First off, the smell of rain actually has a name. It’s called petrichor. This term was coined by Australian mineralogists as they started studying the smell of rain. It’s a combination of the Greek words petra (stone) and ichor (the blood of gods in ancient myth). It turns out two aromas might be embedded in the petrichor scent, geosmin and ozone.
Geosmin is a chemical produced by soil-dwelling bacteria, known as actinomycetes, and is found in moist, forested areas. The bacteria secrete the geosmin when they produce spores.
Researchers at MIT recently discovered how the smell gets into the air. They learned that raindrops trap tiny air bubbles as they hit the ground, which then bounce out through the drop and erupt into the air, producing aerosols. When the raindrops hit soil the geosmin gets trapped in the air bubbles and then released into the air. The authors suspect that the tiny particles spread these smells as well as soil-based bacteria and viruses. The results might lead to a better understanding of how contaminants such as E. coli can be spread via rainfall.
Ozone, the molecule made up of three oxygen atoms bonded together whose name comes from the Greek work ozein (to smell)—also plays a role in the smell, especially after thunderstorms. An electric charge, often in the form of a lightning bolt, splits oxygen and nitrogen molecules in the atmosphere and they recombine into nitric oxide (NO), which then interacts with other chemicals in the atmosphere to produce ozone. Ozone has been described as having a sharp scent like chlorine.
It’s believed that the rain and its odor might be delivering chemical messages to animals. Some biologists suspect that petrichor running into waterways is a cue for freshwater fish to start spawning. Microbiologist Keith Chater at the John Innes Center in England has proposed that geosmin's fragrance may help camels find their way to desert oases. In return, the actinomycetes use the camels as vehicles to spread their spores.
But do these smells send meaningful messages to humans? Studies have revealed that the human nose is extremely sensitive to geosmin in particular—some people can detect it at concentrations as low as 5 parts per trillion. To our ancestors, the first rains represented an important event for survival. The rains lure prey, quench our thirst and transform our dry environments.
The strong tie between smell and memory are known. We learn to associate smells with our experiences. Flooding aside, the smell of rain is probably positive for most of us—relief from the relentless summer heat—which is why my memories of Florida’s afternoon storms are so sweet.