Petrichor is the smell of rain when it falls on dry ground. The aroma comes from a mixture of chemicals, including geosmin from algae and bacteria, ozone from the effect of lightning on air, and volatile oils released by plants.
Key Takeaways: Petrichor and Geosmin
- Petrichor is the earthy smell that occurs when rain falls on dry ground.
- The scent comes from geosmin, plant oils, and ozone.
- Geosmin is produced by soil bacteria such as Streptomyces and by cyanobacteria.
- Humans can detect geosmin at extremely low concentrations, about 5 parts per trillion.
- Raindrop impacts create aerosols that carry scent molecules into the air.
- Ozone from lightning can contribute a sharp, clean smell before rainfall.
What Is Petrichor?
The term “petrichor” comes from the Greek words petra (stone) and ichor (the ethereal blood of the gods). Australian scientists Isabel Joy Bear and Richard G. Thomas introduced the word in their 1964 Nature article to describe the unique, earthy scent experienced when rain falls on dry soil. Prior to this, scientists referred to the scent as “argillaceous odour”. Chemical analysis of air during rain show it contains three groups of chemicals that are not normally present: geosmin, ozone, and certain plant oils.
Lightning produces ozone well in advance of a thunderstorm, so its scent often precedes rain. When raindrops fall, their impact disturbs geosmin and plant oils on rocks and soil. The water also traps tiny air bubbles that travel upwards through the raindrop and burst out in a fizz of aerosols, spreading the petrichor scent.
Geosmin
Geosmin is the primary compound responsible for the smell of rain. The word stems from the Greek words geo (earth) and osme (smell). Geosmin is a bicyclic alcohol produced by certain blue-green algae (cyanobacteria) and bacteria (mainly Actinomyces, such as Streptomyces). The compound has an earthy, musty, or muddy scent. Humans are highly sensitive to smelling or tasting geosmin. Its odor is detectable at levels as low as 5 parts per trillion. Many animals also smell geosmin. For example, camels use the odor for finding water in deserts.
Cyanobacteria and bacteria that produce geosmin release it when they die. When rain falls, it disrupts the surface and spreads geosmin into air, contributing its part to the petrichor scent.
While most people find the aroma of geosmin pleasant in rain, it also has a flavor. It is the molecule that gives beets and some wines their earthy flavor. However, it also gives water and fish (especially freshwater fish) a muddy or musty taste. Water treatment plants often take steps to remove geosmin from drinking water, typically using activated carbon or advanced oxidation. Fish farms purge the flavor by housing fish in clean water prior to harvesting. Adding lemon, vinegar, or another acid to a fish recipe breaks down the compound so that the dish tastes better.
Ozone in Petrichor
Lightning is the main source of the ozone scent of a thunderstorm. The energy of a lightning strike breaks the chemical bonds that hold nitrogen (N2) and oxygen (O2) molecules together. Some of the oxygen molecules combine and form ozone (O3). While a small amount of ozone in air smells like rain, too much is unhealthy and causes health effects, such as worsening asthma.
Volatile Plant Oils in Petrichor
Plants release oils during dry spells that protect them from loosing too much water and also deposit on the ground and prevent seed germination. This minimizes the chance of seedlings sprouting and then dying from lack of moisture. When rain falls, these oils splash up and enter the air, adding another earthy component to petrichor. The principal oils are stearic acid and palmitic acid. Both of these oils are nearly odorless, but they have a slight waxy, soapy aroma.
How Petrichor Forms
Petrichor forms through a combination of biological activity, dry weather conditions, and the physical impact of raindrops.
- Compounds build up during dry weather
Soil bacteria produce geosmin, and plants release protective oils that accumulate on soil and rock surfaces. - Raindrops strike dry ground
When rain begins, droplets hit porous surfaces and trap tiny air bubbles. - Bubbles rise and burst
The bubbles move upward through the droplet and burst at the surface, ejecting microscopic particles. - Aerosols carry scent molecules into the air
These particles contain geosmin and plant oils, forming an aerosol that disperses into the surrounding air. - Wind spreads the scent
Air currents transport the aerosols, allowing people to smell petrichor even at very low concentrations.
Why Rain Smells Stronger After a Dry Spell
The smell of rain is usually strongest after a long period without precipitation.
During dry conditions, geosmin and plant oils accumulate on surfaces. When rain finally falls, a larger amount of these compounds becomes airborne at once. Dry soil also enhances aerosol formation, making it easier for scent particles to enter the air. As a result, the first rainfall after a drought often produces the most intense petrichor.
Why Humans Are So Sensitive to Geosmin
Humans can detect geosmin at concentrations as low as a few parts per trillion. This sensitivity may have evolutionary advantages, helping early humans locate water sources or identify safe environments. Many animals also respond strongly to geosmin, although sensitivity varies by species.
Factors Affecting Petrichor Intensity
Several environmental factors influence how strong the smell of rain appears.
- Soil type: Porous soils release aerosols more easily than compact or saturated soils.
- Rain intensity: Light to moderate rain produces more aerosols, while heavy rain can suppress the effect.
- Temperature: Warm temperatures increase microbial activity and volatilization of compounds.
- Wind: Gentle wind helps disperse scent, while strong wind can dilute it.
- Microbial activity: Higher populations of geosmin-producing organisms increase the intensity of the smell.
Petrichor vs Other Rain-Related Smells
Not all smells associated with rain come from petrichor.
| Source | Smell Description | Cause |
|---|---|---|
| Petrichor | Earthy, musty | Geosmin and plant oils |
| Ozone | Sharp, clean | O₃ formed by lightning |
| Vegetation | Fresh, green | Plant volatile compounds |
| Urban surfaces | Metallic, oily | Pollutants and surface residues |
Applications and Scientific Relevance
Petrichor and geosmin have practical importance in several fields.
- Water treatment: Facilities remove geosmin to prevent unpleasant tastes and odors in drinking water.
- Aquaculture: Fish are placed in clean water before harvesting to eliminate geosmin-related off-flavors.
- Food science: Geosmin contributes to the earthy taste of foods such as beets and certain wines.
- Environmental monitoring: Geosmin levels can indicate microbial activity in soil and water systems.
- Fragrance industry: Chemists attempt to replicate petrichor for use in perfumes and scented products.
Common Misconceptions
- Petrichor is just ozone
Ozone contributes a scent, but petrichor mainly comes from geosmin and plant oils. - Rain itself has a smell
Pure rainwater is nearly odorless. The smell comes from compounds released from surfaces. - Geosmin only appears during rain
Geosmin is always present in soil but becomes airborne when rain disturbs the surface.
FAQs
Why does rain smell good?
Many people associate the earthy scent of geosmin with freshness and water, and humans are extremely sensitive to detecting it.
Can you smell rain before it starts?
Yes. Ozone produced by lightning and airborne particles carried by wind can create a noticeable scent before rainfall begins.
Is petrichor harmful?
Petrichor itself is not harmful, but high levels of ozone can irritate the respiratory system and worsen conditions such as asthma.
Why doesn’t rain always smell?
The effect is strongest after dry conditions. If the ground is already wet, fewer compounds are released into the air.
What causes the earthy smell after rain?
The primary cause is geosmin, a compound produced by soil bacteria and cyanobacteria.
References
- Anderson, W.; G.J. Prescott; S. Packham; J. Mullins; M. Brookes; A. Seaton (2001). “Asthma admissions and thunderstorms: a study of pollen, fungal spores, rainfall, and ozone”. QJM: An International Journal of Medicine. 94 (8): 429–433. doi:10.1093/qjmed/94.8.429
- Bear, Isabel Joy; Thomas, Richard G. (March 1964). “Nature of argillaceous odour”. Nature. 201 (4923): 993–995. doi:10.1038/201993a0
- Gerber, N. N.; Lechevalier, H. A. (1965). “Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes”. Applied Microbiology. 13 (6): 935–938. doi:10.1128/am.13.6.935-938.1965
- Phipson, T.L. (May 16, 1891). “The Odor of the Soil after a Shower”. Scientific American. 64 (20): 308.
- Watson, W.; Juttner, F. (2019). Taste and Odour in Source and Drinking Water: Causes, Controls, and Consequences. IWA Publishing. ISBN 9781780406657.