What Is Petrichor? What Causes “Rain Smell” Anyway?

Understanding Petrichor: What Causes "Rain Smell" Anyway?

If you go outside often, you’ve probably dealt with that weird rain smell. And if you’re like a lot of people, you also probably like the smell. “Why does rain smell good?” is a popular internet search. But did you know that smell has its own actual name (petrichor) and origins? Let’s look into what exactly petrichor is, why it’s called that, and what causes it in the first place.

What is Petrichor?

While you may be forgiven for not knowing that petrichor is rain smell, most links will direct any thought of rain smell to petrichor. So if you’ve ever tried to find anything at all out about the smell of rain, you should know it’s called petrichor. Anyway, petrichor is the smell you get when rain hits dirt. So if you’ve ever lived in a relatively rainy place and been outside after it’s rained, you know what it smells like. 

If you like etymology, as we frequently get into on the blog, petrichor is constructed from Greek. Literally, it translates to “stone vein,” which sounds way cooler than “rain smell.”

Petrichor, though derived from Greek, was first used in 1964 by Australian researchers. This early research went into how petrichor comes from an oil made by plants. It doesn’t smell when dry, but once it’s washed into the soil by rain, it creates the signature smell as soil and rock absorbs the oils. 

Some of the smell may also come from ozone–stronger after lightning storms. That’s because lightning splits the air to create those molecules. 

Our Mystery Oil

It turns out, nobody actually cares about the oil. We care about what it does once it’s in the soil after it gets absorbed. Once that happens, bacteria have a field day with it–because they have field days with everything, it seems. Anyway, as a byproduct of the bacterial consumption, we get geosmin. That term literally translates to “earth smell,” and is something that has its own interesting properties.

Research at Brown gave us some more insight where there originally wasn’t, especially since our 1964 research just kinda threw out “oil makes geosmin let’s move on.” The bacteria responsible for creating geosmin is named Streptomyces coelicolor, and it has a very specific protein that creates geosmin. No, we’re not going to go further down the rabbit hole. Anyway, Streptomyces coelicolor converts farnesyl diphosphate into geosmin–thus giving us our earthly rain smell.

What Makes Geosmin So Special?

If you thought the human nose was super sensitive to smells, get a whiff of this. The human nose can actually detect geosmin in the parts per trillion, specifically five parts per trillion. Which means if you have a trillion molecules of water (or some other crud), you’ll detect the presence of petrichor if there are just five particles in the mix. 

Pretty nutty, is it not?

Smelling Oncoming Rain

It’s likely that not everyone has experienced this phenomena, but if you haven’t, you probably know someone who has. Or you’ve seen it in a movie–with someone saying that they “smell a storm coming” or something similar. 

Now given how the human nose can detect petrichor in the parts per trillion, we’ve laid it out that our noses are super sensitive.

We mentioned ozone earlier, and the human nose is sensitive to that too. We can detect ozone changes in the parts per billion–which is three less zeroes than trillion, but hey. The threshold for most people is somewhere between 5 and 20 parts per billion. When there’s an oncoming storm, especially one that involves lightning, nitrogen and oxygen molecules in the air begin to split and sometimes form ozone. Drafts carry the ozone ahead of the oncoming storm, and so give us our smell.

With all this talk about rain, maybe it’s time to throw some music into the mix. So pick out some rainy music here.