By Megan Campbell ’16, Contributor

Drip, drop, drip, drop.  Rain falling down on your roof.  Drip, drop, drip, drop.  You go outside as the rain comes trickling down and then slows and stops and you take in the scent of the air around you. It smells… good. Earthy, fresh—it smells of natural cleanliness.  But what is it that makes the air after rain smell so good?  Is it simply that the rain has washed away all of the stink and filth of the world, or is it something more?

Petrichor.  The word petrichor is comes from the Greek word petra, meaning stone and ichor—blood that runs through the gods.  The name was created by two scientists during their research to be published as an article in the scientific journal, Nature.  Petrichor, as defined by the Oxford Dictionary, is “a pleasant smell that frequently accompanies the first rain after a long period of warm, dry weather.”  But that doesn’t tell us what petrichor really is.  The scent of petrichor is created by an oil that is released into the air by plants. This oil is created by the plants during dry periods to keep seeds from germinating. The oil is then released into the air when the rain falls as smaller, volatile compounds.  In addition, the oil is released with geosmin created by the bacteria, actinomycetes, otherwise known as actinobacteria.  Geosmin is a byproduct of the bacteria.  These two compounds, geosmin and the oil, then combine to create what we know as petrichor.  So that explains what the smell is, now our question is why does it smell so good to us?  Human beings are extremely sensitive to the smell of petrichor, some being able to smell it at a concentration of 5 parts per trillion.  Not only that, but it is also thought that humans associate the smell of petrichor with the growth of plants and animals, items that humans need to eat.  Animals and plants are much more abundant during times of rain than during time of drought.  So, it makes sense that the scent after a fresh rain is associated with a lot of plant and animal life.  The association between petrichor and abundant life and food and our survival is why petrichor smells so good to us.  So the next time it rains, remember to stop and smell the petrichor.