Petrichor: What Causes the Earthy Smell After Rain?

Australian scientists Isabel Joy Bear and Richard Grenfell Thomas coined term "petrichor" in 1964. The word "petrichor" itself is derived from two Greek words: "petros," meaning "stone," and "ichor," which in Greek mythology referred to the fluid that flowed in the veins of the gods. The term was chosen to emphasize the connection between the earth and the air, which is fundamental to the release of the scent during rainfall.

The scientists published their research on the smell of rain and the chemical compounds responsible for it in the journal Nature. For the study, Bear and Thomas investigated the earthy aroma that arises when rain falls on dry soil. They identified two key compounds, geosmin and 2-methylisoborneol, produced by certain types of soil bacteria, as major contributors to the petrichor scent.

While the smell of rain and the phenomenon of petrichor were known long before Bear and Thomas's research, they were the ones who formally defined and named it in the scientific literature. Their work significantly contributed to our understanding of the chemistry behind the scent of rain.

As it turns out, the smells people associate with rainstorms can be caused by a number of things, including raindrops themselves. (More on that later.) One of the more pleasant rain smells — the one we often notice in the woods — is actually caused by bacteria. Actinomycetes, a type of filamentous bacteria, grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produce spores in the soil.

The wetness and force of rainfall kick these tiny spores up into the air where the moisture after a rain acts as an aerosol (just like an aerosol air freshener). The moist air easily carries the spores to us so we breathe them in. These spores have a distinctive, earthy smell we often associate with rainfall.

The "rain smell" is caused by a chemical in the bacteria called geosin, which is released by the bacteria as they die. Geosin is a type of alcohol molecule with a very strong scent. The bacteria are extremely common and can be found in areas all over the world, which accounts for the universality of this sweet "after-the-rain" smell.

Since the bacteria thrive in moist soil but release the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell. That said, you'll notice it to some degree after most rainstorms.

Breakthrough Raindrop Study

In a study conducted in 2015, researchers at the Massachusetts Institute of Technology (MIT) uncovered a mechanism that may explain how raindrops release aerosols and potentially carry aromatic elements, bacteria and viruses from the soil into the environment. Using high-speed cameras, the scientists observed that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact, much like bubbles in champagne.

These bubbles then shoot upward and burst from the raindrop, creating a cloud of aerosols. The team was able to predict the amount of aerosols released based on the raindrop's velocity and the permeability of the surface. Researchers believe that in natural environments, these aerosols may transport aromatic compounds, as well as microorganisms and chemicals from the soil. Light or moderate rainfall may trigger this aerosol release, which can then be dispersed by wind.

This discovery has potential implications for understanding how soil-based diseases spread and how various compounds are distributed in the environment.

Petrichor is not the only rain-related smell. There's another distinctive scent caused by the acidity of rain. Because of chemicals in the atmosphere, rainwater tends to be somewhat acidic, especially in urban environments. When it comes in contact with organic debris or chemicals on the ground, it can cause some particularly aromatic reactions. It breaks apart soil and releases minerals trapped inside, which react with chemicals, such as gasoline, giving them a stronger smell.

These reactions generally produce more unpleasant smells than bacteria spores, which is why the after-the-rain smell isn't always a good one. Like the smell caused by the bacteria spores, the smell of chemical reactions is most noticeable when it rains following dry periods. This is because once the chemicals on the dry ground have been diluted by one downpour, they don't have the same reaction with the rainwater.

Volatile Oils and Aromatic Materials

Another after-the-rain smell comes from volatile oils that plants and trees release. The oil then collects on surfaces such as rocks. The rain reacts with the oil on the rocks and carries it as a gas through the air. This scent is like bacteria spores in that most people consider it a fresh and pleasant scent. It has even been bottled and sold for its aromatic qualities!

Aside from rain smells, there are all sorts of other scents in the air after it rains. These derive from the aromatic materials that the moisture and impact of rain can stir up — the moist atmosphere following a downpour is particularly good at carrying these particles through the air.

So, when you talk about the after-the-rain smell with a friend, you may mean one thing while your friend is thinking of something else. You'll both agree, however, that the air has a much stronger aroma to it after a good rain.

Humans often enjoy the smell of rain for several reasons, both biological and psychological. Some scientists speculate that humans may have developed an affinity for the smell of rain because it signaled the end of drought and the potential availability of fresh water. In our evolutionary history, access to clean water was crucial for survival, so an attraction to the smell of rain could have been advantageous.

Rain is also often associated with positive experiences and emotions, such as relief from hot weather, the nourishment of crops and the soothing sound of rainfall. These positive associations can make the smell of rain more appealing to humans. Additionally, rain can create a sense of coziness when people are indoors, which can enhance their overall enjoyment of the experience.

On a more intimate note, some people often have specific experiences and associations with rainy weather that can shape their perception of the smell of rain. It may call to mind pleasant childhood memories, cozy days spent indoors or romantic moments, all of which make it easier to understand why some people think of rain as such a pleasant smell.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.