What is the smell of rain on dry ground called?

Answer

It comes from plant oils and a compound called geosmin.

Context

The word petrichor was coined in 1964 by two Australian scientists, Isabel Joy Bear and Richard Thomas, in a paper in the journal Nature. They built it from Greek words: petra, meaning stone, and ichor, the fluid said to flow in the veins of the gods.

Two things are behind the smell. During dry spells, plants release oils that collect in soil and on rocks. Soil bacteria, especially the group called actinomycetes, also produce a compound called geosmin, which smells earthy. When raindrops hit a porous surface they trap tiny air bubbles that burst and throw fine droplets, called aerosols, into the air, carrying these smells to your nose.

Humans are extremely sensitive to geosmin and can notice it at very low concentrations, which may be why the smell is so noticeable. This is a separate smell from the sharp scent people notice before a storm, which is often linked to ozone carried down by winds.

Bear and Thomas originally extracted the fragrant oil by steaming rocks and clay-rich soil, which is why their 1964 paper dealt with an odour linked to argillaceous (clay-bearing) material. They proposed that oils built up on dry ground and were released when moisture displaced them, and they also reported that such oils could slow seed germination, suggesting a possible role for plants in limiting competition during drought.

A later explanation of how the smell travels came from high-speed imaging of raindrops hitting porous surfaces, published in 2015. It found that bubbles rising through the drop burst at the surface and eject aerosols, with the effect varying with rain intensity and soil type; light to moderate rain appeared to release the most. Geosmin is made by microbes rather than by plants, so the full scent is a blend, not a single chemical. Some people also report smelling coming rain, a perception sometimes attributed to wind carrying these compounds ahead of a storm, though the evidence is less settled.