We’ve all heard someone say “I smell rain,” as storm clouds gather. And we’ve probably heard experts say “rain has no smell.” As odd as it may seem, both answers are correct…well, sort of.
When a raindrop comes in contact with the earth (or any porous surface) it traps miniscule pockets of air. These air “bubbles” speed upwards very quickly and explode at the top surface of the drop. This, of course, takes place in milliseconds. Upon bursting, the bubbles release microscopic particles called aerosols.
The amount of these aerosol particles relies on how many raindrops hit the surface. The speed and number of the raindrops and the quality of the soil will determine how many of the aerosol droplets are released.
If close to the rain, we smell this rich earthy mineral odor right away. If farther away, temperature variations can cause wind to deliver it to our noses, even from many miles away.
The scent is called petrichor – a composite of two Greek words: “petra” meaning “stone,” and “ichor” meaning “the blood of the gods.”
Petrichor is basically composed of two substances: one is an oil that plants secrete during dry periods. The oil inhibits germination of seed. Plants hold onto this oil until it rains, then they release the oils. These oils are encompassed in the tiny aerosol bubbles.
The second element is that rich, earthy smell, caused by microscopic, bacteria-like creatures called actinomycetes, which are released when the rain drop hits fertile soil. Actinomycetes are great nitrogen-fixing organisms. This means they can trap nitrogen from ozone and from the air and help soil retain it for later use by plants. Actinomycetes exist in large numbers (read millions) in a teaspoon of good, fertile soil, and are part of an ecosystem that creates strong roots systems in turf grass and other plants. One family of actinomycetes, Streptomyces, provides us with many of the important antibiotics used in medicine.
A third element is ozone, which may or may not be present trapped in the aerosol. Ozone is formed during a thunderstorm. Lightning can split oxygen and nitrogen molecules which then forms nitric oxide. Rain brings this form of nitrogen directly into the ground. That’s why people say “everything looks greener” after a thunderstorm. It probably is greener, as plants take in lots of nitrogen from the rain, which helps them create more chlorophyll, thus greening up the plant leaves.
When someone says they smell rain coming, it’s probable that wind from a coming storm or rain event is carrying ozone, actinomycetes and oils from the soil and oils from plants. This goes into a person’s nostrils and is interpreted as “smelling rain.”
Enjoy the smell. It’s beneficial to humans, other animals and to the soil itself.