The scent of rain is a distinct sensory experience that often arrives just before the first drops fall. This common observation is rooted in a fascinating interplay of biology, atmospheric chemistry, and fluid dynamics. That fresh, earthy aroma is the result of specific chemical compounds being mobilized and carried to your nose by the shifting winds that precede a storm.
Petrichor: The Earthy Aroma
The unique, earthy scent most people associate with rainfall is known as petrichor. This name was coined by two Australian researchers, Isabel Bear and Richard Thomas, in a 1964 paper published in the journal Nature. The term is derived from the Greek words petra (“stone”) and ichor (the fluid that flowed in the veins of the gods).
The researchers discovered the smell originates from a yellowish oil that accumulates in rocks and soil during long dry periods. This oil is a complex mixture, partially secreted by certain plants, which collects on porous surfaces. When rain arrives, it interacts with this concentrated mixture of plant oils and other compounds, releasing their aroma into the air.
Geosmin: The Biological Source
One of the primary chemical components responsible for the earthy note in petrichor is geosmin. This molecule is a metabolic byproduct produced by soil-dwelling bacteria known as Actinobacteria, particularly those in the genus Streptomyces. These microbes thrive in moist soil but produce and release geosmin as they form spores during dry periods, causing the compound to accumulate.
The human nose is exceptionally sensitive to geosmin, capable of detecting it at concentrations as low as 5 parts per trillion. This remarkable sensitivity suggests an evolutionary advantage, possibly helping ancestors locate sources of water, given that the compound signals a moist environment.
Ozone: The Electric Scent
Separate from the earthy aroma of petrichor is a secondary, sharper, and often metallic smell experienced specifically before a thunderstorm. This clean scent is caused by ozone ($\text{O}_3$), a triatomic form of oxygen. Ozone is naturally produced in the atmosphere when strong electrical discharges, such as lightning, occur.
Lightning generates immense heat and energy, which splits atmospheric oxygen ($\text{O}_2$) and nitrogen ($\text{N}_2$) molecules into individual atoms. Some of these free oxygen atoms then recombine with existing oxygen molecules to form ozone. Because the human nose can detect ozone at concentrations as low as 10 parts per billion, the scent is carried from high altitudes to the ground by the storm’s strong downdrafts, signaling the approaching electrical activity.
The Mechanism of Release
The physical process that transfers these aromatic compounds from the ground into the air relies on the formation of aerosols, which are tiny airborne particles. When a raindrop strikes a porous surface, such as dry soil or pavement, it traps small pockets of air at the point of impact.
These trapped air pockets rapidly rise through the falling water droplet, behaving like bubbles in a glass of champagne. As the bubbles reach the surface, they burst, ejecting a cloud of microscopic aerosol particles into the atmosphere. These minute particles contain the dissolved geosmin, plant oils, and other compounds from the soil, effectively carrying the scent aloft.
High-speed camera studies show that this aerosolization process is most effective during light or moderate rainfall. Heavy rain hits the ground too forcefully to allow the bubbles to form and burst efficiently.