The pleasant scent of rain after a dry spell is called "petrichor."
Humans love the smell of rain, a delicious medley of aromas known collectively as “petrichor.” But where does that smell come from? Well, the word itself (coined by Australian scientists in 1964) gives a clue. “Ichor” is the name for the fluid that forms the blood of the gods in Greek mythology, while “petros” in Greek means “stone” — which is one of the places this sweet-smelling chemistry happens. When rain hits porous soil and rocks after a long dry spell, small bacteria called actinobacteria (primarily Streptomyces species) release earthy-smelling organic compounds known as geosmin (a type of alcohol) into the air and into our nostrils. Plants also secrete oils during dry spells that then release odors when it rains. Human noses are highly sensitive to this mixture of smells and can sniff it out better than a shark can smell blood in the sea. Some scientists theorize that early humans relied on this keen sense of smell to find clean sources of water.
Bacteria and plant oils aren’t the only reasons rainstorms come with a signature aroma, however. Lightning can sometimes split two oxygen atoms (O2), which reform with other oxygen molecules to form ozone (O3), a word derived from the Greek for “to smell.” Because of powerful downdrafts, ozone can be carried on the wind for miles, tingling the nostrils of animals and humans alike.
The human nose is a remarkable evolutionary creation. While not as sensitive as a dog’s or as flexible as an elephant’s, its 400 receptors can sniff out the chemical properties of a particular smell and help create neural responses linked with a memory or emotion. But for years, science has underestimated just how good the human nose really is. Since 1927, the common belief was that humans could smell around 10,000 distinct smells, a pitiful number compared to our other senses (human eyes can see around a million colors). However, new research in 2014 discovered that the sniffing ability of the human nose far surpassed that number. The study in question started by showing that humans have a hard time distinguishing between two scents whose chemical mixtures overlap by more than 50%. By extrapolating how many mixtures reside below that 50% ceiling, scientists were able to determine the lower limit of humanity’s sense of smell, which comes out to around 1 trillion odors. In other words, certainly nothing to sniff at.
