What is Petrichor? The smell of the Earth when it rains

Have you heard of Petrichor? If you have, chances are it might have been from Doctor Who, Risk of Rain, Saga, Phish, or The 1975 - things from popular culture that referenced this unique word. But what exactly is Petrichor, and how is this uniquely Earthy scent created?

What is Petrichor?

Historically, it is from the Greek roots "petra" (meaning stone) and "ichor" (roughly translating to "the fluid that flows in the veins of the gods"). It refers to the distinct earthy scent that fills the air when rain falls on dry soil. Its smell is uniquely nostalgic and evokes a sense of renewal.

What causes the smell?

At the heart of this unique aroma is a compound called geosmin, which is produced by a bacteria in the soil called an actinomycete. These actinomycetes thrive in damp soil, so when the soil begins to dry out due to a lack of rain, the bacteria begin to die off and release geosmin spores. When raindrops hit the dry ground, they disturb these bacteria, releasing geosmin into the air. This compound is incredibly potent, with humans being able to detect it at concentrations as low as 5 parts per trillion. To put that in perspective, humans are more capable of detecting geosmin in the air than sharks are at detecting blood in the water.

Dry soil on the forest floor before the rain

In addition to geosmin, the characteristic smell is also influenced by ozone in the air. Ozone is most often associated with the smell before a thunderstorm. Why? When lightning strikes, it ionizes the surrounding air molecules, breaking apart oxygen molecules (O2) into individual oxygen atoms (O). These oxygen atoms recombine with oxygen molecules and form ozone (O3). While this process primarily occurs in the upper atmosphere, lightning can produce ground-level ozone. You smell the ozone as a storm approaches due to the wind currents that carry these particles to you in advance of an oncoming storm.

Finally, another contributing factor are the volatile oils that get washed off plant leaves by the rain. Much like geosmin, these particles are released into the air. These oils often accumulate during dry periods and not only contribute complexity to the Petrichor aroma but are also the reason why rain can smell different depending on where you live (I'm looking at you, Arizona and New Mexico, which have a very distinct Petrichor smell as a result of the Creosote Bush).

Petrichor Candle by Alatreyu
Petrichor candle by Alatreyu, $28 or pay what you can

Modern Appeal:

Beyond its scientific intricacies, Petrichor holds a special place in our hearts due to its nostalgic and calming effects. The smell of Petrichor reminds me of childhood memories spent playing in the rain or walking through the freshly dampened woods (I grew up in central Massachusetts). It's a scent that symbolizes nature's renewal and the promise of a fresh start.

I wanted to create a candle that honored and celebrated this uniquely Earthy scent. It is a beautiful reminder of nature's intricate and complex beauty and a nostalgic celebration of my childhood adventures, playing in the woods and running around in the rain.

Our sensitivity to the smell:

It is believed that humans developed the ability to smell Petrichor (mainly geosmin) with such sensitivity for several reasons.

Firstly, it was a means to detect sources of water. Geosmin is often associated with moist environments, such as soil, lakes, and rivers. Sensitivity to geosmin may have provided an evolutionary advantage by helping our ancestors locate sources of water, a critical resource for survival.

Additionally, the smell often accompanies changes in weather, particularly the onset of rain after a dry period. Sensitivity to geosmin may have helped early humans recognize and respond to environmental changes, such as water availability for drinking, agriculture, and other activities.