Petrichor - The Smell of Rain

Do you know one of these hot summer days where all of a sudden the rain starts? Usually I don’t like rain. Everything gets wet and I feel uncomfortable. But on warm summer days I do love rain! When it starts to rain I run outside and have a long walk. There is this certain small in the air which I can hardly describe. It’s a very pleasant scent that makes me feel good. Did you know that this kind of smell, the smell of summer rain has actually a name? It’s called petrichor and we will investigate where this term originates from and why we can smell the rain.

Fig.1 Dry soil Credits
In 1964 the Australian scientists I.J. Bear and R.G. Thomas introduced the term petrichor for the very first time. They wanted to find out what is responsible for the pleasant smell of rain and did a research. Was it the rain, soil or maybe something else that produced the scent?

Raindrops are basically water and we know that H20 doesn’t have an aroma at all. Go take a bottle of water and try it. You won’t smell anything. So the rain itself can’t be responsible for the scent. I.J. Bear and R.G. Thomas found out that after a period of drought the effect is most powerful. Plants secrete essential oils that are absorbed by the ground (clay, soils etc.). Besides that actinobacteria release a metabolic product, an organic compound which give us the earthy flavor: Geosmin. These compounds are released during rain.

Fig.2 Chemical structure of Geosmin Credits

Scientists at the Massachusetts Institute of Technology in Cambridge went further and filmed raindrops as they hit different surfaces. The experiment can be seen in the video below. The results were that raindrops release the essential oils when they hit the soil. The moment the drop hits the dust particles which are mixed with the oils, the particles shot up and are released as an aerosol into the air. Now comes the crazy part. Not only the essential oils and dust are tossed into the air but also viruses and bacteria can get into the air and threat us.

Fig.3 Video of the MIT experiment Credits

Smell without rain?

Sometimes there is a scent even when it didn’t rain. Some people swear to know that this is an indication for a coming storm. In fact they are right. Lightning strikes can split oxygen and nitrogen molecules in the air. By splitting them they recombine to nitric oxide and ozone. Ozone however has a specific smell. Though the wind these compounds can spread and carry the scent of rain.
In order to release the oils light rain is needed. If the rain is too heavy the bubbles which are shown in the video can’t be formed and they won’t create an aerosol.