adventure journal dan eckert via creative commons

If you’re from California or John Wesley Powell’s arid West, you’ll recognize the five reactionary stages to a rainstorm.

Stage one: Surprise. Did I just hear a raindrop?
Stage two:
Spirituality. Thank (insert god, mother nature, whomever you like), it’s raining!
Stage three:
Wonder. Are my windows rolled up?
Stage four:
Wishful thinking. I hope it never ends.
Stage five:
Smell. Ah, what is that?

You know that smell: earthy, peaty, and yet somehow fresh at the same time. It has the vibrancy of fresh sliced cucumbers on the one hand and the richness of dark roast, steaming espresso on the other. Against all odds and unappealing gustatory metaphors, it works. It’s both delightful and sumptuous.

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What is the smell that we love the world over? It’s called petrichor, as dubbed by scientists Isabel Joy Bear and Roderick G. Thomas. The duo got to the bottom of this global phenomenon way back in 1964 in their paper “The Nature of Agrillaceous Odor,” Nature, Issue 201. Petrichor, which etymologically speaking is Greek for stone + lifeblood of the gods, is a drunken cocktail of plant oils, bacterial-produced compounds, and ozone (sort of).

The oils are flora’s contraception. In arid climates, plants are competitive little buggers, so some species will secrete these oils into the air to prevent seed germination. The oils succumb to gravity and are absorbed by nearby soil and rocks.

The compound that mixes with the oils is called geosmin, which is a byproduct of specific bacteria that break down soils. Geosmin is the most powerful olfactory element in the goodness that is petrichor. Humans are particularly sensitive to smelling geosmin, down to a few parts per trillion. You may also recognize it’s scent from beets and freshly turned garden soil.

The third component, ozone, is a little more up to interpretation. Ozone is always present, but it’s around in higher concentrations during lightning events. The scent, akin to a muted chlorine, can precede a storm as the front and center odor. It can also be detected, on its own, after a storm. But when ozone concentration is heavy in just the right blend with the plant oils and bacterial compound, the soothing, heady petrichor effect is in full force.

Even though Bear and Thomas identified petrichor more than 50 years ago, it was just in spring of 2015 that the mechanics of delivering petrichor from the soil to our pleasantly tickled noses were identified. Scientists at MIT used ultra slow-mo, ultra zoomed-in video to capture the moment a raindrop hits the dirt.

When a raindrop slaps the ground, it’s not just a bounce of water that ricochets back into the air. The videos showed that depending on the strength of the rain (i.e. light versus heavy rain shower) and the compaction of the soil, there are actually microscopic aerosol bubbles created by the force of the impact. In those gaseous bubbles is the magical release of petrichor.

This also leads right down the golden path as to why the petrichor scent is so much more pronounced in arid climates. Dry areas go hand in hand with dry, compacted, and non-porous soils. In a porous soil, fewer aerosol bubbles will be created at impact because the raindrop will be absorbed more readily. The more aerosol, the stronger the scent.

Don’t be too sad, Pacific Northwest. Nature does tend to lean toward equilibrium, after all. Your aerosol numbers may be less impressive to start, meaning you may not have as lingering a petrichor effect, but geosmin is more prolific in wet soils. So there’s that.

Photo by Dan Eckert.


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