozone has a clean, slightly metallic, electrical smell. Under the right atmospheric conditions — particularly with a cold front bringing storm air downward — ozone can be detected at ground level before a storm arrives.

This is the smell of the air changing, not the smell of rain itself. But it often arrives first, mixing with the later, earthier petrichor when the rain actually falls.

The distinction is why people sometimes describe two different “rain smells” — one sharp and clean before the storm, one earthy and warm after the first drops hit.

The Mechanism: MIT Aerosols, 2015

Until 2015, it was unclear exactly how geosmin and petrichor oils got from the soil into the air at concentrations humans could detect.

Researchers at MIT — Young Soo Joung and Cullen Buie — filmed raindrops hitting soil surfaces using high-speed cameras and discovered a previously undescribed mechanism. When a raindrop strikes a porous surface, it traps tiny air bubbles inside the soil. These bubbles shoot upward through the water film and burst, releasing aerosols — microscopic droplets — laden with geosmin and other soil compounds.

The faster the rain, the more aerosols. Lighter rain produced more aerosol per raindrop than heavy downpours (heavy rain saturates the soil and reduces the bubble-trapping effect). The smell was strongest, they found, in the first minutes of light rain after a dry period — matching common experience.

The aerosols travel on the air. You detect them before the wetness reaches you.

Why Does It Smell Good?

Geosmin in water is a warning signal. Geosmin in rain is pleasant. The same molecule, two different responses.

The leading hypothesis for the pleasantness of petrichor is evolutionary: for much of human history, rain was survival. It determined food availability, water access, agricultural success, migration timing. Groups that responded positively to the smell of rain — groups that were motivated to seek it out, to stay in regions where it fell, to move toward it — may have survived more reliably than those who were indifferent.

This does not mean petrichor causes any behavior in modern humans. It means that a positive hedonic response to the smell of rain may be old enough that it’s been selected into human olfactory wiring, as default rather than learned.

Cross-cultural surveys suggest petrichor is consistently rated as one of the most pleasant environmental odors. It appears in perfumes, candles, and air fresheners across many cultures. The molecule is so commercially interesting that a synthetic version — cyclopentyl geosmin — has been developed for fragrance use.

What You’re Smelling

The sequence of a rain approach, chemically described:

Before the storm: ozone from lightning-driven atmospheric chemistry, descending with the front.

First drops on dry soil: aerosol release via raindrop impact — geosmin from Streptomyces, plant drought-oils from clay surfaces, both aerosolized and carried on the air.

The smell as it reaches you: a combination of all three, weighted toward the earthy-bacterial notes of geosmin if the soil has been dry, weighted toward clean-sharp ozone if a thunderstorm is approaching.

You’ve been smelling bacteria all along. They are everywhere — billions per gram of healthy soil — quietly cycling nitrogen, breaking down organic matter, and incidentally producing one of the most universally recognized and well-liked smells in the world.

The rain doesn’t make the earth smell. The earth has always smelled. The rain just lifts it into the air.