Why does rain smell so distinctive the moment it starts?
― The raindrops themselves are launching that smell into the air
A summer evening, and the first fat drops of rain begin to fall — and with them comes that earthy smell. It's so unmistakable you can tell it's raining without even opening a window. But that's not the smell of rain. The source of the smell has been sitting on the ground for a long time already. The raindrop is simply the courier that delivers it to your nose.
It's a summer evening after several days of clear skies. The sky suddenly darkens, and fat raindrops start falling, one by one.
You're not wet anywhere yet. And still, a faint smell drifts up into your nose — earthy, slightly green, unmistakable.
Here's the odd part: the smell is strongest in the first few minutes, stronger than once the rain has settled into a steady fall. That's our first clue to the answer.
The smell has two separate ingredients
The ground is full of tiny living things you can't see. Some of them produce a compound with a very distinctive odor. During a run of dry days, that compound clings to soil particles and quietly builds up.
When a raindrop hits dry ground, air trapped in the tiny gaps between soil particles gets sealed in as a small bubble. When the bubble bursts, invisibly fine droplets shoot upward, carrying the smell into the air.
In other words, the "maker" of the smell compound and the "carrier" that delivers it are two different things. Neither alone produces that smell. Let's look at each in turn.
The smell's source has been waiting in the soil all along
Soil is home to huge numbers of bacteria. One particular group is known to produce a compound with a very distinctive smell. It's called geosmin, a name said to come from words meaning "earth smell."
Geosmin forms inside damp soil. When clear days continue, it gets carried to the surface along with moisture and ends up clinging to the surface of dry soil particles. As shown on the left of Figure 1, dry ground is effectively a reservoir that has been storing up the source of the smell.
What's remarkable is how sensitive the human nose is to this particular compound. Even an extremely tiny amount is said to register as "the smell of earth." Just how tiny that amount is, we'll actually calculate in the final collapsible section.
The raindrop turns into a "spray bottle" that strikes the ground
So how does the smell compound clinging to the soil surface actually reach your nose? This is where the impact of the raindrop comes in.
Dry soil, or a porous surface like unglazed pottery, is riddled with countless tiny gaps. When a raindrop lands there, water seals off those gaps, and the air inside has nowhere to escape. As shown in the middle of Figure 1, that air gets trapped as a small bubble beneath a film of water.
Because a bubble is lighter than water, it rises to the surface of the film — and bursts through it. The force of that burst launches invisibly fine droplets upward. It's the same thing that happens when bubbles in a carbonated drink burst and you feel a fine spray on your face.
Those launched droplets carry dissolved smell compounds that were sitting on the soil surface. Being light, the droplets simply ride along on air currents. That's how the smell gets carried in one leap from the ground up to the height of your nose.
This also explains why the smell is strongest right when the rain starts. Once the rain keeps falling, the ground becomes waterlogged and no air is left in the gaps in the soil. With no bubbles forming, no droplets get launched. With no courier left to carry it, the smell quietly fades away.
Sometimes you notice a smell before the rain even starts. One reason is that wind can carry ahead the smell created by rain already falling somewhere in the distance. Another is thought to be a relative of oxygen formed inside thunderclouds, which descends along with downdrafts. That one is said to smell less earthy and more sharp and pungent.
When phytoplankton multiply in a summer lake or reservoir, the same smell compound can dissolve into the water. It poses no health risk, but because even a tiny trace is noticeable, Japan's drinking water quality standards set an upper limit for it. Water treatment plants take specific countermeasures, such as using activated carbon to absorb the smell.
Summary
The smell of rain isn't something the rain brought with it. It's the smell the ground had been storing up during the dry spell, launched into the air, bubbles and all, by the falling raindrops. That's why the first few minutes smell strongest, and why the smell actually fades the longer the rain keeps falling. That smell is like a brief dispatch from the ground itself.
What you're smelling isn't the smell of rain.
It's the smell of the ground, dug up by raindrops.
For how the raindrop itself comes into being, see Clouds are made of water, so why doesn't it just fall out? For what happens as a droplet dries, see Why does a coffee drop leave a ring-shaped stain when it dries? And for how the ground gets wet even without rain, see Why is the morning grass wet even when it hasn't rained? The mechanism by which we sense smell as "taste" is covered in Why does food lose its taste when you have a cold? And the way your nose becomes less sensitive to a smell the longer you're exposed to it is explained in Why do you suddenly notice the fragrant olive's scent "one particular morning"?
- After a few clear days in a row, bring your nose close to the dry soil in a garden or planter and take a smell. There should be almost no smell at all.
- Next, sprinkle just a little water on with a watering can, then smell it again right away. That same soil now gives off that unmistakable smell clearly. Just enough water to dampen the surface is plenty.
- Once rain starts falling, compare the strength of the smell over a paved path versus over soil or grass. You should find the smell is stronger over ground with more gaps in it.
Don't bring your face too close to the soil, and wash your hands afterward.
Want to go deeper? ― Terms, formulas, and how this connects to the textbooksWe've labeled which level each part belongs to, from middle-school science through university-level courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry"
- High school+High-school advanced content, or textbook sidebar material
- UniversityNot covered in high school — university-level specialist content (surface chemistry, olfactory physiology)
- ResearchNot yet settled even at university level — an active area of research
Middle schoolTerminology: this phenomenon has a name
- Geosmin: the compound behind the smell of earth. It's produced by soil bacteria and by certain freshwater algae.
- Actinomycetes: a group of bacteria abundant in soil. Many grow into thin, branching, thread-like forms, and they're the best-known producers of geosmin.
- Petrichor: the word for that whole distinctive smell that rises when rain falls on dry ground. It's said to have been coined by researchers in the 1960s.
- Aerosol: extremely fine liquid or solid particles suspended in air. What this article calls the "launched droplets" are exactly this.
Middle schoolHigh schoolChecking the numbers: how little does it take before you can't smell it?
Let's put "even an extremely tiny amount can be detected" to a numerical test. We'll use units of weight — nanograms and milligrams — and units of volume — liters and cubic meters. A nanogram is one gram divided by a billion; a milligram is one gram divided by a thousand. We'll spell these out in words rather than using symbols.
| In symbols | m = c × V |
| In words | weight needed to notice it = lowest noticeable concentration × volume of water |
| Where it comes from | This is just the definition of concentration (weight dissolved per volume), turned around. We're finding the amount that would make the whole pool's concentration exactly equal to the "detection threshold" for the smell. |
| Symbol | Meaning and unit |
| m | weight of smell compound needed for the whole pool (nanograms) |
| c | lowest concentration people can notice (nanograms per liter) |
| V | volume of water (liters) |
| Concentration at which people can notice the smell | roughly 10 nanograms per liter |
| Volume of water in a 25-meter pool | roughly 400 cubic meters |
| How many liters in a cubic meter | 1000 liters |
| How many nanograms in a milligram | 1,000,000 nanograms |
| How many liters of water in the pool | 400 × 1000 = 400,000 |
| Weight needed to notice it (nanograms) | 400,000 × 10 = 4,000,000 |
| Converting to milligrams | 4,000,000 ÷ 1,000,000 = 4 |
| Converting to grams | 4 ÷ 1000 = 0.004 |
Just 4 milligrams in an entire school swimming pool of water — about the same as a few grains of sand. And yet the human nose still notices it as "earthy." The droplets a raindrop launches are too small to see, and yet the smell comes through clearly — that's how sharp this sensitivity really is.
High schoolHigh school+Why does dry soil build up the smell?
High schoolThe smell compound is held in place clinging to the surface of soil particles. A substance sticking in a thin layer to a solid surface is called adsorption, which also comes up in Basic Chemistry with the example of activated carbon. Soil particles have rough, uneven surfaces, so their surface area is huge relative to their apparent size, which makes them very good at trapping such compounds.
High school+When water arrives, water molecules compete with the compound for space on the soil surface. Geosmin only dissolves in water to a limited degree, so when water arrives it tends to get pushed off the surface. The displaced molecules move into the thin film of water and ride along on the droplets that fly off when a bubble bursts. Drier ground smells stronger both because more of the compound has built up and because air remains in the gaps, making bubbles easier to form.
UniversityHow a bursting bubble launches droplets
When a bubble bursts at a water surface, it's known to produce two kinds of droplets. One kind comes from the bursting film itself shattering into fine particles. The other comes from water rushing into the depression left by the collapsed bubble, shooting up a thin jet of water from the center that then breaks apart. The second kind has long been studied in research on sea spray, and its size is set by a balance of surface tension, viscosity, and gravity. The same mechanism is thought to apply when a raindrop hits porous ground, and high-speed photography has shown a single raindrop producing hundreds of individual droplets. These droplets travel as fine airborne particles, that is, as an aerosol. The whole smell of rain is called petrichor. How strongly the smell rises is thought to depend on the type of ground and the intensity of the rain, both of which affect how easily these bubbles form.
📖 Go further: Petrichor (Wikipedia, Japanese) ／ Joung & Buie (2015), a high-speed-photography paper on aerosol generation by raindrops hitting soil
ResearchWhat's still not fully understood
- Why the human nose is so sensitive to this particular compound. Several theories have been proposed — that it helped in finding water sources, or in avoiding spoiled food — but no way has been found to confirm which, if any, is correct.
- Why soil bacteria produce this compound in the first place. Some reports suggest it's a lure to attract small soil-dwelling animals, which then carry the bacteria's spores away on their bodies over long distances. But whether that fully explains it is still under debate.
- What exactly makes up the pre-rain smell. Gases descending from thunderclouds, distant smells carried by wind, compounds released by plants — several candidates have been proposed. Because they all mix together in the real outdoors, it hasn't been possible to separate out how much each one contributes.
In other words, the content of this article, too, is "the best explanation currently available." The broad picture of where the smell comes from and how it's carried is well established, but questions remain about its meaning and finer details.
Connections to the textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, Field 2, "Balance in Nature" | Soil microbes producing the compound |
| High school | Basic Chemistry, "States of matter and dissolution" | Solubility in water, adsorption to a surface |
| High school+ | Advanced chemistry, "Surface tension and colloids" | Bubbles forming and bursting to launch droplets |
| University | Surface chemistry, fluid dynamics, olfactory physiology | How bubbles launch droplets, sensitivity of smell perception |
| Research | Soil microbiology, atmospheric aerosols | Why bacteria produce it, what makes up the pre-rain smell |
| ― | Everyday connections | Why the smell is strongest right when rain starts, musty tap water |
- Bear, I. J. and Thomas, R. G. "Nature of Argillaceous Odour", Nature, 1964 (naming of petrichor)
- Joung, Y. S. and Buie, C. R. "Aerosol generation by raindrop impact on soil", Nature Communications, 2015
- Gerber, N. N. and Lechevalier, H. A. "Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes", Applied Microbiology, 1965
- Ministry of Health, Labour and Welfare, "Ordinance on Water Quality Standards" (厚生労働省「水質基準に関する省令」— list of water quality standard items and values, geosmin entry)
- Japan Water Works Association (日本水道協会), Drinking Water Quality Standards Guidebook (『水道水質基準ガイドブック』— explanation of musty-odor compounds and water treatment)
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding, not precise values. When heavy rain is forecast, please follow the latest information and instructions from official sources, such as the Japan Meteorological Agency or your local government.