If you were to list all the smells you know, many good and bad smells would come to mind. The good smells would probably include the aroma of freshly baked cookies, or maybe the smell of a campfire. For many people, the smell of rain would also make the list.
The sweet, earthy smell of rain is a favorite of many people. Our fondness for the smell of rain is so great that we even purchase perfumes, colognes, soaps and other scented products that smell like rain.
But even better than that bottled-up version is the real deal: stepping outside during or just after a rainfall and sniffing in the sweet smell of rain. The smell of rain even has its own special term — petrichor. The term was coined in the 1960s by two Australian scientists who studied the phenomenon behind the odor.
The reason rain smells has to do with biology, chemistry and timing. A rainstorm that occurs after rain has recently fallen doesn't produce the smell so many of us love. Instead, we are most likely to smell rain when it falls after a dry period.
The earthy smell of rain develops because plants secrete oils during dry periods. These oils then accumulate in the soil and rocks around the plants. When rain falls, the oily compounds combine with other compounds to create the smell we have all come to know.
One of these compounds is called geosmin. It is a chemical produced by bacteria called actinomycetes that live in soil. When it rains, spores produced by the actinomycetes are pushed up into the air, releasing the geosmin and creating the distinctive smell of rains.
Geosmin can be found in most healthy soils, usually in high quantities. Although it's always there, you don't smell it when it is dry because it's the rain that releases it into the air.
Although most people enjoy the smell of geosmin, its taste is another story. Geosmin is sometimes present in small quantities in mineral water and wine, and many people do not like the taste.
If you've noticed that the smell of a thunderstorm is a bit different than an average rainstorm, you aren't wrong. When a thunderstorm is rolling in, it sometimes seems like you can both smell and see it coming. That's because of ozone, which is a form of oxygen. Ozone comes from natural sources as well as fertilizers and pollutants. During a storm, a lightning strike splits atmospheric nitrogen and oxygen into separate particles. Then some of these particles combine with nitric oxide, which can then create ozone.
Ozone is always in the atmosphere, but we can smell it during a storm because down drafts push it to ground level from higher up in the atmosphere. This is why it is possible to actually be able to smell rain coming.
Ozone is said to smell like different things to different people, but it's often described as smelling sweet or clean or having a metallic odor. The word ozone is derived from the Greek word ozein, which means "to smell."
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