Why does rain smell like rain?

Why does rain smell like rain?

By Steven Law, KSL.com Contributor | Posted - Oct. 22, 2012 at 12:06 p.m.



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SALT LAKE CITY — A rainstorm has several smells distinctly associated with it. The first smell people often notice during a rainstorm, even when the storm may still be many miles away, is ozone. To put it simply, ozone smells like really fresh air, and there’s a reason for that.

Ozone is a form of oxygen created when an electrical charge — in this case lightning — splits atmospheric nitrogen and oxygen molecules into separate atoms, says tropospheric chemist Louisa Emmons at the National Center for Atmospheric Research. But, says Emmons, it’s in the molecules' nature to recombine, and in doing so they sometimes recombine into a molecule made up of three oxygen atoms, or O3 (normal oxygen is made up of two atoms, O2). And O3 — or ozone — smells different than O2, which we are so used to we can’t even smell.

The second thing we’ll smell, as the storm reaches us, is the scent of rain on dry earth. And it even has a name. It’s called petrichor, constructed from Greek:

Ozone, a big component of the smell of rain.
Ozone, a big component of the smell of rain.

petra meaning stone, and ichor, which was the fluid that flowed through the veins of the gods in Greek mythology. The term petrichor was coined by two Australian researchers, Isabel Joy Bear and R.G. Thomas, in an article that appeared in the journal Nature in 1964.Specifically, what you’re smelling when the rain first falls is the spores of certain actinobacteria, writes Bear and Thomas in Nature. Actinobacteria are common in most types of soil. Actinobacteria play a major role in the decomposition of organic materials, such as cellulose and chitin, breaking down the plant matter and returning it to the carbon cycle.

Some members of actinobacteria reproduce by creating spores, known as actinomycetes. The impact of the rain hitting the soil disturbs and displaces the actinomycetes, and the lightweight spores are easily carried and wafted back into the air, where we can smell them, writes Bear and Thomas. That’s what you smell when the rain first begins to fall.

That “rain smell” is most noticeable during the first rain after a dry spell when there are more dried actinomycetes lying on the surface of the soil.

After the storm has passed you’ll notice another smell, the musty-earth smell of damp ground. What you’re smelling this time is the once-dried oil from certain plants being released into the air, writes Daisy

Actinobacteria, another component of the smell of rain.
Actinobacteria, another component of the smell of rain.

Yuhas in the July 2012 Scientific American.

Many plants coat their stems and leaves with oils during dry periods to protect themselves from harsh effects of the sun. They also coat their seeds with these same protective oils. It’s a defense mechanism against drought. The seeds won’t germinate while they are covered with the oil. When the rain finally comes, the protective but water-soluble oils are washed from the seeds and they can then germinate in the newly moistened ground.

These oils often get absorbed by the surrounding soils and rocks, especially when the plant dies and falls back into the earth. The oils dry up and also fall back into the soil. When the soil is dampened with rain, these dried oils are freshened and the smell is released back into the air, writes Yuhas.

While raining, a second compound, called geosmin, is released into the air, says Yuhas. Geosmin is a metabolic by-product of blue-green algae and bacteria.

If you have a science subject you'd like Steven Law to explore in a future article, send him your idea at curious_things@hotmail.com.

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