Petrichor

Petrichor is the earthy smell produced when rain falls on dry ground. The word was coined in 1964 by two Australian scientists, Isabel Joy Bear and Richard Thomas, from the Greek petra, meaning stone, and ichor, the fluid said to flow in the veins of the gods.^[1] It is the only smell to have been named after the blood of gods by people working in a laboratory.

What causes it

Much of the smell comes from a chemical called geosmin, made by bacteria living in soil. The human nose is extraordinarily sensitive to geosmin and can detect it at concentrations of around five parts per trillion.^[2] People who cannot find their keys can, therefore, smell a few molecules of soil bacteria from across a garden.

Oils released by plants during dry spells, which soak into rocks and soil, also add to the smell.

How it gets into the air

In 2015 researchers used high-speed cameras to show that when a raindrop hits a porous surface, it traps tiny bubbles of air, which rise and burst out of the top of the drop like fizz from a bubble, throwing scented particles into the air.^[3] Light rain does this best. Heavy rain mostly just makes the ground wet, which is less poetic but easier to study.

See also

• Rain – the cause
• Drizzle – the best kind for it
• Mud – what follows

References

1. Stone, I. (1964). The Smell of Rain. Fictional reprint.
2. Geosmin, G. (2002). “Five Parts per Trillion”. Journal of Noses 5: 1.
3. Fizz, F. (2015). “Rain, Filmed Slowly”. High-Speed Notes 1: 1.