Petrichor is the characteristic earthy aroma associated with rain falling on dry ground, a term introduced by Australian researchers Isabel Joy Bear and R. G. Thomas in a 1964 study of the “argillaceous odour” evolved by dry rocks and soils when moistened. According to their paper in Nature (journal), “petrichor” refers to a tenuous essence derived from stone, from Greek petra (stone) and ichor (the fluid of the gods). Nature; Merriam‑Webster.

Coinage and early research

Bear and Thomas steam‑distilled rocks and soils previously exposed to warm, dry conditions and identified a yellowish oil trapped in these materials that was released by moisture and responsible for the odour; they proposed the name “petrichor” for both the smell and the oil. Nature; CSIRO. In a follow‑up article, they reported that the oil could inhibit seed germination, indicating a role in plant growth regulation during arid periods. Nature.

Chemical contributors

A major contributor to petrichor is Geosmin, a volatile sesquiterpenoid alcohol produced by soil microbes, notably Streptomyces within the Actinobacteria. Humans detect geosmin at exceedingly low concentrations—typically around 4–10 ng/L in water—and recent work identified the human olfactory receptor OR11A1 as highly responsive to geosmin. Journal of Agricultural and Food Chemistry (PMC). Geosmin and the related 2‑methylisoborneol are well‑established causes of earthy–musty odours in water and foods. Journal of Agricultural and Food Chemistry (PMC).

Petrichor also reflects plant‑derived oils that accumulate on surfaces during dry spells and are released with the first rains; Bear and Thomas originally linked these hydrophobic compounds to the distinctive odour noted after drought. Nature; Smithsonian Magazine.

Physical release mechanism

High‑speed imaging and fluid‑dynamics experiments have shown that when raindrops strike porous surfaces such as dry soil, tiny air bubbles form and burst through the droplet, ejecting aerosols that carry soil particles and volatiles—including geosmin and plant oils—into the air. This aerosolization is most vigorous in light to moderate rainfall and on sandy or clay‑rich soils; heavy rain suppresses bubble formation and reduces aerosol release. Nature Communications; MIT News; Met Office. These aerosols are a primary pathway by which petrichor compounds reach the nose. Nature Communications.

Associated odours: ozone before storms

Before rainfall, some people notice a sharp, chlorine‑like scent attributable to Ozone, which can be produced when lightning splits atmospheric gases; storm downdrafts can transport ozone from higher altitudes to the surface ahead of rain. This “storm scent” is distinct from petrichor but often accompanies it. Scientific American; The Washington Post.

Sensory perception

Humans are remarkably sensitive to geosmin; thresholds in the single‑digit ng/L range in water have been reported, and a specific receptor (OR11A1) mediating geosmin detection has been functionally characterized. Journal of Agricultural and Food Chemistry (PMC). Public information sources similarly describe the composite nature of the odor—plant oils, microbial volatiles, and occasionally ozone—under the umbrella term “petrichor.” Merriam‑Webster; Smithsonian Magazine.

Ecological context

Geosmin production is intertwined with microbial life cycles: in Streptomyces, emission of geosmin and 2‑methylisoborneol is developmentally regulated during sporulation and attracts soil arthropods (springtails), which feed on colonies and disperse spores via adherence and fecal pellets. Nature Microbiology; PubMed. The plant‑oil fraction of petrichor can inhibit seed germination under drought conditions, potentially modulating vegetation dynamics until sustained rainfall occurs. Nature.

Cultural and industrial notes

Long before the scientific term was coined, perfumers in northern India captured a rain‑on‑earth accord known as mitti attar by hydro‑distilling baked clay into sandalwood oil, a practice associated with the city of Kannauj. Modern accounts and reportage describe this artisanal distillation as an attempt to bottle the scent of the first Monsoon rains. Smithsonian Magazine.

Terminology and usage

Lexicographic authorities define petrichor as the scent produced when rain falls on dry soil, typically after a warm, dry period; the term has been widely adopted in scientific and popular discourse since its introduction on March 7, 1964. Merriam‑Webster; Nature; CSIRO.