What is the smell of rain and why do some of us love it so much?

Many of us are aware that the smell of rain after a long period of sunshine has a very distinct smell. But did you know that there’s a name for this? It’s called petrichor and it is the result of bacteria and chemicals in the soil.

The term was coined in the 1960s by Isabel Bear and RG Thomas, two Australian scientists working at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Melbourne. The name stems from Greek roots: “petra” meaning stone, and “ichor” which translates to the blood of the gods in Greek mythology. The characteristic smell is produced by the actinomycete bacteria found in soils. These bacteria secrete a molecule known as geosmin and when rain hits the ground, geosmin is released into the air making it detectable to humans.

The characteristic smell is produced by the actinomycete bacteria found in soils

There is also evidence that geosmin may be related to the plant molecules known as terpenes. Terpenes are molecules produced by many plants that give them their scent. A current theory is that when it rains, plant leaf hairs get damaged and release scent chemicals into the air. It is also possible that terpenes are released from dry plant matter, and when these are crushed, by being stepped on for example, the scent is released.

Whilst bacteria and possibly plants are involved in producing this fresh smell, lightning also has a role. During a thunderstorm, a molecule known as ozone is produced due to electrical discharges in the atmosphere splitting nitrogen and oxygen apart to form nitric oxide. In conjugation with other chemicals, atmosphere ozone can be produced from nitric oxide, creating that sharp smell.

During a thunderstorm, a molecule known as ozone is produced due to electrical discharges in the atmosphere splitting nitrogen and oxygen apart to form nitric oxide

As well as producing lightning, thunderstorms and rain greatly improve the air quality. Dust, aerosols and other pollutants are removed out of the air, possibly contributing to this pleasant smell. Whilst many of us enjoy this smell, we are not so pleased when we eat it. Geosmin can make its way into our water and food, and despite it not being toxic or harmful to humans, it can put us off drinks such as wine and mineral water by creating a slightly bitter and acidic taste.

Recently, the perfume industry has shown interest in making a synthetic version of this smell as many of us find it desirable. Whilst we all agree that it is a pleasant smell, anthropologists at the University of Queensland are investigating why so many of us find it appealing. The answer lies with evolution. The current theory suggests that ancient cultures, who relied on the rain for the growth of crops and an increase in game animals for survival, passed on an imprint that we all share today. Referred to as ‘cultural synesthesia’,  it may explain why so many of us find the smell of rain appealing.