The distinct aroma that often follows a rain shower is a familiar sensory experience. This earthy, fresh scent is a natural phenomenon. This unique fragrance results from a complex interaction of environmental factors and specific chemical compounds released when raindrops meet the ground.
Petrichor: The Official Name
The scientific term for the smell after rain is “petrichor.” This word was coined in 1964 by two Australian researchers, Isabel Joy Bear and R.G. Thomas. They published their findings in the journal Nature, formally defining the phenomenon. The term “petrichor” is derived from ancient Greek words: “petra,” meaning stone, and “ichor,” which in Greek mythology refers to the ethereal fluid that flows in the veins of the gods. This etymology emphasizes the connection between the earth, specifically rocks and soil, and the unique scent released into the air by rain.
The Main Players Behind the Scent
The scent of petrichor is primarily composed of several chemical compounds, each contributing a distinct note to the overall aroma.
A significant contributor is geosmin, an organic compound produced by soil bacteria, particularly Streptomyces. These microorganisms thrive in warm, damp soil and release geosmin, which has a potent, earthy, or musty smell. The human nose is incredibly sensitive to geosmin, capable of detecting it at concentrations as low as 5 parts per trillion.
Another component of petrichor comes from volatile plant oils. During dry spells, plants secrete these oils, which accumulate on surfaces like rocks and soil. These compounds are released into the air when rain makes contact with these surfaces, contributing a waxy or slightly soapy undertone.
Ozone (O3) also plays a role, particularly during thunderstorms. Lightning contains electrical charges powerful enough to split oxygen and nitrogen molecules in the atmosphere. Some of these atoms then recombine to form ozone, which has a sharp, clean, and sometimes pungent scent, often described as similar to chlorine. This ozone can be carried by downdrafts from the storm, contributing to the “fresh” smell experienced before or during rain.
How Rain Intensifies the Aroma
Rain intensifies the aroma of petrichor through a physical process that disperses these compounds into the air. When raindrops strike a porous surface, they trap tiny air bubbles. These bubbles then shoot upwards through the raindrop, similar to fizz in a carbonated drink. Upon reaching the surface of the raindrop, these bubbles burst, releasing microscopic particles called aerosols into the atmosphere. These aerosols contain the various aromatic compounds, including geosmin and plant oils.
The release of these scented aerosols is most prominent during light to moderate rainfall, as heavy rain’s impact speed can repress bubble creation. Increased humidity helps these aerosols remain suspended and travel further, making the scent more widespread and noticeable. Changes in atmospheric pressure before and during rain can also contribute by drawing trapped scents from the ground into the air.