Why Does Rain Smell?

My college-aged daughter called me odd the other day. She said it was not meant to be an insult. One of the things she mentioned was my quirky aversion to things like mustard, mayonnaise, and salad dressings. I am sure my sensitivity to smells qualifies me for her odd designation too. On rainy days, I often smell the rain. I bet many of you often smell rain too. The “smell” of rain is called petrichor. Here’s what you need to know about it.

Though petrichor is the used to describe the scent of rain, the smell actually describes oils and chemicals released from the ground. As I previously wrote, “The basic chemistry of the process is related to decomposed organic material fused with soil, rock, and minerals in an interesting chemical brew.” The word is derived from two Greek words – “petra” (stones of the Earth) and “ichor” (a mythological term related to the blood of the gods).

Legendary science journalist Andrew Revkin recently tipped me to a very helpful website hosted by the American Chemical Society (ACS). The website says, “Petrichor is the term coined by Australian scientists in 1964 to describe the unique, earthy smell associated with rain….caused by the water from the rain, along with certain compounds like ozone, geosmin, and plant oils.”

I suspect a couple of those terms may not be familiar to you. As an atmospheric scientist, I can handle ozone. It is an societal ally in the stratosphere where it protects us from harmful ultraviolet radiation, but it can be a pollutant at the surface. When lightning occurs, the ACS website notes, “Diatomic molecules of oxygen and nitrogen (containing 2 atoms) are split, and rearrange to create nitric oxide (NO) and ozone, or O3.” Ozone, transported by rain droplets to the surface, adds to the petrichor scent.

So what is geosmin? The same website goes on to say, “Actinomycetes, a type of bacteria found in soil, secrete a compound called geosmin , which is released from soil intothe air by raindrops.” Our nose can apparently detect geosmin at concentrations less than 5 parts per trillion according to ACS writer. Plant oils like stearic acid and palmitic acid, which are released during rainfall, also contribute to the smell of petrichor.

Interestingly, while researching material for this essay, I learned that there is a song by the group Phish called “Petrichor.” There is also a wine vineyard by the same name. Who knew?

Hopefully you have a better appreciation for those times when you think to youself, “Hmmm, it smells like rain.”