After it rains, most of us notice a distinctive refreshing smell that lingers in the air for a while. The strange thing is that most of the time, we didn’t notice that there was a bad odor in the air, but when we walk through a rainstorm, or just step outside after it has rained, slowly hits us…that fresh, earthy smell. There is no way to say exactly what it smells like, because it’s a smell like no other smell I know…it just smells like rain. Strangely, before it hits the ground, rain is just water. It has no smell at all.
The big change happens after it hits the ground, and it might shock you to find out what the cause of that wonderful transformation actually is. After the drops hit the ground and interact with dirt, the fresh and almost sweet fragrance of rain is let go. Now, as near as the scientists have been able to determine, they believe they’ve identified the exact mechanism that releases this aroma into the environment. The smell actually has a name. It’s called “Petrichor” and comes from the Greek words “Petra” or “stone” and “ichor” or a fluid that flows like blood in veins. The phenomenon was first characterized (as the familiar smell after a light rain) by two Australian scientists in 1964, but until now, researchers didn’t understand the physical mechanism behind it.
There are actually several causes for this, and this is where it gets a little…well, creepy…maybe. The most common is a bacteria called Actinomycetes, which is a filamentous type of bacteria that’s smells more strongly in woodland or areas with soil where it grows when it’s damp and warm. When the bacteria dries, spores are produced and that is what we can smell, because the force of the rainfall disperses the spores into the air, rather like an aerosol air freshener. The smell produced by these spores is a kind of earthy, refreshing smell. It’s most noticeable when it rains after a long spell of dryness, but will still occur after most instances of rainfall. Now, I suppose we could become grossed out, but there is nothing we can do about it, so we might as well just enjoy the smell. Besides that, there are bacteria, spores, and pollen in the air we breathe all the time, so it’s not like this is some “new assault” on our systems. There are also more triggers for smells after 
rainstorms. One such trigger is acidity caused by chemicals in the atmosphere, which mostly occurs in heavy urban areas. Another smell that’s common and very similar to actinomycetes, and has even been bottled and sold, is from the evaporative oils that plants and trees produce. That probably doesn’t turn our stomachs quite as much, so if the idea of bacteria bothers you, just make up your mind to believe that the smell comes from evaporative plant oils, and you should feel better about it.
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