Petrichor and the Secret of Rain's Happy Smell
—Eli Schaffer, Environmental Educator
Image: science.howstuffworks.com
Although March was in like a lamb, and out like a lion this year -throwing our adage guide to spring into disarray- April seems to be falling back in line and setting the stage for lots of May flowers. The rainy days we've had lately will help the plants around us show off their warm weather outfits of green, pink, blue and many other colors. I love going on walks outside after it rains, and one aspect of the post-precipitation outside world always captures my attention: that earthy smell. As an environmental educator, I can't let an opportunity for pursuing curiosity like this pass me by. What causes that smell anyways? And why does it smell so good?
In 1964, two researchers, Isabel Bear and Roderick Thomas, gave the smell that rises from the ground after it rains the name petrichor. They found that when rain falls to the ground, it causes some of the material that was on the surface of the forest floor to get absorbed by the water and thrown out into the air as an aerosol. It is kind of like each rain drop turning into a tiny can of hairspray pointed straight up.
So rain causes stuff on the ground to get put into the air, but what is on the ground that causes the post-rain air to have that delightful petrichor smell? It turns out that two different natural chemical compounds are present in the petrichor-ific air. Plants exude an oil when it is dry that causes the plant to use less water. This oil gets deposited onto the dirt and rocks around the plant. When it rains, the oil is dispersed into the air, contributing to petrichor.
The second chemical compound is called geosmin and creates the smell best known as "earthy." Geosmin comes from the spores of a soil bacterium called actinomycetes. Scientists are not sure why, but the human nose has an extreme sensitivity to geosmin and can detect it at concentrations below 5 parts per trillion! That's the equivalent of one teaspoon of water in 200 Olympic swimming pools! A theory to explain why the geosmin chemical smells so enjoyable to mammals, is that over millions of years, the bacterium evolved to attract animals to the water containing its spores so the animal would carry and then deposit the spores in a new location. The bacterium wouldn't be the only thing to benefit though, because it would be pretty handy for animals to sense where the rain had just fallen. I imagine ancient humans rejoicing at the smell of fresh rain after weeks or months of drought and finally knowing where to fill up their water jugs. The petrichor smell is so enjoyable that perfume manufacturers recreate it.
Sometimes, the easiest things to overlook, such as the fact that you enjoy the smell after rain or even the fact that there is a smell after rain, are actually the result of amazing processes that interact with and impact our world just beneath the surface of our lives...or in this case, right under our noses.
Image: dailyhappyquotes.com
