Most people can readily identify the smell of rain. It’s more than the sense of moisture in the air—depending on where you live, a light shower might smell sweet, musty and earthy when it hits the soil or it might carry the stench of warm garbage and hot concrete. Whatever the mix of odors is, we have a name for it: petrichor. Petrichor is a mash-up of two greek roots: ichor, which the Atlantic translates as the "ethereal essence" that courses through the veins of gods, and petros, or stones.
Australian scientists first described petrichor in 1964. Given what it smelled like, they figured that its molecules came from decaying plant and animal matter—oils, hydrocarbons and alcohols—that attached themselves to mineral and clay surfaces. Somehow rain drops would release those compounds into the air for us to smell. Now, researchers from MIT have captured this phenomenon on video.
They deployed high-speed cameras to watch water droplets hit different surfaces and saw them trap tiny air bubbles. "As in a glass of champagne, the bubbles then shoot upward, ultimately bursting from the drop in a fizz of aerosols," the MIT News Office explains. Those aerosols can carry with them all the compounds we smell, including some microbes, the researchers say. Moderate or light rains on sandy or clay soils produce the most aerosols, they found. They published their work in Nature Communications.
“This finding should be a good reference for future work, illuminating microbes and chemicals existing inside soil and other natural materials, and how they can be delivered in the environment, and possibly to humans," Youngsoo Joung, a postdoctoral student and one of the researchers, says in the statement. It could even explain how some microbes have been found high in the atmosphere—breezes can pick up the aerosols containing bacteria, reports Rachel Feltman for the Washington Post.
Rain and storms bring other smells, as well. Lightning’s charge creates ozone high in the atmosphere, and thunderstorms' powerful downdrafts deliver it to us, along with the sharp tang of its scent, explains Daisy Yuhas for Scientific American. After the rains fall, the heavy aroma of damp earth and must fills the air. This smell, called geosmin, is produced by bacteria that make their homes in decaying matter and soil. It also lends beets their earthy flavor and can taint wine.
Human noses aren’t the only ones that perk up when rain falls. Yuhas writes;
Some biologists suspect that petrichor running into waterways acts as a cue to freshwater fish, signaling spawning time. Microbiologist Keith Chater at the John Innes Center in England has proposed that geosmin's fragrance may be a beacon, helping camels find their way to desert oases. In return, the bacteria that produce geosmin use the camels as carriers for their spores.
The heady aroma, however it reaches our nostrils, is sure to evoke memories. Smell is wired a little bit differently than our other senses, Natalie Angier writes for the New York Times. Instead of sending new signals to the thalamus, which is serves as a "structural way station" before the signals go to the regions of the brain that can interpret the input, odor receptors send messages to the olfactory cortex. We don’t get a chance to decode the smells before we experience them. Smell is tied in with feelings, as this olfactory cortex lives in the part of the brain where emotional memories are stored.
So whether rainfall reminds you of summer soccer games, puddle-splashing with siblings or a terrifying storm, thank (or blame) the planets, microbes and minerals that give petrichor such a distinctive odor.